# Find the missing permutation

Find the missing permutation
You are encouraged to solve this task according to the task description, using any language you may know.
```ABCD
CABD
ACDB
DACB
BCDA
ACBD
CDAB
DABC
CDBA
ABDC
BDCA
DCBA
BACD
BDAC
CBDA
DBCA
DCAB
```

Listed above are all of the permutations of the symbols   A,   B,   C,   and   D,   except   for one permutation that's   not   listed.

Find that missing permutation.

Methods
• Obvious method:
```        enumerate all permutations of   A,  B,  C,  and  D,
and then look for the missing permutation.
```
• alternate method:
```        Hint:  if all permutations were shown above,  how many
times would  A  appear in each position?
What is the  parity  of this number?
```
• another alternate method:
```        Hint:  if you add up the letter values of each column,
does a missing letter   A,  B,  C,  and  D   from each
column cause the total value for each column to be unique?
```

## 360 Assembly

Translation of: BBC BASIC

Very compact version, thanks to the clever Perl 6 "xor" algorithm.

`*        Find the missing permutation - 19/10/2015PERMMISX CSECT         USING  PERMMISX,R15       set base register         LA     R4,0               i=0         LA     R6,1               step         LA     R7,23              toLOOPI    BXH    R4,R6,ELOOPI       do i=1 to hbound(perms)         LA     R5,0               j=0         LA     R8,1               step         LA     R9,4               toLOOPJ    BXH    R5,R8,ELOOPJ       do j=1 to hbound(miss)         LR     R1,R4              i         SLA    R1,2               *4         LA     R3,PERMS-5(R1)     @perms(i)         AR     R3,R5              j         LA     R2,MISS-1(R5)      @miss(j)         XC     0(1,R2),0(R3)      miss(j)=miss(j) xor substr(perms(i),j,1)         B      LOOPJELOOPJ   B      LOOPIELOOPI   XPRNT  MISS,15            print buffer         XR     R15,R15            set return code         BR     R14                return to callerPERMS    DC     C'ABCD',C'CABD',C'ACDB',C'DACB',C'BCDA',C'ACBD'         DC     C'ADCB',C'CDAB',C'DABC',C'BCAD',C'CADB',C'CDBA'         DC     C'CBAD',C'ABDC',C'ADBC',C'BDCA',C'DCBA',C'BACD'         DC     C'BADC',C'BDAC',C'CBDA',C'DBCA',C'DCAB'MISS     DC     4XL1'00',C' is missing'  buffer         YREGS         END    PERMMISX`
Output:
`DBAC is missing`

`with Ada.Text_IO;procedure Missing_Permutations is   subtype Permutation_Character is Character range 'A' .. 'D';    Character_Count : constant :=      1 + Permutation_Character'Pos (Permutation_Character'Last)        - Permutation_Character'Pos (Permutation_Character'First);    type Permutation_String is     array (1 .. Character_Count) of Permutation_Character;    procedure Put (Item : Permutation_String) is   begin      for I in Item'Range loop         Ada.Text_IO.Put (Item (I));      end loop;   end Put;    Given_Permutations : array (Positive range <>) of Permutation_String :=     ("ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD",      "ADCB", "CDAB", "DABC", "BCAD", "CADB", "CDBA",      "CBAD", "ABDC", "ADBC", "BDCA", "DCBA", "BACD",      "BADC", "BDAC", "CBDA", "DBCA", "DCAB");    Count     : array (Permutation_Character, 1 .. Character_Count) of Natural      := (others => (others => 0));   Max_Count : Positive := 1;    Missing_Permutation : Permutation_String;begin   for I in Given_Permutations'Range loop      for Pos in 1 .. Character_Count loop         Count (Given_Permutations (I) (Pos), Pos)   :=           Count (Given_Permutations (I) (Pos), Pos) + 1;         if Count (Given_Permutations (I) (Pos), Pos) > Max_Count then            Max_Count := Count (Given_Permutations (I) (Pos), Pos);         end if;      end loop;   end loop;    for Char in Permutation_Character loop      for Pos in 1 .. Character_Count loop         if Count (Char, Pos) < Max_Count then            Missing_Permutation (Pos) := Char;         end if;      end loop;   end loop;    Ada.Text_IO.Put_Line ("Missing Permutation:");   Put (Missing_Permutation);end Missing_Permutations;`

## Aime

`voidpaste(record r, index x, text p, integer a){    p = insert(p, -1, a);    x.delete(a);    if (~x) {        x.vcall(paste, -1, r, x, p);    } else {        r[p] = 0;    }    x[a] = 0;} integermain(void){    record r;    list l;    index x;     l.bill(0, "ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD", "ADCB",           "CDAB", "DABC", "BCAD", "CADB", "CDBA", "CBAD", "ABDC", "ADBC",           "BDCA", "DCBA", "BACD", "BADC", "BDAC", "CBDA", "DBCA", "DCAB");     x['A'] = x['B'] = x['C'] = x['D'] = 0;     x.vcall(paste, -1, r, x, "");     l.ucall(r_delete, 1, r);     o_(r.low, "\n");     return 0;}`
Output:
`DBAC`

## AppleScript

Translation of: JavaScript
(Statistical versions)

Taking the third approach from the task description, and composing with functional primitives:

Yosemite OS X onwards (uses NSString for sorting):

`use framework "Foundation" -- ( sort ) -- RAREST LETTER IN EACH COLUMN ----------------------------------------------on run    intercalate("", ¬        map(composeAll({¬            head, ¬            curry(minimumBy)'s |λ|(comparing(|length|)), ¬            group, ¬            sort}), ¬            transpose(map(chars, ¬                |words|("ABCD CABD ACDB DACB BCDA ACBD " & ¬                    "ADCB CDAB DABC BCAD CADB CDBA " & ¬                    "CBAD ABDC ADBC BDCA DCBA BACD " & ¬                    "BADC BDAC CBDA DBCA DCAB")))))     --> "DBAC"end run -- GENERIC FUNCTIONS ---------------------------------------------------------- -- chars :: String -> [String]on chars(s)    characters of send chars -- Ordering  :: (-1 | 0 | 1)-- compare :: a -> a -> Orderingon compare(a, b)    if a < b then        -1    else if a > b then        1    else        0    end ifend compare -- comparing :: (a -> b) -> (a -> a -> Ordering)on comparing(f)    script        on |λ|(a, b)            tell mReturn(f) to compare(|λ|(a), |λ|(b))        end |λ|    end scriptend comparing -- composeAll :: [(a -> a)] -> (a -> a)on composeAll(fs)    script        on |λ|(x)            script                on |λ|(f, a)                    mReturn(f)'s |λ|(a)                end |λ|            end script             foldr(result, x, fs)        end |λ|    end scriptend composeAll -- curry :: (Script|Handler) -> Scripton curry(f)    script        on |λ|(a)            script                on |λ|(b)                    |λ|(a, b) of mReturn(f)                end |λ|            end script        end |λ|    end scriptend curry -- foldl :: (a -> b -> a) -> a -> [b] -> aon foldl(f, startValue, xs)    tell mReturn(f)        set v to startValue        set lng to length of xs        repeat with i from 1 to lng            set v to |λ|(v, item i of xs, i, xs)        end repeat        return v    end tellend foldl -- foldr :: (b -> a -> a) -> a -> [b] -> aon foldr(f, startValue, xs)    tell mReturn(f)        set v to startValue        set lng to length of xs        repeat with i from lng to 1 by -1            set v to |λ|(item i of xs, v, i, xs)        end repeat        return v    end tellend foldr -- group :: Eq a => [a] -> [[a]]on group(xs)    script eq        on |λ|(a, b)            a = b        end |λ|    end script     groupBy(eq, xs)end group -- groupBy :: (a -> a -> Bool) -> [a] -> [[a]]on groupBy(f, xs)    set mf to mReturn(f)     script enGroup        on |λ|(a, x)            if length of (active of a) > 0 then                set h to item 1 of active of a            else                set h to missing value            end if             if h is not missing value and mf's |λ|(h, x) then                {active:(active of a) & x, sofar:sofar of a}            else                {active:{x}, sofar:(sofar of a) & {active of a}}            end if        end |λ|    end script     if length of xs > 0 then        set dct to foldl(enGroup, {active:{item 1 of xs}, sofar:{}}, tail(xs))        if length of (active of dct) > 0 then            sofar of dct & {active of dct}        else            sofar of dct        end if    else        {}    end ifend groupBy -- head :: [a] -> aon head(xs)    if length of xs > 0 then        item 1 of xs    else        missing value    end ifend head -- intercalate :: Text -> [Text] -> Texton intercalate(strText, lstText)    set {dlm, my text item delimiters} to {my text item delimiters, strText}    set strJoined to lstText as text    set my text item delimiters to dlm    return strJoinedend intercalate -- length :: [a] -> Inton |length|(xs)    length of xsend |length| -- map :: (a -> b) -> [a] -> [b]on map(f, xs)    tell mReturn(f)        set lng to length of xs        set lst to {}        repeat with i from 1 to lng            set end of lst to |λ|(item i of xs, i, xs)        end repeat        return lst    end tellend map -- minimumBy :: (a -> a -> Ordering) -> [a] -> a on minimumBy(f, xs)    if length of xs < 1 then return missing value    tell mReturn(f)        set v to item 1 of xs        repeat with x in xs            if |λ|(x, v) < 0 then set v to x        end repeat        return v    end tellend minimumBy -- Lift 2nd class handler function into 1st class script wrapper -- mReturn :: Handler -> Scripton mReturn(f)    if class of f is script then        f    else        script            property |λ| : f        end script    end ifend mReturn -- sort :: [a] -> [a]on sort(xs)    ((current application's NSArray's arrayWithArray:xs)'s ¬        sortedArrayUsingSelector:"compare:") as listend sort -- tail :: [a] -> [a]on tail(xs)    if length of xs > 1 then        items 2 thru -1 of xs    else        {}    end ifend tail -- transpose :: [[a]] -> [[a]]on transpose(xss)    script column        on |λ|(_, iCol)            script row                on |λ|(xs)                    item iCol of xs                end |λ|            end script             map(row, xss)        end |λ|    end script     map(column, item 1 of xss)end transpose -- words :: String -> [String]on |words|(s)    words of send |words|`
Output:
`"DBAC"`

## AutoHotkey

`IncompleteList := "ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC BCAD CADB CDBA CBAD ABDC ADBC BDCA DCBA BACD BADC BDAC CBDA DBCA DCAB" CompleteList := Perm( "ABCD" )Missing := "" Loop, Parse, CompleteList, `n, `r  If !InStr( IncompleteList , A_LoopField )    Missing .= "`n" A_LoopField MsgBox Missing Permutation(s):%Missing% ;------------------------------------------------- ; Shortened version of [VxE]'s permutation function; http://www.autohotkey.com/forum/post-322251.html#322251Perm( s , dL="" , t="" , p="") {   StringSplit, m, s, % d := SubStr(dL,1,1) , %t%   IfEqual, m0, 1, return m1 d p   Loop %m0%   {      r := m1      Loop % m0-2         x := A_Index + 1, r .= d m%x%      L .= Perm(r, d, t, m%m0% d p)"`n" , mx := m1      Loop % m0-1         x := A_Index + 1, m%A_Index% := m%x%      m%m0% := mx   }   return substr(L, 1, -1)}`

## AWK

This reads the list of permutations as standard input and outputs the missing one.

