Remove duplicate elements

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Task
Remove duplicate elements
You are encouraged to solve this task according to the task description, using any language you may know.
Given an Array, derive a sequence of elements in which all duplicates are removed.

There are basically three approaches seen here:

  • Put the elements into a hash table which does not allow duplicates. The complexity is O(n) on average, and O(n2) worst case. This approach requires a hash function for your type (which is compatible with equality), either built-in to your language, or provided by the user.
  • Sort the elements and remove consecutive duplicate elements. The complexity of the best sorting algorithms is O(n log n). This approach requires that your type be "comparable", i.e., have an ordering. Putting the elements into a self-balancing binary search tree is a special case of sorting.
  • Go through the list, and for each element, check the rest of the list to see if it appears again, and discard it if it does. The complexity is O(n2). The up-shot is that this always works on any type (provided that you can test for equality).

Contents

[edit] ACL2

(remove-duplicates xs)

[edit] Applesoft BASIC

100 DIM L$(15)
110 L$(0) = "NOW"
120 L$(1) = "IS"
130 L$(2) = "THE"
140 L$(3) = "TIME"
150 L$(4) = "FOR"
160 L$(5) = "ALL"
170 L$(6) = "GOOD"
180 L$(7) = "MEN"
190 L$(8) = "TO"
200 L$(9) = "COME"
210 L$(10) = "TO"
220 L$(11) = "THE"
230 L$(12) = "AID"
240 L$(13) = "OF"
250 L$(14) = "THE"
260 L$(15) = "PARTY."
 
300 N = 15
310 GOSUB 400
320 FOR I = 0 TO N
330 PRINT L$(I) " " ;
340 NEXT
350 PRINT
360 END
 
400 REMREMOVE DUPLICATES
410 FOR I = N TO 1 STEP -1
420 I$ = L$(I)
430 FOR J = 0 TO I - 1
440 EQ = I$ = L$(J)
450 IF NOT EQ THEN NEXT J
460 IF EQ THEN GOSUB 500
470 NEXT I
480 RETURN
 
500 REMREMOVE ELEMENT
510 L$(I) = L$(N)
520 L$(N) = ""
530 N = N - 1
540 RETURN

[edit] Ada

Works with: GNAT version GPL 2007
with Ada.Containers.Ordered_Sets;
with Ada.Text_IO; use Ada.Text_IO;
 
procedure Unique_Set is
package Int_Sets is new Ada.Containers.Ordered_Sets(Integer);
use Int_Sets;
Nums : array (Natural range <>) of Integer := (1,2,3,4,5,5,6,7,1);
Unique : Set;
Set_Cur : Cursor;
Success : Boolean;
begin
for I in Nums'range loop
Unique.Insert(Nums(I), Set_Cur, Success);
end loop;
Set_Cur := Unique.First;
loop
Put_Line(Item => Integer'Image(Element(Set_Cur)));
exit when Set_Cur = Unique.Last;
Set_Cur := Next(Set_Cur);
end loop;
end Unique_Set;

[edit] APL

Works with: Dyalog APL

The primitive monad ∪ means "unique", so:

∪ 1 2 3 1 2 3 4 1
1 2 3 4
Works with: APL2
w←1 2 3 1 2 3 4 1
((⍳⍨w)=⍳⍴w)/w
1 2 3 4

[edit] AppleScript

unique({1, 2, 3, "a", "b", "c", 2, 3, 4, "b", "c", "d"})
 
on unique(x)
set R to {}
repeat with i in x
if i is not in R then set end of R to i's contents
end repeat
return R
end unique

[edit] AutoHotkey

Built in Sort has an option to remove duplicates

a = 1,2,1,4,5,2,15,1,3,4
Sort, a, a, NUD`,
MsgBox % a ; 1,2,3,4,5,15

[edit] AWK

We produce an array a with duplicates from a string; then index a second array b with the contents of a, so that duplicates make only one entry; then produce a string with the keys of b, which is finally output.