`{   split(\$1,a,"");   for (i=1;i<=4;++i) {     t[i,a[i]]++;   } }END {  for (k in t) {    split(k,a,SUBSEP)    for (l in t) {      split(l, b, SUBSEP)      if (a[1] == b[1] && t[k] < t[l]) {        s[a[1]] = a[2]        break      }    }  }  print s[1]s[2]s[3]s[4]}`
Output:

DBAC

## BBC BASIC

`      DIM perms\$(22), miss&(4)      perms\$() = "ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD", "ADCB", \      \  "CDAB", "DABC", "BCAD", "CADB", "CDBA", "CBAD", "ABDC", "ADBC", \      \  "BDCA", "DCBA", "BACD", "BADC", "BDAC", "CBDA", "DBCA", "DCAB"       FOR i% = 0 TO DIM(perms\$(),1)        FOR j% = 1 TO DIM(miss&(),1)          miss&(j%-1) EOR= ASCMID\$(perms\$(i%),j%)        NEXT      NEXT      PRINT \$\$^miss&(0) " is missing"      END`
Output:
```DBAC is missing
```

## Burlesque

` ln"ABCD"[email protected]\/\\ `

(Feed permutations via STDIN. Uses the naive method).

Version calculating frequency of occurences of each letter in each row and thus finding the missing permutation by choosing the letters with the lowest frequency:

` ln)XXtp)><)F:)<]u[/v\[ `

## C

`#include <stdio.h> #define N 4const char *perms[] = {	"ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD", "ADCB", "CDAB",	"DABC", "BCAD", "CADB", "CDBA", "CBAD", "ABDC", "ADBC", "BDCA",	"DCBA", "BACD", "BADC", "BDAC", "CBDA", "DBCA", "DCAB",}; int main(){	int i, j, n, cnt[N];	char miss[N]; 	for (n = i = 1; i < N; i++) n *= i; /* n = (N-1)!, # of occurrence */ 	for (i = 0; i < N; i++) {		for (j = 0; j < N; j++) cnt[j] = 0; 		/* count how many times each letter occur at position i */		for (j = 0; j < sizeof(perms)/sizeof(const char*); j++)			cnt[perms[j][i] - 'A']++; 		/* letter not occurring (N-1)! times is the missing one */		for (j = 0; j < N && cnt[j] == n; j++); 		miss[i] = j + 'A';	}	printf("Missing: %.*s\n", N, miss); 	return 0; }`
Output:
`Missing: DBAC`

## C++

`#include <algorithm>#include <vector>#include <set>#include <iterator>#include <iostream>#include <string> static const std::string GivenPermutations[] = {  "ABCD","CABD","ACDB","DACB",  "BCDA","ACBD","ADCB","CDAB",  "DABC","BCAD","CADB","CDBA",  "CBAD","ABDC","ADBC","BDCA",  "DCBA","BACD","BADC","BDAC",  "CBDA","DBCA","DCAB"};static const size_t NumGivenPermutations = sizeof(GivenPermutations) / sizeof(*GivenPermutations); int main(){    std::vector<std::string> permutations;    std::string initial = "ABCD";    permutations.push_back(initial);     while(true)    {        std::string p = permutations.back();        std::next_permutation(p.begin(), p.end());        if(p == permutations.front())            break;        permutations.push_back(p);    }     std::vector<std::string> missing;    std::set<std::string> given_permutations(GivenPermutations, GivenPermutations + NumGivenPermutations);    std::set_difference(permutations.begin(), permutations.end(), given_permutations.begin(),        given_permutations.end(), std::back_inserter(missing));    std::copy(missing.begin(), missing.end(), std::ostream_iterator<std::string>(std::cout, "\n"));    return 0;}`

## C#

### By permutating

Works with: C# version 2+
`using System;using System.Collections.Generic; namespace MissingPermutation{    class Program    {        static void Main()        {            string[] given = new string[] { "ABCD", "CABD", "ACDB", "DACB",                                             "BCDA", "ACBD", "ADCB", "CDAB",                                             "DABC", "BCAD", "CADB", "CDBA",                                             "CBAD", "ABDC", "ADBC", "BDCA",                                             "DCBA", "BACD", "BADC", "BDAC",                                             "CBDA", "DBCA", "DCAB" };             List<string> result = new List<string>();            permuteString(ref result, "", "ABCD");             foreach (string a in result)                            if (Array.IndexOf(given, a) == -1)                    Console.WriteLine(a + " is a missing Permutation");        }         public static void permuteString(ref List<string> result, string beginningString, string endingString)        {            if (endingString.Length <= 1)            {                                 result.Add(beginningString + endingString);            }            else            {                for (int i = 0; i < endingString.Length; i++)                {                                         string newString = endingString.Substring(0, i) + endingString.Substring(i + 1);                    permuteString(ref result, beginningString + (endingString.ToCharArray())[i], newString);                                    }            }        }    }}`

### By xor-ing the values

Works with: C# version 3+
`using System;using System.Linq; public class Test{    public static void Main()    {        var input = new [] {"ABCD","CABD","ACDB","DACB","BCDA",            "ACBD","ADCB","CDAB","DABC","BCAD","CADB",            "CDBA","CBAD","ABDC","ADBC","BDCA","DCBA",            "BACD","BADC","BDAC","CBDA","DBCA","DCAB"};         int[] values = {0,0,0,0};        foreach (string s in input)            for (int i = 0; i < 4; i++)                values[i] ^= s[i];        Console.WriteLine(string.Join("", values.Select(i => (char)i)));    }}`

## Clojure

` (use 'clojure.math.combinatorics)(use 'clojure.set) (def given (apply hash-set (partition 4 5 "ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC BCAD CADB CDBA CBAD ABDC ADBC BDCA DCBA BACD BADC BDAC CBDA DBCA DCAB" )))(def s1 (apply hash-set (permutations "ABCD")))  	   (def missing (difference s1 given)) `

Here's a version based on the hint in the description. freqs is a sequence of letter frequency maps, one for each column. There should be 6 of each letter in each column, so we look for the one with 5.

`(def abcds ["ABCD" "CABD" "ACDB" "DACB" "BCDA" "ACBD" "ADCB" "CDAB"             "DABC" "BCAD" "CADB" "CDBA" "CBAD" "ABDC" "ADBC" "BDCA"             "DCBA" "BACD" "BADC" "BDAC" "CBDA" "DBCA" "DCAB"]) (def freqs (->> abcds (apply map vector) (map frequencies))) (defn v->k [fqmap v] (->> fqmap (filter #(-> % second (= v))) ffirst)) (->> freqs (map #(v->k % 5)) (apply str) println)`

## CoffeeScript

` missing_permutation = (arr) ->  # Find the missing permutation in an array of N! - 1 permutations.   # We won't validate every precondition, but we do have some basic  # guards.  if arr.length == 0    throw Error "Need more data"  if arr.length == 1      return [arr[0][1] + arr[0][0]]   # Now we know that for each position in the string, elements should appear  # an even number of times (N-1 >= 2).  We can use a set to detect the element appearing  # an odd number of times.  Detect odd occurrences by toggling admission/expulsion  # to and from the set for each value encountered.  At the end of each pass one element  # will remain in the set.  result = ''  for pos in [0...arr[0].length]      set = {}      for permutation in arr          c = permutation[pos]          if set[c]            delete set[c]          else            set[c] = true      for c of set        result += c        break  result given = '''ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC BCAD CADB CDBA   CBAD ABDC ADBC BDCA DCBA BACD BADC BDAC CBDA DBCA DCAB''' arr = (s for s in given.replace('\n', ' ').split ' ' when s != '') console.log missing_permutation(arr) `
Output:
``` > coffee missing_permute.coffee
DBAC
```

## Common Lisp

`(defparameter *permutations*  '("ABCD" "CABD" "ACDB" "DACB" "BCDA" "ACBD" "ADCB" "CDAB" "DABC" "BCAD" "CADB" "CDBA"    "CBAD" "ABDC" "ADBC" "BDCA" "DCBA" "BACD" "BADC" "BDAC" "CBDA" "DBCA" "DCAB")) (defun missing-perm (perms)  (let* ((letters (loop for i across (car perms) collecting i))	 (l (/ (1+ (length perms)) (length letters))))    (labels ((enum (n) (loop for i below n collecting i))	     (least-occurs (pos)	       (let ((occurs (loop for i in perms collecting (aref i pos))))		 (cdr (assoc (1- l) (mapcar #'(lambda (letter)						(cons (count letter occurs) letter))					    letters))))))      (concatenate 'string (mapcar #'least-occurs (enum (length letters)))))))`
Output:
```ROSETTA> (missing-perm *permutations*)
"DBAC"```

## D

`void main() {    import std.stdio, std.string, std.algorithm, std.range, std.conv;     immutable perms = "ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC                       BCAD CADB CDBA CBAD ABDC ADBC BDCA DCBA BACD                       BADC BDAC CBDA DBCA DCAB".split;     // Version 1: test all permutations.    immutable permsSet = perms                         .map!representation                         .zip(true.repeat)                         .assocArray;    auto perm = perms[0].dup.representation;    do {        if (perm !in permsSet)            writeln(perm.map!(c => char(c)));    } while (perm.nextPermutation);     // Version 2: xor all the ASCII values, the uneven one    // gets flushed out. Based on Perl 6 (via Go).    enum len = 4;    char[len] b = 0;    foreach (immutable p; perms)        b[] ^= p[];    b.writeln;     // Version 3: sum ASCII values.    immutable rowSum = perms[0].sum;    len    .iota    .map!(i => to!char(rowSum - perms.transversal(i).sum % rowSum))    .writeln;     // Version 4: a checksum, Java translation. maxCode will be 36.    immutable maxCode = reduce!q{a * b}(len - 1, iota(3, len + 1));     foreach (immutable i; 0 .. len) {        immutable code = perms.map!(p => perms[0].countUntil(p[i])).sum;         // Code will come up 3, 1, 0, 2 short of 36.        perms[0][maxCode - code].write;    }}`
Output:
```DBAC
DBAC
DBAC
DBAC```

## EchoLisp

` ;; use the obvious methos(lib 'list) ; for (permutations) function ;; input(define perms '(ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC BCAD CADB CDBA CBAD ABDC ADBC BDCA DCBA BACD BADC BDAC CBDA DBCA DCAB)) ;; generate all permutations(define all-perms (map list->string (permutations '(A B C D))))   → all-perms ;; {set} substraction(set-substract (make-set all-perms) (make-set perms))  → { DBAC } `

## Elixir

`defmodule RC do  def find_miss_perm(head, perms) do    all_permutations(head) -- perms  end   defp all_permutations(string) do    list = String.split(string, "", trim: true)    Enum.map(permutations(list), fn x -> Enum.join(x) end)  end   defp permutations([]), do: [[]]  defp permutations(list), do: (for x <- list, y <- permutations(list -- [x]), do: [x|y])end perms = ["ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD", "ADCB", "CDAB", "DABC", "BCAD", "CADB", "CDBA",         "CBAD", "ABDC", "ADBC", "BDCA", "DCBA", "BACD", "BADC", "BDAC", "CBDA", "DBCA", "DCAB"] IO.inspect RC.find_miss_perm( hd(perms), perms )`
Output:
```["DBAC"]
```

## Erlang

The obvious method. It seems fast enough (no waiting time).

` -module( find_missing_permutation ). -export( [difference/2, task/0] ). difference( Permutate_this, Existing_permutations ) -> all_permutations( Permutate_this ) -- Existing_permutations. task() -> difference( "ABCD", existing_permutations() ).   all_permutations( String ) -> [[A, B, C, D] || A <- String, B <- String, C <- String, D <- String, is_different([A, B, C, D])]. existing_permutations() -> ["ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD", "ADCB", "CDAB", "DABC", "BCAD", "CADB", "CDBA", "CBAD", "ABDC", "ADBC", "BDCA", "DCBA", "BACD", "BADC", "BDAC", "CBDA", "DBCA", "DCAB"]. is_different( [_H] ) -> true;is_different( [H | T] ) -> not lists:member(H, T) andalso is_different( T ). `
Output:
```6> find_the_missing_permutation:task().
["DBAC"]
```

## ERRE

` PROGRAM MISSING CONST N=4 DIM PERMS\$[23] BEGIN  PRINT(CHR\$(12);) ! CLS  DATA("ABCD","CABD","ACDB","DACB","BCDA","ACBD","ADCB")  DATA("CDAB","DABC","BCAD","CADB","CDBA","CBAD","ABDC","ADBC")  DATA("BDCA","DCBA","BACD","BADC","BDAC","CBDA","DBCA","DCAB")   FOR I%=1 TO UBOUND(PERMS\$,1) DO    READ(PERMS\$[I%])  END FOR   SOL\$="...."   FOR I%=1 TO N DO    CH\$=CHR\$(I%+64)    COUNT%=0    FOR Z%=1 TO N DO       COUNT%=0       FOR J%=1 TO UBOUND(PERMS\$,1) DO          IF CH\$=MID\$(PERMS\$[J%],Z%,1) THEN COUNT%=COUNT%+1 END IF       END FOR       IF COUNT%<>6 THEN           !\$RCODE="MID\$(SOL\$,Z%,1)=CH\$"       END IF    END FOR  END FOR  PRINT("Solution is: ";SOL\$)END PROGRAM `
Output:
```Solution is: DBAC
```

## Factor

Permutations are read in via STDIN.

`USING: io math.combinatorics sequences sets ; "ABCD" all-permutations lines diff first print`
Output:
```DBAC
```

## Forth

Tested with: GForth, VFX Forth, SwiftForth, Win32 Forth. Should work with any ANS Forth system.

Method: Read the permutations in as hexadecimal numbers, exclusive ORing them together gives the answer. (This solution assumes that none of the permutations is defined as a Forth word.)

` hex ABCD     CABD xor ACDB xor DACB xor BCDA xor ACBD xor ADCB xor CDAB xor DABC xor BCAD xor CADB xor CDBA xor CBAD xor ABDC xor ADBC xor BDCA xor DCBA xor BACD xor BADC xor BDAC xor CBDA xor DBCA xor DCAB xor cr .( Missing permutation: ) u. decimal`
Output:
`Missing permutation: DBAC  ok`

## Fortran

Work-around to let it run properly with some bugged versions (e.g. 4.3.2) of gfortran: remove the parameter attribute to the array list.

`program missing_permutation   implicit none  character (4), dimension (23), parameter :: list =                    &    & (/'ABCD', 'CABD', 'ACDB', 'DACB', 'BCDA', 'ACBD', 'ADCB', 'CDAB', &    &   'DABC', 'BCAD', 'CADB', 'CDBA', 'CBAD', 'ABDC', 'ADBC', 'BDCA', &    &   'DCBA', 'BACD', 'BADC', 'BDAC', 'CBDA', 'DBCA', 'DCAB'/)  integer :: i, j, k   do i = 1, 4    j = minloc ((/(count (list (:) (i : i) == list (1) (k : k)), k = 1, 4)/), 1)    write (*, '(a)', advance = 'no') list (1) (j : j)  end do  write (*, *) end program missing_permutation`
Output:
`DBAC`

## FreeBASIC

### Simple count

`' version 30-03-2017' compile with: fbc -s console Data "ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD"Data "ADCB", "CDAB", "DABC", "BCAD", "CADB", "CDBA"Data "CBAD", "ABDC", "ADBC", "BDCA", "DCBA", "BACD"Data "BADC", "BDAC", "CBDA", "DBCA", "DCAB" ' ------=< MAIN >=------ Dim As ulong total(3, Asc("A") To Asc("D"))  ' total(0 to 3, 65 to 68)Dim As ULong i, j, n = 24 \ 4   ' n! \ nDim As String tmp For i = 1 To 23    Read tmp    For j = 0 To 3        total(j, tmp[j]) += 1    NextNext tmp = Space(4)For i = 0 To 3    For j = Asc("A") To Asc("D")        If total(i, j) <> n Then         tmp[i] = j        End If    NextNext Print "The missing permutation is : "; tmp ' empty keyboard bufferWhile InKey <> "" : WendPrint : Print "hit any key to end program"SleepEnd`
Output:
`The missing permutation is : DBAC`

`' version 30-03-2017' compile with: fbc -s console Data "ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD"Data "ADCB", "CDAB", "DABC", "BCAD", "CADB", "CDBA"Data "CBAD", "ABDC", "ADBC", "BDCA", "DCBA", "BACD"Data "BADC", "BDAC", "CBDA", "DBCA", "DCAB" ' ------=< MAIN >=------ Dim As ULong total(3)  ' total(0 to 3)Dim As ULong i, j, n = 24 \ 4   ' n! \ nDim As ULong total_val = (Asc("A") + Asc("B") + Asc("C") + Asc("D")) * nDim As String tmp For i = 1 To 23    Read tmp    For j = 0 To 3        total(j) += tmp[j]    NextNext tmp = Space(4)For i = 0 To 3    tmp[i] = total_val - total(i)Next Print "The missing permutation is : "; tmp ' empty keyboard bufferWhile Inkey <> "" : WendPrint : Print "hit any key to end program"SleepEnd`
`output is same as the first version`

### Using Xor

`' version 30-03-2017' compile with: fbc -s console Data "ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD"Data "ADCB", "CDAB", "DABC", "BCAD", "CADB", "CDBA"Data "CBAD", "ABDC", "ADBC", "BDCA", "DCBA", "BACD"Data "BADC", "BDAC", "CBDA", "DBCA", "DCAB" ' ------=< MAIN >=------ Dim As ULong i,j Dim As String tmp, missing = chr(0, 0, 0, 0) ' or string(4, 0) For i = 1 To 23    Read tmp    For j = 0 To 3        missing[j] Xor= tmp[j]    NextNext Print "The missing permutation is : "; missing ' empty keyboard bufferWhile Inkey <> "" : WendPrint : Print "hit any key to end program"SleepEnd`
`Output is the same as the first version`

## GAP

`# our deficient listL :=[ "ABCD", "CABD", "ACDB", "DACB", "BCDA",  "ACBD", "ADCB", "CDAB", "DABC", "BCAD",  "CADB", "CDBA", "CBAD", "ABDC", "ADBC",  "BDCA", "DCBA", "BACD", "BADC", "BDAC",  "CBDA", "DBCA", "DCAB" ]; # convert L to permutations on 1..4u := List(L, s -> List([1..4], i -> Position("ABCD", s[i]))); # set difference (with all permutations)v := Difference(PermutationsList([1..4]), u); # convert back to letterss := "ABCD";List(v, p -> List(p, i -> s[i]));`

## Go

Alternate method suggested by task description:

`package main import (    "fmt"    "strings") var given = strings.Split(`ABCDCABDACDBDACBBCDAACBDADCBCDABDABCBCADCADBCDBACBADABDCADBCBDCADCBABACDBADCBDACCBDADBCADCAB`, "\n") func main() {    b := make([]byte, len(given[0]))    for i := range b {        m := make(map[byte]int)        for _, p := range given {            m[p[i]]++        }        for char, count := range m {            if count&1 == 1 {                b[i] = char                break            }        }    }    fmt.Println(string(b))}`

Xor method suggested by Perl 6 contributor:

`func main() {    b := make([]byte, len(given[0]))    for _, p := range given {        for i, c := range []byte(p) {            b[i] ^= c        }    }    fmt.Println(string(b))}`
Output:
in either case:
```DBAC
```

## Groovy

Solution:

`def fact = { n -> [1,(1..<(n+1)).inject(1) { prod, i -> prod * i }].max() }def missingPermsmissingPerms = {List elts, List perms ->    perms.empty ? elts.permutations() : elts.collect { e ->        def ePerms = perms.findAll { e == it[0] }.collect { it[1..-1] }        ePerms.size() == fact(elts.size() - 1) ? [] \            : missingPerms(elts - e, ePerms).collect { [e] + it }    }.sum()}`

Test:

`def e = 'ABCD' as Listdef p = ['ABCD', 'CABD', 'ACDB', 'DACB', 'BCDA', 'ACBD', 'ADCB', 'CDAB', 'DABC', 'BCAD', 'CADB', 'CDBA',        'CBAD', 'ABDC', 'ADBC', 'BDCA', 'DCBA', 'BACD', 'BADC', 'BDAC', 'CBDA', 'DBCA', 'DCAB'].collect { it as List } def mp = missingPerms(e, p)mp.each { println it }`
Output:
`[D, B, A, C]`

#### Difference between two lists

Works with: GHC version 7.10.3
`import Data.List ((\\), permutations, nub)import Control.Monad (join) missingPerm  :: Eq a  => [[a]] -> [[a]]missingPerm = (\\) =<< permutations . nub . join deficientPermsList :: [String]deficientPermsList =  [ "ABCD"  , "CABD"  , "ACDB"  , "DACB"  , "BCDA"  , "ACBD"  , "ADCB"  , "CDAB"  , "DABC"  , "BCAD"  , "CADB"  , "CDBA"  , "CBAD"  , "ABDC"  , "ADBC"  , "BDCA"  , "DCBA"  , "BACD"  , "BADC"  , "BDAC"  , "CBDA"  , "DBCA"  , "DCAB"  ] main :: IO ()main = print \$ missingPerm deficientPermsList`
Output:
`["DBAC"]`

#### Character frequency in each column

Another, more statistical, approach is to return the least common letter in each of the four columns. (If all permutations were present, letter frequencies would not vary).

`import Data.List (minimumBy, group, sort, transpose)import Data.Ord (comparing) missingPerm  :: Ord a  => [[a]] -> [a]missingPerm = ((head . minimumBy (comparing length) . group . sort) <\$>) . transpose deficientPermsList :: [String]deficientPermsList =  [ "ABCD"  , "CABD"  , "ACDB"  , "DACB"  , "BCDA"  , "ACBD"  , "ADCB"  , "CDAB"  , "DABC"  , "BCAD"  , "CADB"  , "CDBA"  , "CBAD"  , "ABDC"  , "ADBC"  , "BDCA"  , "DCBA"  , "BACD"  , "BADC"  , "BDAC"  , "CBDA"  , "DBCA"  , "DCAB"  ] main :: IO ()main = print \$ missingPerm deficientPermsList`
Output:
`"DBAC"`

## Icon and Unicon

`link strings    # for permutes procedure main()givens := set![ "ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD", "ADCB", "CDAB", "DABC", "BCAD", "CADB",                 "CDBA", "CBAD", "ABDC", "ADBC", "BDCA", "DCBA", "BACD", "BADC", "BDAC", "CBDA", "DBCA", "DCAB"] every insert(full := set(), permutes("ABCD"))  # generate all permutationsgivens := full--givens                         # and difference write("The difference is : ")every write(!givens, " ")end`

The approach above generates a full set of permutations and calculates the difference. Changing the two commented lines to the three below will calculate on the fly and would be more efficient for larger data sets.

`every x := permutes("ABCD") do                    # generate all permutations    if member(givens,x) then delete(givens,x)      # remove givens as they are generated   else insert(givens,x)                          # add back any not given`

A still more efficient version is:

`link strings procedure main()    givens := set("ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD",                  "ADCB", "CDAB", "DABC", "BCAD", "CADB", "CDBA",                  "CBAD", "ABDC", "ADBC", "BDCA", "DCBA", "BACD",                  "BADC", "BDAC", "CBDA", "DBCA", "DCAB")     every p := permutes("ABCD") do         if not member(givens, p) then write(p) end`

## J

Solution:

`permutations=: A.~ [email protected][email protected]#missingPerms=: -.~ permutations @ {.`

Use:

```data=: >;: 'ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC BCAD CADB CDBA'

missingPerms data
DBAC```

### Alternatives

Or the above could be a single definition that works the same way:

`missingPerms=: -.~ (A.~ [email protected][email protected]#) @ {.   `

Or the equivalent explicit (cf. tacit above) definition:

`missingPerms=: monad define  item=. {. y  y -.~ item A.~ i.! #item)`

Or, the solution could be obtained without defining an independent program:

`   data -.~ 'ABCD' A.~ i.!4DBAC`

Here, `'ABCD'` represents the values being permuted (their order does not matter), and `4` is how many of them we have.

Yet another alternative expression, which uses parentheses instead of the passive operator (`~`), would be:

`   ((i.!4) A. 'ABCD') -. dataDBAC`

Of course the task suggests that the missing permutation can be found without generating all permutations. And of course that is doable:

`   'ABCD'{~,[email protected](= <./)@(#/.~)@('ABCD' , ])"1 |:permsDBAC`

However, that's actually a false economy - not only does this approach take more code to implement (at least, in J) but we are already dealing with a data structure of approximately the size of all permutations. So what is being saved by this supposedly "more efficient" approach? Not much... (Still, perhaps this exercise is useful as an illustration of some kind of advertising concept?)

We could use parity, as suggested in the task hints:

`   ,(~.#~2|(#/.~))"1|:dataDBAC`

We could use arithmetic, as suggested in the task hints:

`   ({.data){~|(->./)+/({.i.])dataDBAC`

## Java

optimized Following needs: Utils.java

`import java.util.ArrayList; import com.google.common.base.Joiner;import com.google.common.collect.ImmutableSet;import com.google.common.collect.Lists; public class FindMissingPermutation {	public static void main(String[] args) {		Joiner joiner = Joiner.on("").skipNulls();		ImmutableSet<String> s = ImmutableSet.of("ABCD", "CABD", "ACDB",				"DACB", "BCDA", "ACBD", "ADCB", "CDAB", "DABC", "BCAD", "CADB",				"CDBA", "CBAD", "ABDC", "ADBC", "BDCA", "DCBA", "BACD", "BADC",				"BDAC", "CBDA", "DBCA", "DCAB"); 		for (ArrayList<Character> cs : Utils.Permutations(Lists.newArrayList(				'A', 'B', 'C', 'D')))			if (!s.contains(joiner.join(cs)))				System.out.println(joiner.join(cs));	}}`
Output:
`DBAC`

Alternate version, based on checksumming each position:

`public class FindMissingPermutation{  public static void main(String[] args)  {    String[] givenPermutations = { "ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD",                                   "ADCB", "CDAB", "DABC", "BCAD", "CADB", "CDBA",                                   "CBAD", "ABDC", "ADBC", "BDCA", "DCBA", "BACD",                                   "BADC", "BDAC", "CBDA", "DBCA", "DCAB" };    String characterSet = givenPermutations[0];    // Compute n! * (n - 1) / 2    int maxCode = characterSet.length() - 1;    for (int i = characterSet.length(); i >= 3; i--)      maxCode *= i;    StringBuilder missingPermutation = new StringBuilder();    for (int i = 0; i < characterSet.length(); i++)    {      int code = 0;      for (String permutation : givenPermutations)        code += characterSet.indexOf(permutation.charAt(i));      missingPermutation.append(characterSet.charAt(maxCode - code));    }    System.out.println("Missing permutation: " + missingPermutation.toString());  }}`

## JavaScript

### ES5

#### Imperative

The permute() function taken from http://snippets.dzone.com/posts/show/1032

`permute = function(v, m){ //v1.0    for(var p = -1, j, k, f, r, l = v.length, q = 1, i = l + 1; --i; q *= i);    for(x = [new Array(l), new Array(l), new Array(l), new Array(l)], j = q, k = l + 1, i = -1;        ++i < l; x[2][i] = i, x[1][i] = x[0][i] = j /= --k);    for(r = new Array(q); ++p < q;)        for(r[p] = new Array(l), i = -1; ++i < l; !--x[1][i] && (x[1][i] = x[0][i],            x[2][i] = (x[2][i] + 1) % l), r[p][i] = m ? x[3][i] : v[x[3][i]])            for(x[3][i] = x[2][i], f = 0; !f; f = !f)                for(j = i; j; x[3][--j] == x[2][i] && (x[3][i] = x[2][i] = (x[2][i] + 1) % l, f = 1));    return r;}; list = [ 'ABCD', 'CABD', 'ACDB', 'DACB', 'BCDA', 'ACBD', 'ADCB', 'CDAB',        'DABC', 'BCAD', 'CADB', 'CDBA', 'CBAD', 'ABDC', 'ADBC', 'BDCA',        'DCBA', 'BACD', 'BADC', 'BDAC', 'CBDA', 'DBCA', 'DCAB']; all = permute(list[0].split('')).map(function(elem) {return elem.join('')}); missing = all.filter(function(elem) {return list.indexOf(elem) == -1});print(missing);  // ==> DBAC`

#### Functional

`(function (strList) {     // [a] -> [[a]]    function permutations(xs) {        return xs.length ? (            chain(xs, function (x) {                return chain(permutations(deleted(x, xs)), function (ys) {                    return [[x].concat(ys).join('')];                })            })) : [[]];    }     // Monadic bind/chain for lists    // [a] -> (a -> b) -> [b]    function chain(xs, f) {        return [].concat.apply([], xs.map(f));    }     // a -> [a] -> [a]    function deleted(x, xs) {        return xs.length ? (            x === xs[0] ? xs.slice(1) : [xs[0]].concat(                deleted(x, xs.slice(1))            )        ) : [];    }     // Provided subset    var lstSubSet = strList.split('\n');     // Any missing permutations    // (we can use fold/reduce, filter, or chain (concat map) here)    return chain(permutations('ABCD'.split('')), function (x) {        return lstSubSet.indexOf(x) === -1 ? [x] : [];    }); })(    'ABCD\nCABD\nACDB\nDACB\nBCDA\nACBD\nADCB\nCDAB\nDABC\nBCAD\nCADB\n\CDBA\nCBAD\nABDC\nADBC\nBDCA\nDCBA\nBACD\nBADC\nBDAC\nCBDA\nDBCA\nDCAB');`
Output:
`["DBAC"]`

### ES6

#### Statistical

##### Using a dictionary
`(() => {    'use strict';     // transpose :: [[a]] -> [[a]]    let transpose = xs =>        xs[0].map((_, iCol) => xs            .map((row) => row[iCol]));      let xs = 'ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB' +        ' DABC BCAD CADB CDBA CBAD ABDC ADBC BDCA DCBA' +        ' BACD BADC BDAC CBDA DBCA DCAB'     return transpose(xs.split(' ')            .map(x => x.split('')))        .map(col => col.reduce((a, x) => ( // count of each character in each column            a[x] = (a[x] || 0) + 1,            a        ), {}))        .map(dct => { // character with frequency below mean of distribution ?            let ks = Object.keys(dct),                xs = ks.map(k => dct[k]),                mean = xs.reduce((a, b) => a + b, 0) / xs.length;             return ks.reduce(                (a, k) => a ? a : (dct[k] < mean ? k : undefined),                undefined            );        })        .join(''); // 4 chars as single string     // --> 'DBAC'})();`
Output:
`DBAC`

##### Composing functional primitives
`(() => {    'use strict';     // MISSING PERMUTATION ---------------------------------------------------     // missingPermutation :: [String] -> String    const missingPermutation = xs =>        map(            // Rarest letter,            compose([                sort,                group,                curry(minimumBy)(comparing(length)),                head            ]),             // in each column.            transpose(map(stringChars, xs))        )        .join('');      // GENERIC FUNCTIONAL PRIMITIVES -----------------------------------------     // transpose :: [[a]] -> [[a]]    const transpose = xs =>        xs[0].map((_, iCol) => xs.map(row => row[iCol]));     // sort :: Ord a => [a] -> [a]    const sort = xs => xs.sort();     // group :: Eq a => [a] -> [[a]]    const group = xs => groupBy((a, b) => a === b, xs);     // groupBy :: (a -> a -> Bool) -> [a] -> [[a]]    const groupBy = (f, xs) => {        const dct = xs.slice(1)            .reduce((a, x) => {                const                    h = a.active.length > 0 ? a.active[0] : undefined,                    blnGroup = h !== undefined && f(h, x);                 return {                    active: blnGroup ? a.active.concat(x) : [x],                    sofar: blnGroup ? a.sofar : a.sofar.concat([a.active])                };            }, {                active: xs.length > 0 ? [xs[0]] : [],                sofar: []            });        return dct.sofar.concat(dct.active.length > 0 ? [dct.active] : []);    };     // length :: [a] -> Int    const length = xs => xs.length;     // comparing :: (a -> b) -> (a -> a -> Ordering)    const comparing = f =>        (x, y) => {            const                a = f(x),                b = f(y);            return a < b ? -1 : a > b ? 1 : 0        };     // minimumBy :: (a -> a -> Ordering) -> [a] -> a    const minimumBy = (f, xs) =>        xs.reduce((a, x) => a === undefined ? x : (            f(x, a) < 0 ? x : a        ), undefined);     // head :: [a] -> a    const head = xs => xs.length ? xs[0] : undefined;     // map :: (a -> b) -> [a] -> [b]    const map = (f, xs) => xs.map(f)     // compose :: [(a -> a)] -> (a -> a)    const compose = fs => x => fs.reduce((a, f) => f(a), x);     // curry :: ((a, b) -> c) -> a -> b -> c    const curry = f => a => b => f(a, b);     // stringChars :: String -> [Char]    const stringChars = s => s.split('');      // TEST ------------------------------------------------------------------     return missingPermutation(["ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD",        "ADCB", "CDAB", "DABC", "BCAD", "CADB", "CDBA", "CBAD", "ABDC", "ADBC",        "BDCA", "DCBA", "BACD", "BADC", "BDAC", "CBDA", "DBCA", "DCAB"    ]);     // -> "DBAC"})();`
Output:
`DBAC`

## jq

Works with: jq version 1.4

The following assumes that a file, Find_the_missing_permutation.txt, has the text exactly as presented in the task description.

To find the missing permutation, we can for simplicity invoke jq twice:

``` jq -R . Find_the_missing_permutation.txt | jq -s -f Find_the_missing_permutation.jq
```

The first invocation simply converts the raw text into a stream of JSON strings; these are then processed by the following program, which implements the parity-based approach.

The program will handle permutations of any set of uppercase letters. The letters need not be consecutive. Note that the following encoding of letters is used: A => 0, B => 1, ....

Infrastructure:

If your version of jq has transpose/0, the definition given here (which is the same as in Matrix_Transpose#jq) may be omitted.

`def transpose:  if (.[0] | length) == 0 then []  else [map(.[0])] + (map(.[1:]) | transpose)  end ; # Input:  an array of integers (based on the encoding of A=0, B=1, etc)#         corresponding to the occurrences in any one position of the#         letters in the list of permutations.# Output: a tally in the form of an array recording in position i the#         parity of the number of occurrences of the letter corresponding to i.# Example: given [0,1,0,1,2], the array of counts of 0, 1, and 2 is [2, 2, 1],#          and thus the final result is [0, 0, 1].def parities:  reduce .[] as \$x ( []; .[\$x] = (1 + .[\$x]) % 2); # Input: an array of parity-counts, e.g. [0, 1, 0, 0]# Output: the corresponding letter, e.g. "B".def decode:  [index(1) + 65] | implode; # encode a string (e.g. "ABCD") as an array (e.g. [0,1,2,3]):def encode_string: [explode[] - 65];`

`map(encode_string) | transpose | map(parities | decode) | join("")`
Output:
`\$ jq -R . Find_the_missing_permutation.txt | jq -s -f Find_the_missing_permutation.jq"DBAC"`

## Julia

Works with: Julia version 0.6

## Obvious method

Calculate all possible permutations and return the first not included in the array.

`using Combinatoricsfunction missingperm(arr::Vector)    allperms = permutations(arr[1])    for perm in allperms        perm = convert(eltype(arr), perm)        if perm ∉ arr return perm end    endend arr = ["ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD", "ADCB", "CDAB", "DABC", "BCAD",       "CADB", "CDBA", "CBAD", "ABDC", "ADBC", "BDCA", "DCBA", "BACD", "BADC", "BDAC",       "CBDA", "DBCA", "DCAB"]@show missingperm(arr)`
Output:
`missingperm(arr) = "DBAC"`

## Alternative method

Translation of: Python
`function missingperm1(arr::Vector{<:AbstractString})    missperm = string()    for pos in 1:length(arr[1])        s = Set()        for perm in arr            c = perm[pos]            if c ∈ s pop!(s, c) else push!(s, c) end        end        missperm *= first(s)    end    return misspermend using BenchmarkTools @btime missingperm(arr)@btime missingperm1(arr)`
Output:
```  8.818 μs (213 allocations: 10.34 KiB)
6.915 μs (24 allocations: 2.06 KiB)```

As you con see from the benchmark the second function is more efficient in term of both time and (most of all) space.

## K

`   split:{1_'(&x=y)_ x:y,x}    g: ("ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC BCAD CADB")   g,:(" CDBA CBAD ABDC ADBC BDCA DCBA BACD BADC BDAC CBDA DBCA DCAB")   p: split[g;" "];    / All permutations of "ABCD"   perm:{:[1<x;,/(>:'(x,x)#1,x#0)[;0,'1+_f x-1];,!x]}   p2:[email protected](perm(#a:"ABCD"));    / Which permutations in p are there in p2?   p2 _lin p1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1 1 1    / Invert the result   ~p2 _lin p0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0    / It's the 20th permutation that is missing   &~p2 _lin p,20    [email protected]&~p2 _lin p"DBAC"`

Alternative approach:

` table:{[email protected]<b:([email protected]*:'a),'#:'a:=x},/"ABCD"@&:'{5=(table p[;x])[;1]}'!4"DBAC"`

Third approach (where p is the given set of permutations):

` ,/[email protected]&~(p2:{[email protected]@&n=(#?:)'m:!n#n:#x}[*p]) _lin p `

## Kotlin

`// version 1.1.2 fun <T> permute(input: List<T>): List<List<T>> {    if (input.size == 1) return listOf(input)    val perms = mutableListOf<List<T>>()    val toInsert = input[0]    for (perm in permute(input.drop(1))) {        for (i in 0..perm.size) {            val newPerm = perm.toMutableList()            newPerm.add(i, toInsert)            perms.add(newPerm)        }    }    return perms} fun <T> missingPerms(input: List<T>, perms: List<List<T>>) = permute(input) - perms fun main(args: Array<String>) {    val input = listOf('A', 'B', 'C', 'D')    val strings = listOf(        "ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD", "ADCB", "CDAB",        "DABC", "BCAD", "CADB", "CDBA", "CBAD", "ABDC", "ADBC", "BDCA",        "DCBA", "BACD", "BADC", "BDAC", "CBDA", "DBCA", "DCAB"    )    val perms = strings.map { it.toList() }    val missing = missingPerms(input, perms)    if (missing.size == 1)        print("The missing permutation is \${missing[0].joinToString("")}")    else {        println("There are \${missing.size} missing permutations, namely:\n")        for (perm in missing) println(perm.joinToString(""))    }}`
Output:
```The missing permutation is DBAC
```

## Lua

Using the popular Penlight extension module - https://luarocks.org/modules/steved/penlight

`local permute, tablex = require("pl.permute"), require("pl.tablex")local permList, pStr = {    "ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD", "ADCB", "CDAB",    "DABC", "BCAD", "CADB", "CDBA", "CBAD", "ABDC", "ADBC", "BDCA",    "DCBA", "BACD", "BADC", "BDAC", "CBDA", "DBCA", "DCAB"}for perm in permute.iter({"A","B","C","D"}) do    pStr = table.concat(perm)    if not tablex.find(permList, pStr) then print(pStr) endend`
Output:
`DBAC`

## Maple

`lst := ["ABCD","CABD","ACDB","DACB","BCDA","ACBD","ADCB","CDAB","DABC","BCAD","CADB","CDBA","CBAD","ABDC","ADBC","BDCA","DCBA","BACD","BADC","BDAC","CBDA","DBCA","DCAB"]:perm := table():for letter in "ABCD" do	perm[letter] := 0:end do:for item in lst do	for letter in "ABCD" do		perm[letter] += StringTools:-FirstFromLeft(letter, item):	end do:end do:print(StringTools:-Join(ListTools:-Flatten([indices(perm)], 4)[sort(map(x->60-x, ListTools:-Flatten([entries(perm)],4)),'output=permutation')], "")):`
Output:
`"DBAC"`

## Mathematica / Wolfram Language

`ProvidedSet = {"ABCD" , "CABD" , "ACDB" , "DACB" , "BCDA" , "ACBD", "ADCB" , "CDAB", "DABC", "BCAD" , "CADB", "CDBA" , "CBAD" , "ABDC", "ADBC" , "BDCA",  "DCBA" , "BACD", "BADC", "BDAC" , "CBDA", "DBCA", "DCAB"}; Complement[StringJoin /@ [email protected]@[email protected]#, #] &@ProvidedSet  ->{"DBAC"}`

## MATLAB

This solution is designed to work on a column vector of strings. This will not work with a cell array or row vector of strings.

`function perm = findMissingPerms(list)     permsList = perms(list(1,:)); %Generate all permutations of the 4 letters    perm = []; %This is the functions return value if the list is not missing a permutation     %Normally the rest of this would be vectorized, but because this is    %done on a vector of strings, the vectorized functions will only access    %one character at a time. So, in order for this to work we have to use    %loops.    for i = (1:size(permsList,1))         found = false;         for j = (1:size(list,1))            if (permsList(i,:) == list(j,:))                found = true;                break            end        end         if not(found)            perm = permsList(i,:);            return        end     end %for   end %fingMissingPerms`
Output:
`>> list = ['ABCD';'CABD';'ACDB';'DACB';'BCDA';'ACBD';'ADCB';'CDAB';'DABC';'BCAD';'CADB';'CDBA';'CBAD';'ABDC';'ADBC';'BDCA';'DCBA';'BACD';'BADC';'BDAC';'CBDA';'DBCA';'DCAB'] list = ABCDCABDACDBDACBBCDAACBDADCBCDABDABCBCADCADBCDBACBADABDCADBCBDCADCBABACDBADCBDACCBDADBCADCAB >> findMissingPerms(list) ans = DBAC`

## Nim

Translation of: Python
`import strutils proc missingPermutation(arr): string =  result = ""  if arr.len == 0: return  if arr.len == 1: return arr[0][1] & arr[0][0]   for pos in 0 .. <arr[0].len:    var s: set[char] = {}    for permutation in arr:      let c = permutation[pos]      if c in s: s.excl c      else:      s.incl c    for c in s: result.add c const given = """ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC BCAD CADB CDBA  CBAD ABDC ADBC BDCA DCBA BACD BADC BDAC CBDA DBCA DCAB""".split() echo missingPermutation(given)`
Output:
`DBAC`

## OCaml

some utility functions:

`(* insert x at all positions into li and return the list of results *)let rec insert x = function  | [] -> [[x]]  | a::m as li -> (x::li) :: (List.map (fun y -> a::y) (insert x m)) (* list of all permutations of li *)let permutations li =   List.fold_right (fun a z -> List.concat (List.map (insert a) z)) li [[]] (* convert a string to a char list *)let chars_of_string s =  let cl = ref [] in  String.iter (fun c -> cl := c :: !cl) s;  (List.rev !cl) (* convert a char list to a string *)let string_of_chars cl =  String.concat "" (List.map (String.make 1) cl)`

`let deficient_perms = [  "ABCD";"CABD";"ACDB";"DACB";  "BCDA";"ACBD";"ADCB";"CDAB";  "DABC";"BCAD";"CADB";"CDBA";  "CBAD";"ABDC";"ADBC";"BDCA";  "DCBA";"BACD";"BADC";"BDAC";  "CBDA";"DBCA";"DCAB";  ] let it = chars_of_string (List.hd deficient_perms) let perms = List.map string_of_chars (permutations it) let results = List.filter (fun v -> not(List.mem v deficient_perms)) perms let () = List.iter print_endline results`

Alternate method : if we had all permutations, each letter would appear an even number of times at each position. Since there is only one permutation missing, we can find where each letter goes by looking at the parity of the number of occurences of each letter. The following program works with permutations of at least 3 letters:

`let array_of_perm s =	let n = String.length s in	Array.init n (fun i -> int_of_char s.[i] - 65);; let perm_of_array a =	let n = Array.length a in	let s = String.create n in	Array.iteri (fun i x ->		s.[i] <- char_of_int (x + 65)	) a;	s;; let find_missing v =	let n = String.length (List.hd v) in	let a = Array.make_matrix n n 0	and r = ref v in	List.iter (fun s ->		let u = array_of_perm s in		Array.iteri (fun i x -> x.(u.(i)) <- x.(u.(i)) + 1) a	) v;	let q = Array.make n 0 in	Array.iteri (fun i x ->		Array.iteri (fun j y ->			if y mod 2 != 0 then q.(i) <- j		) x	) a;	perm_of_array q;; find_missing deficient_perms;;(* - : string = "DBAC" *)`

## Octave

`given = [ 'ABCD';'CABD';'ACDB';'DACB'; ...          'BCDA';'ACBD';'ADCB';'CDAB'; ...          'DABC';'BCAD';'CADB';'CDBA'; ...          'CBAD';'ABDC';'ADBC';'BDCA'; ...          'DCBA';'BACD';'BADC';'BDAC'; ...          'CBDA';'DBCA';'DCAB' ];val = 4.^(3:-1:0)';there = 1+(toascii(given)-toascii('A'))*val;every = 1+perms(0:3)*val; bits = zeros(max(every),1);bits(every) = 1;bits(there) = 0;missing = dec2base(find(bits)-1,'ABCD') `

## Oz

Using constraint programming for this problem may be a bit overkill...

`declare  GivenPermutations =  ["ABCD" "CABD" "ACDB" "DACB" "BCDA" "ACBD" "ADCB" "CDAB" "DABC" "BCAD" "CADB" "CDBA"   "CBAD" "ABDC" "ADBC" "BDCA" "DCBA" "BACD" "BADC" "BDAC" "CBDA" "DBCA" "DCAB"]   %% four distinct variables between "A" and "D":  proc {Description Root}     Root = {FD.list 4 &A#&D}     {FD.distinct Root}     {FD.distribute naiv Root}  end   AllPermutations = {SearchAll Description}in  for P in AllPermutations do     if {Not {Member P GivenPermutations}} then        {System.showInfo "Missing: "#P}     end  end`

## PARI/GP

`v=["ABCD","CABD","ACDB","DACB","BCDA","ACBD","ADCB","CDAB","DABC","BCAD","CADB","CDBA","CBAD","ABDC","ADBC","BDCA","DCBA","BACD","BADC","BDAC","CBDA","DBCA","DCAB"];v=apply(u->permtonum(apply(n->n-64,Vec(Vecsmall(u)))),v);t=numtoperm(4, binomial(4!,2)-sum(i=1,#v,v[i]));Strchr(apply(n->n+64,t))`
Output:
`%1 = "DBAC"`

## Pascal

like c, summation, and Perl 6 XORing

`program MissPerm;{\$MODE DELPHI} //for result const  maxcol = 4;type  tmissPerm = 1..23;  tcol = 1..maxcol;  tResString = String[maxcol];const  Given_Permutations : array [tmissPerm] of tResString =     ('ABCD', 'CABD', 'ACDB', 'DACB', 'BCDA', 'ACBD',      'ADCB', 'CDAB', 'DABC', 'BCAD', 'CADB', 'CDBA',      'CBAD', 'ABDC', 'ADBC', 'BDCA', 'DCBA', 'BACD',      'BADC', 'BDAC', 'CBDA', 'DBCA', 'DCAB');  chOfs =  Ord('A')-1;var  SumElemCol: array[tcol,tcol] of NativeInt;function fib(n: NativeUint): NativeUint;var  i : NativeUint;Begin  result := 1;  For i := 2 to n do    result:= result*i;end; function CountOccurences: tresString;//count the number of every letter in every column//should be (colmax-1)! => 6//the missing should count (colmax-1)! -1 => 5var  fibN_1 : NativeUint;  row, col: NativeInt;Begin  For row := low(tmissPerm) to High(tmissPerm) do    For col := low(tcol) to High(tcol) do      inc(SumElemCol[col,ORD(Given_Permutations[row,col])-chOfs]);   //search the missing  fibN_1 := fib(maxcol-1)-1;  setlength(result,maxcol);  For col := low(tcol) to High(tcol) do    For row := low(tcol) to High(tcol) do      IF SumElemCol[col,row]=fibN_1 then        result[col]:= chr(row+chOfs);end; function CheckXOR: tresString;var  row,col: NativeUint;Begin  setlength(result,maxcol);  fillchar(result[1],maxcol,#0);  For row := low(tmissPerm) to High(tmissPerm) do    For col := low(tcol) to High(tcol) do      result[col] := chr(ord(result[col]) XOR ord(Given_Permutations[row,col]));end; Begin  writeln(CountOccurences,' is missing');  writeln(CheckXOR,' is missing');end.`
Output:
```DBAC is missing
DBAC is missing```

## Perl

Because the set of all permutations contains all its own rotations, the first missing rotation is the target.

`sub check_perm {    my %hash; @hash{@_} = ();    for my \$s (@_) { exists \$hash{\$_} or return \$_        for map substr(\$s,1) . substr(\$s,0,1), (1..length \$s); }} # Check and display@perms = qw(ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC BCAD CADB CDBA            CBAD ABDC ADBC BDCA DCBA BACD BADC BDAC CBDA DBCA DCAB);print check_perm(@perms), "\n";`
Output:
`DBAC`

## Perl 6

`my @givens = <ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC BCAD CADB CDBA                CBAD ABDC ADBC BDCA DCBA BACD BADC BDAC CBDA DBCA DCAB>; my @perms = <A B C D>.permutations.map: *.join; .say when none(@givens) for @perms;`
Output:
`DBAC`

Of course, all of these solutions are working way too hard, when you can just xor all the bits, and the missing one will just pop right out:

`say [~^] <ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC BCAD CADB CDBA          CBAD ABDC ADBC BDCA DCBA BACD BADC BDAC CBDA DBCA DCAB>;`
Output:
`DBAC`

## Phix

`constant perms = {"ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD", "ADCB", "CDAB",                  "DABC", "BCAD", "CADB", "CDBA", "CBAD", "ABDC", "ADBC", "BDCA",                  "DCBA", "BACD", "BADC", "BDAC", "CBDA", "DBCA", "DCAB"} -- 1: sum of letterssequence r = repeat(0,4)for i=1 to length(perms) do    r = sq_add(r,perms[i])end forr = sq_sub(max(r)+'A',r)puts(1,r&'\n')-- based on the notion that missing = sum(full)-sum(partial) would be true,--  and that sum(full) would be like {M,M,M,M} rather than a mix of numbers.-- the final step is equivalent to eg {1528,1530,1531,1529} --                        max-r[i] -> {   3,   1,   0,   2}--                        to chars -> {   D,   B,   A,   C}-- (but obviously both done in one line) -- 2: the xor trickr = repeat(0,4)for i=1 to length(perms) do    r = sq_xor_bits(r,perms[i])end forputs(1,r&'\n')-- (relies on the missing chars being present an odd number of times, non-missing chars an even number of times) -- 3: find least frequent lettersr = "    "for i=1 to length(r) do    sequence count = repeat(0,4)    for j=1 to length(perms) do        count[perms[j][i]-'A'+1] += 1    end for    r[i] = smallest(count,1)+'A'-1end forputs(1,r&'\n')-- (relies on the assumption that a full set would have each letter occurring the same number of times in each position)-- (smallest(count,1) returns the index position of the smallest, rather than it's value) -- 4: test all permutationsfor i=1 to factorial(4) do    r = permute(i,"ABCD")    if not find(r,perms) then exit end ifend forputs(1,r&'\n')-- (relies on brute force(!) - but this is the only method that could be made to cope with >1 omission)`
Output:
```DBAC
DBAC
DBAC
DBAC
```

## PHP

`<?php\$finalres = Array();function permut(\$arr,\$result=array()){	global  \$finalres;	if(empty(\$arr)){		\$finalres[] = implode("",\$result);	}else{		foreach(\$arr as \$key => \$val){			\$newArr = \$arr;			\$newres = \$result;			\$newres[] = \$val;			unset(\$newArr[\$key]);			permut(\$newArr,\$newres);				}	}}\$givenPerms = Array("ABCD","CABD","ACDB","DACB","BCDA","ACBD","ADCB","CDAB","DABC","BCAD","CADB","CDBA","CBAD","ABDC","ADBC","BDCA","DCBA","BACD","BADC","BDAC","CBDA","DBCA","DCAB");\$given = Array("A","B","C","D");permut(\$given);print_r(array_diff(\$finalres,\$givenPerms)); // Array ( [20] => DBAC )  `

## PicoLisp

`(setq *PermList   (mapcar chop      (quote         "ABCD" "CABD" "ACDB" "DACB" "BCDA" "ACBD" "ADCB" "CDAB"         "DABC" "BCAD" "CADB" "CDBA" "CBAD" "ABDC" "ADBC" "BDCA"         "DCBA" "BACD" "BADC" "BDAC" "CBDA" "DBCA" "DCAB" ) ) ) (let (Lst (chop "ABCD")  L Lst)   (recur (L)  # Permute      (if (cdr L)         (do (length L)            (recurse (cdr L))            (rot L) )         (unless (member Lst *PermList)  # Check            (prinl Lst) ) ) ) )`
Output:
`DBAC`

## PowerShell

Works with: PowerShell version 4.0
` function permutation (\$array) {    function generate(\$n, \$array, \$A) {        if(\$n -eq 1) {            \$array[\$A] -join ''        }        else{            for( \$i = 0; \$i -lt (\$n - 1); \$i += 1) {                generate (\$n - 1) \$array \$A                if(\$n % 2 -eq 0){                    \$i1, \$i2 = \$i, (\$n-1)                    \$temp = \$A[\$i1]                    \$A[\$i1] = \$A[\$i2]                    \$A[\$i2] = \$temp                }                else{                    \$i1, \$i2 = 0, (\$n-1)                    \$temp = \$A[\$i1]                    \$A[\$i1] = \$A[\$i2]                    \$A[\$i2] = \$temp                }            }            generate (\$n - 1) \$array \$A        }    }    \$n = \$array.Count    if(\$n -gt 0) {        (generate \$n \$array (0..(\$n-1)))    } else {\$array}}\$perm = permutation @('A','B','C', 'D')\$find = @("ABCD""CABD""ACDB""DACB""BCDA""ACBD""ADCB""CDAB""DABC""BCAD""CADB""CDBA""CBAD""ABDC""ADBC""BDCA""DCBA""BACD""BADC""BDAC""CBDA""DBCA""DCAB")\$perm | where{-not \$find.Contains(\$_)} `

Output:

```DBAC
```

## PureBasic

`Procedure in_List(in.s)  Define.i i, j  Define.s a  Restore data_to_test  For i=1 To 3*8-1    Read.s a    If in=a      ProcedureReturn #True    EndIf  Next i  ProcedureReturn #FalseEndProcedure Define.c z, x, c, vIf OpenConsole()  For z='A' To 'D'    For x='A' To 'D'      If z=x:Continue:EndIf      For c='A' To 'D'        If c=x Or c=z:Continue:EndIf        For v='A' To 'D'          If v=c Or v=x Or v=z:Continue:EndIf          Define.s test=Chr(z)+Chr(x)+Chr(c)+Chr(v)          If Not in_List(test)            PrintN(test+" is missing.")          EndIf         Next      Next    Next  Next  PrintN("Press Enter to exit"):Input()EndIf DataSectiondata_to_test:  Data.s "ABCD","CABD","ACDB","DACB","BCDA","ACBD","ADCB","CDAB"  Data.s "DABC","BCAD","CADB","CDBA","CBAD","ABDC","ADBC","BDCA"  Data.s "DCBA","BACD","BADC","BDAC","CBDA","DBCA","DCAB"EndDataSection`

Based on the Permutations task, the solution could be:

`If OpenConsole()  NewList a.s()  findPermutations(a(), "ABCD", 4)  ForEach a()    Select a()      Case "ABCD","CABD","ACDB","DACB","BCDA","ACBD","ADCB","CDAB","DABC"      Case "BCAD","CADB","CDBA","CBAD","ABDC","ADBC","BDCA","DCBA","BACD"      Case "BADC","BDAC","CBDA","DBCA","DCAB"      Default        PrintN(A()+" is missing.")    EndSelect  Next   Print(#CRLF\$ + "Press ENTER to exit"): Input()EndIf`

## Python

### Python: Calculate difference when compared to all permutations

Works with: Python version 2.6+
`from itertools import permutations given = '''ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC BCAD CADB CDBA           CBAD ABDC ADBC BDCA DCBA BACD BADC BDAC CBDA DBCA DCAB'''.split() allPerms = [''.join(x) for x in permutations(given[0])] missing = list(set(allPerms) - set(given)) # ['DBAC']`

### Python:Counting lowest frequency character at each position

Here is a solution that is more in the spirit of the challenge, i.e. it never needs to generate the full set of expected permutations.

` def missing_permutation(arr):  "Find the missing permutation in an array of N! - 1 permutations."   # We won't validate every precondition, but we do have some basic  # guards.  if len(arr) == 0: raise Exception("Need more data")  if len(arr) == 1:      return [arr[0][1] + arr[0][0]]   # Now we know that for each position in the string, elements should appear  # an even number of times (N-1 >= 2).  We can use a set to detect the element appearing  # an odd number of times.  Detect odd occurrences by toggling admission/expulsion  # to and from the set for each value encountered.  At the end of each pass one element  # will remain in the set.  missing_permutation = ''  for pos in range(len(arr[0])):      s = set()      for permutation in arr:          c = permutation[pos]          if c in s:            s.remove(c)          else:            s.add(c)      missing_permutation += list(s)[0]  return missing_permutation given = '''ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC BCAD CADB CDBA           CBAD ABDC ADBC BDCA DCBA BACD BADC BDAC CBDA DBCA DCAB'''.split() print missing_permutation(given) `

### Python:Counting lowest frequency character at each position: functional

Uses the same method as explained directly above, but calculated in a more functional manner:

`>>> from collections import Counter>>> given = '''ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC BCAD CADB CDBA           CBAD ABDC ADBC BDCA DCBA BACD BADC BDAC CBDA DBCA DCAB'''.split()>>> ''.join(Counter(x).most_common()[-1][0] for x in zip(*given))'DBAC'>>> `
Explanation

It is rather obfuscated, but can be explained by showing these intermediate results and noting that `zip(*x)` transposes x; and that at the end of the list created by the call to `most_common()` is the least common character.

`>>> from pprint import pprint as pp>>> pp(list(zip(*given)), width=120)[('A', 'C', 'A', 'D', 'B', 'A', 'A', 'C', 'D', 'B', 'C', 'C', 'C', 'A', 'A', 'B', 'D', 'B', 'B', 'B', 'C', 'D', 'D'), ('B', 'A', 'C', 'A', 'C', 'C', 'D', 'D', 'A', 'C', 'A', 'D', 'B', 'B', 'D', 'D', 'C', 'A', 'A', 'D', 'B', 'B', 'C'), ('C', 'B', 'D', 'C', 'D', 'B', 'C', 'A', 'B', 'A', 'D', 'B', 'A', 'D', 'B', 'C', 'B', 'C', 'D', 'A', 'D', 'C', 'A'), ('D', 'D', 'B', 'B', 'A', 'D', 'B', 'B', 'C', 'D', 'B', 'A', 'D', 'C', 'C', 'A', 'A', 'D', 'C', 'C', 'A', 'A', 'B')]>>> pp([Counter(x).most_common() for x in zip(*given)])[[('C', 6), ('B', 6), ('A', 6), ('D', 5)], [('D', 6), ('C', 6), ('A', 6), ('B', 5)], [('D', 6), ('C', 6), ('B', 6), ('A', 5)], [('D', 6), ('B', 6), ('A', 6), ('C', 5)]]>>> pp([Counter(x).most_common()[-1] for x in zip(*given)])[('D', 5), ('B', 5), ('A', 5), ('C', 5)]>>> pp([Counter(x).most_common()[-1][0] for x in zip(*given)])['D', 'B', 'A', 'C']>>> ''.join([Counter(x).most_common()[-1][0] for x in zip(*given)])'DBAC'>>> `

## R

This uses the "combinat" package, which is a standard R package:

` library(combinat) permute.me <- c("A", "B", "C", "D")perms  <- permn(permute.me)  # list of all permutationsperms2 <- matrix(unlist(perms), ncol=length(permute.me), byrow=T)  # matrix of all permutationsperms3 <- apply(perms2, 1, paste, collapse="")  # vector of all permutations incomplete <- c("ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD", "ADCB", "CDAB",                 "DABC", "BCAD", "CADB", "CDBA", "CBAD", "ABDC", "ADBC", "BDCA",                 "DCBA", "BACD", "BADC", "BDAC", "CBDA", "DBCA", "DCAB") setdiff(perms3, incomplete) `
Output:
```[1] "DBAC"
```

## Racket

` #lang racket (define almost-all  '([A B C D] [C A B D] [A C D B] [D A C B] [B C D A] [A C B D] [A D C B]    [C D A B] [D A B C] [B C A D] [C A D B] [C D B A] [C B A D] [A B D C]    [A D B C] [B D C A] [D C B A] [B A C D] [B A D C] [B D A C] [C B D A]    [D B C A] [D C A B]))  ;; Obvious method:(for/first ([p (in-permutations (car almost-all))]            #:unless (member p almost-all))  p);; -> '(D B A C)  ;; For permutations of any set(define charmap  (for/hash ([x (in-list (car almost-all))] [i (in-naturals)])    (values x i)))(define size (hash-count charmap)) ;; Illustrating approach mentioned in the task description.;; For each position, character with odd parity at that position. (require data/bit-vector) (for/list ([i (in-range size)])  (define parities (make-bit-vector size #f))  (for ([permutation (in-list almost-all)])    (define n (hash-ref charmap (list-ref permutation i)))    (bit-vector-set! parities n (not (bit-vector-ref parities n))))  (for/first ([(c i) charmap] #:when (bit-vector-ref parities i))    c));; -> '(D B A C) `

## RapidQ

` Dim PList as QStringListPList.addItems "ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD", "ADCB", "CDAB"PList.additems "DABC", "BCAD", "CADB", "CDBA", "CBAD", "ABDC", "ADBC", "BDCA"PList.additems "DCBA", "BACD", "BADC", "BDAC", "CBDA", "DBCA", "DCAB" Dim NumChar(4, 65 to 68) as integerDim MPerm as string 'Create table with occurencesFor x = 0 to PList.Itemcount -1    for y = 1 to 4        Inc(NumChar(y, asc(PList.Item(x)[y])))    nextnext 'When a char only occurs 5 times it's the missing onefor x = 1 to 4    for y = 65 to 68        MPerm = MPerm + iif(NumChar(x, y)=5, chr\$(y), "")    nextnext showmessage MPerm '= DBAC`

## REXX

`/*REXX pgm finds one or more missing permutations from an internal list & displays them.*/          list = 'ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC BCAD CADB CDBA',                 'CBAD ABDC ADBC BDCA DCBA BACD BADC BDAC CBDA DBCA DCAB'@.=                                              /* [↓]  needs to be as long as  THINGS.*/@abcU  = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'            /*an uppercase (Latin/Roman) alphabet. */things = 4                                       /*number of unique letters to be used. */bunch  = 4                                       /*number letters to be used at a time. */                 do j=1  for things              /* [↓]  only get a portion of alphabet.*/                 \$.j=substr(@abcU,j,1)           /*extract just one letter from alphabet*/                 end   /*j*/                     /* [↑]  build a letter array for speed.*/call permSet 1                                   /*invoke PERMSET subroutine recursively*/exit                                             /*stick a fork in it,  we're all done. *//*──────────────────────────────────────────────────────────────────────────────────────*/permSet: procedure expose \$. @. bunch list things;    parse arg ?         if ?>bunch  then do;  _=                                   do m=1  for bunch           /*build a permutation.   */                                   _=_ || @.m                  /*add permutation──►list.*/                                   end   /*m*/                                                               /* [↓]  is in the list?  */                          if wordpos(_,list)==0  then say _  ' is missing from the list.'                          end                     else do x=1  for things                   /*build a permutation.   */                                   do k=1  for ?-1                                   if @.k==\$.x then iterate x  /*was permutation built? */                                   end  /*k*/                          @.?=\$.x                              /*define as being built. */                          call permSet  ?+1                    /*call subr. recursively.*/                          end   /*x*/         return`

output

```DBAC  is missing from the list.
```

## Ring

` list = "ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC BCAD CADB CDBA CBAD ABDC ADBC BDCA DCBA BACD BADC BDAC CBDA DBCA DCAB" for a = ascii("A") to ascii("D")    for b = ascii("A") to ascii("D")        for c = ascii("A") to ascii("D")            for d = ascii("A") to ascii("D")                 x = char(a) + char(b) + char(c)+ char(d)                if a!=b and a!=c and a!=d and b!=c and b!=d and c!=d                   if substr(list,x) = 0 see x + " missing" + nl ok ok            next        next    next next  `

Output:

```DBAC missing
```

## Ruby

Works with: Ruby version 2.0+
`given = %w{  ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC BCAD CADB CDBA  CBAD ABDC ADBC BDCA DCBA BACD BADC BDAC CBDA DBCA DCAB} all = given[0].chars.permutation.collect(&:join) puts "missing: #{all - given}"`
Output:
```missing: ["DBAC"]
```

## Run BASIC

`list\$ = "ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC BCAD CADB CDBA CBAD ABDC ADBC BDCA DCBA BACD BADC BDAC CBDA DBCA DCAB" for a = asc("A") to asc("D")  for b = asc("A") to asc("D")    for c = asc("A") to asc("D")      for d = asc("A") to asc("D")        x\$ = chr\$(a) + chr\$(b) + chr\$(c)+ chr\$(d)        for i = 1 to 4                                            ' make sure each letter is unique          j = instr(x\$,mid\$(x\$,i,1))          if instr(x\$,mid\$(x\$,i,1),j + 1) <> 0 then goto [nxt]        next i       if instr(list\$,x\$) = 0 then print x\$;" missing"            ' found missing permutation[nxt] next d    next c  next bnext a`
Output:
`DBAC missing`

## Scala

Library: Scala
Works with: Scala version 2.8
`def fat(n: Int) = (2 to n).foldLeft(1)(_*_)def perm[A](x: Int, a: Seq[A]): Seq[A] = if (x == 0) a else {  val n = a.size  val fatN1 = fat(n - 1)  val fatN = fatN1 * n  val p = x / fatN1 % fatN  val (before, Seq(el, after @ _*)) = a splitAt p  el +: perm(x % fatN1, before ++ after)}def findMissingPerm(start: String, perms: Array[String]): String = {  for {    i <- 0 until fat(start.size)    p = perm(i, start).mkString  } if (!perms.contains(p)) return p  ""}val perms = """ABCDCABDACDBDACBBCDAACBDADCBCDABDABCBCADCADBCDBACBADABDCADBCBDCADCBABACDBADCBDACCBDADBCADCAB""".stripMargin.split("\n")println(findMissingPerm(perms(0), perms))`

### Scala 2.9.x

Works with: Scala version 2.9.1
`println("missing perms: "+("ABCD".permutations.toSet  --"ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC BCAD CADB CDBA CBAD ABDC ADBC BDCA DCBA BACD BADC BDAC CBDA DBCA DCAB".stripMargin.split(" ").toSet))`

## Seed7

`\$ include "seed7_05.s7i"; const func string: missingPermutation (in array string: perms) is func  result    var string: missing is "";  local    var integer: pos is 0;    var set of char: chSet is (set of char).EMPTY_SET;    var string: permutation is "";    var char: ch is ' ';  begin    if length(perms) <> 0 then      for key pos range perms[1] do        chSet := (set of char).EMPTY_SET;        for permutation range perms do          ch := permutation[pos];          if ch in chSet then            excl(chSet, ch);          else            incl(chSet, ch);          end if;        end for;        missing &:= min(chSet);      end for;    end if;  end func; const proc: main is func  begin    writeln(missingPermutation([] ("ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD",           "ADCB", "CDAB", "DABC", "BCAD", "CADB", "CDBA", "CBAD", "ABDC", "ADBC",           "BDCA", "DCBA", "BACD", "BADC", "BDAC", "CBDA", "DBCA", "DCAB")));  end func;`
Output:
```DBAC
```

## Sidef

Translation of: Perl
`func check_perm(arr) {    var hash = Hash()    hash.set_keys(arr...)    arr.each { |s|        {            var t = (s.substr(1) + s.substr(0, 1))            hash.has_key(t) || return t        } * s.len    }} var perms = %w(ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC BCAD CADB CDBA               CBAD ABDC ADBC BDCA DCBA BACD BADC BDAC CBDA DBCA DCAB) say check_perm(perms)`
Output:
```DBAC
```

## Tcl

Library: Tcllib (Package: struct::list)
` package require struct::list set have { \    ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC BCAD CADB CDBA CBAD ABDC \    ADBC BDCA DCBA BACD BADC BDAC CBDA DBCA DCAB \} struct::list foreachperm element {A B C D} {	set text [join \$element ""]	if {\$text ni \$have} {		puts "Missing permutation(s): \$text"	}} `

## Ursala

The permutation generating function is imported from the standard library below and needn't be reinvented, but its definition is shown here in the interest of comparison with other solutions.

`permutations = ~&itB^?a\~&aNC *=ahPfatPRD refer ^C/~&a ~&ar&& ~&arh2falrtPXPRD`

The `~&j` operator computes set differences.

`#import std#show+ main = ~&j/permutations'ABCD' -[ABCDCABDACDBDACBBCDAACBDADCBCDABDABCBCADCADBCDBACBADABDCADBCBDCADCBABACDBADCBDACCBDADBCADCAB]-`
Output:
```DBAC
```

## VBScript

Uses the 3rd method approach by adding the columns.

` arrp = Array("ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD",_      "ADCB", "CDAB", "DABC", "BCAD", "CADB", "CDBA",_      "CBAD", "ABDC", "ADBC", "BDCA", "DCBA", "BACD",_      "BADC", "BDAC", "CBDA", "DBCA", "DCAB") Dim col(4) 'supposes that a complete column have 6 of each letter.target = (6*Asc("A")) + (6*Asc("B")) + (6*Asc("C")) + (6*Asc("D")) missing = "" For i = 0 To UBound(arrp)	For j = 1 To 4            col(j) = col(j) + Asc(Mid(arrp(i),j,1))	NextNext For k = 1 To 4	n = target - col(k)	missing = missing & Chr(n)Next WScript.StdOut.WriteLine missing `
Output:
`DBAC`

## XPL0

The list of permutations is input by using a command line like this: missperm <missperm.txt

`code HexIn=26, HexOut=27;int  P, I;[P:= 0;for I:= 1 to 24-1 do P:= P xor HexIn(1);HexOut(0, P);]`
Output:
```0000DBAC
```

## zkl

Since I just did the "generate the permutations" task, I'm going to use it to do the brute force solution.

`var data=L("ABCD","CABD","ACDB","DACB","BCDA","ACBD","ADCB","CDAB",           "DABC","BCAD","CADB","CDBA","CBAD","ABDC","ADBC","BDCA",           "DCBA","BACD","BADC","BDAC","CBDA","DBCA","DCAB");Utils.Helpers.permute(["A".."D"]).apply("concat").copy().remove(data.xplode());`

Copy creates a read/write list from a read only list. xplode() pushes all elements of data as parameters to remove.

Output:
```L("DBAC")
```

## ZX Spectrum Basic

`10 LET l\$="ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC BCAD CADB CDBA CBAD ABDC ADBC BDCA DCBA BACD BADC BDAC CBDA DBCA DCAB"20 LET length=LEN l\$30 FOR a= CODE "A" TO  CODE "D"40 FOR b= CODE "A" TO  CODE "D"50 FOR c= CODE "A" TO  CODE "D"60 FOR d= CODE "A" TO  CODE "D"70 LET x\$=""80 IF a=b OR a=c OR a=d OR b=c OR b=d OR c=d THEN GO TO 14090 LET x\$=CHR\$ a+CHR\$ b+CHR\$ c+CHR\$ d100 FOR i=1 TO length STEP 5110 IF x\$=l\$(i TO i+3) THEN GO TO 140120 NEXT i130 PRINT x\$;" is missing"140 NEXT d: NEXT c: NEXT b: NEXT a`