$ awk 'BEGIN{split("a b c d c b a",a);for(i in a)b[a[i]]=1;r="";for(i in b)r=r" "i;print r}'
a b c d

[edit] BBC BASIC

      DIM list$(15)
list$() = "Now", "is", "the", "time", "for", "all", "good", "men", \
\ "to", "come", "to", "the", "aid", "of", "the", "party."
num% = FNremoveduplicates(list$())
FOR i% = 0 TO num%-1
PRINT list$(i%) " " ;
NEXT
PRINT
END
 
DEF FNremoveduplicates(l$())
LOCAL i%, j%, n%, i$
n% = 1
FOR i% = 1 TO DIM(l$(), 1)
i$ = l$(i%)
FOR j% = 0 TO i%-1
IF i$ = l$(j%) EXIT FOR
NEXT
IF j%>=i% l$(n%) = i$ : n% += 1
NEXT
= n%

Output:

Now is the time for all good men to come aid of party.

[edit] Bracmat

Here are three solutions. The first one (A) uses a hash table, the second (B) uses a pattern for spotting the elements that have a copy further on in the list and only adds those elements to the answer that don't have copies further on. The third solution (C) utilises an mechanism that is very typical of Bracmat, namely that sums (and also products) always are transformed to a normalised form upon evaluation. Normalisation means that terms are ordered in a unique way and that terms that are equal, apart from a numerical factor, are replaced by a single term with a numerical factor that is the sum of the numerical factors of each term. The answer is obtained by replacing all numerical factors by 1 as the last step.

The list contains atoms and also a few non-atomic expressions. The hash table needs atomic keys, so we apply the str function when searching and inserting elements.

2 3 5 7 11 13 17 19 cats 222 (-100.2) "+11" (1.1) "+7" (7.) 7 5 5 3 2 0 (4.4) 2:?LIST
 
(A=
( Hashing
= h elm list
. new$hash:?h
& whl
' ( !arg:%?elm ?arg
& ( (h..find)$str$!elm
| (h..insert)$(str$!elm.!elm)
)
)
& :?list
& (h..forall)
$ (
= .!arg:(?.?arg)&!arg !list:?list
)
& !list
)
& put$("Solution A:" Hashing$!LIST \n,LIN)
);
 
(B=
( backtracking
= answr elm
.  :?answr
&  !arg
 :  ?
(  %?`elm
 ?
( !elm ?
| &!answr !elm:?answr
)
& ~
)
| !answr
)
& put$("Solution B:" backtracking$!LIST \n,LIN)
);
 
(C=
( summing
= sum car LIST
.  !arg:?LIST
& 0:?sum
& whl
' ( !LIST:%?car ?LIST
& (.!car)+!sum:?sum
)
& whl
' ( !sum:#*(.?el)+?sum
& !el !LIST:?LIST
)
& !LIST
)
& put$("Solution C:" summing$!LIST \n,LIN)
);
 
( !A
& !B
& !C
&
)

Only solution B produces a list with the same order of elements as in the input.

Solution A: 19 (4.4) 17 11 13 (1.1) (7.) 222 +11 7 5 3 2 0 cats (-100.2) +7
Solution B: 11 13 17 19 cats 222 (-100.2) +11 (1.1) +7 (7.) 7 5 3 0 (4.4) 2
Solution C: (7.) (4.4) (1.1) (-100.2) cats 222 19 17 13 11 7 5 3 2 0 +7 +11

[edit] Brat

some_array = [1 1 2 1 'redundant' [1 2 3] [1 2 3] 'redundant']
 
unique_array = some_array.unique

[edit] C

[edit] O(n^2) version, using linked lists

Since there's no way to know ahead of time how large the new data structure will need to be, we'll return a linked list instead of an array.

#include <stdio.h>
#include <stdlib.h>
 
struct list_node {int x; struct list_node *next;};
typedef struct list_node node;
 
node * uniq(int *a, unsigned alen)
{if (alen == 0) return NULL;
node *start = malloc(sizeof(node));
if (start == NULL) exit(EXIT_FAILURE);
start->x = a[0];
start->next = NULL;
 
for (int i = 1 ; i < alen ; ++i)
{node *n = start;
for (;; n = n->next)
{if (a[i] == n->x) break;
if (n->next == NULL)
{n->next = malloc(sizeof(node));
n = n->next;
if (n == NULL) exit(EXIT_FAILURE);
n->x = a[i];
n->next = NULL;
break;}}}
 
return start;}
 
int main(void)
{int a[] = {1, 2, 1, 4, 5, 2, 15, 1, 3, 4};
for (node *n = uniq(a, 10) ; n != NULL ; n = n->next)
printf("%d ", n->x);
puts("");
return 0;}

Output:

1 2 4 5 15 3

[edit] O(n^2) version, pure arrays

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>
 
/* Returns `true' if element `e' is in array `a'. Otherwise, returns `false'.
* Checks only the first `n' elements. Pure, O(n).
*/

bool elem(int *a, size_t n, int e)
{
for (size_t i = 0; i < n; ++i)
if (a[i] == e)
return true;
 
return false;
}
 
/* Removes the duplicates in array `a' of given length `n'. Returns the number
* of unique elements. In-place, order preserving, O(n ^ 2).
*/

size_t nub(int *a, size_t n)
{
size_t m = 0;
 
for (size_t i = 0; i < n; ++i)
if (!elem(a, m, a[i]))
a[m++] = a[i];
 
return m;
}
 
/* Out-place version of `nub'. Pure, order preserving, alloc < n * sizeof(int)
* bytes, O(n ^ 2).
*/

size_t nub_new(int **b, int *a, size_t n)
{
int *c = malloc(n * sizeof(int));
memcpy(c, a, n * sizeof(int));
int m = nub(c, n);
*b = malloc(m * sizeof(int));
memcpy(*b, c, m * sizeof(int));
free(c);
return m;
}
 
int main(void)
{
int a[] = {1, 2, 1, 4, 5, 2, 15, 1, 3, 4};
int *b;
 
size_t n = nub_new(&b, a, sizeof(a) / sizeof(a[0]));
 
for (size_t i = 0; i < n; ++i)
printf("%d ", b[i]);
puts("");
 
free(b);
return 0;
}

Output:

1 2 4 5 15 3

[edit] Sorting method

Using qsort and return uniques in-place:
#include <stdio.h>
#include <stdlib.h>
 
int icmp(const void *a, const void *b)
{
#define _I(x) *(const int*)x
return _I(a) < _I(b) ? -1 : _I(a) > _I(b);
#undef _I
}
 
/* filter items in place and return number of uniques. if a separate
list is desired, duplicate it before calling this function */

int uniq(int *a, int len)
{
int i, j;
qsort(a, len, sizeof(int), icmp);
for (i = j = 0; i < len; i++)
if (a[i] != a[j]) a[++j] = a[i];
return j + 1;
}
 
int main()
{
int x[] = {1, 2, 1, 4, 5, 2, 15, 1, 3, 4};
int i, len = uniq(x, sizeof(x) / sizeof(x[0]));
for (i = 0; i < len; i++) printf("%d\n", x[i]);
 
return 0;
}

Output:

1
2
3
4
5
15

[edit] C++

This version uses std::set, which requires its element type be comparable using the < operator.

#include <set>
#include <iostream>
using namespace std;
 
int main() {
typedef set<int> TySet;
int data[] = {1, 2, 3, 2, 3, 4};
 
TySet unique_set(data, data + 6);
 
cout << "Set items:" << endl;
for (TySet::iterator iter = unique_set.begin(); iter != unique_set.end(); iter++)
cout << *iter << " ";
cout << endl;
}

This version uses hash_set, which is part of the SGI extension to the Standard Template Library. It is not part of the C++ standard library. It requires that its element type have a hash function.

Works with: GCC
#include <ext/hash_set>
#include <iostream>
using namespace std;
 
int main() {
typedef __gnu_cxx::hash_set<int> TyHash;
int data[] = {1, 2, 3, 2, 3, 4};
 
TyHash unique_set(data, data + 6);
 
cout << "Set items:" << endl;
for (TyHash::iterator iter = unique_set.begin(); iter != unique_set.end(); iter++)
cout << *iter << " ";
cout << endl;
}

This version uses unordered_set, which is part of the TR1, which is likely to be included in the next version of C++. It is not part of the C++ standard library. It requires that its element type have a hash function.

Works with: GCC
#include <tr1/unordered_set>
#include <iostream>
using namespace std;
 
int main() {
typedef tr1::unordered_set<int> TyHash;
int data[] = {1, 2, 3, 2, 3, 4};
 
TyHash unique_set(data, data + 6);
 
cout << "Set items:" << endl;
for (TyHash::iterator iter = unique_set.begin(); iter != unique_set.end(); iter++)
cout << *iter << " ";
cout << endl;
}

Alternative method working directly on the array:

#include <iostream>
#include <iterator>
#include <algorithm>
 
// helper template
template<typename T> T* end(T (&array)[size]) { return array+size; }
 
int main()
{
int data[] = { 1, 2, 3, 2, 3, 4 };
std::sort(data, end(data));
int* new_end = std::unique(data, end(data));
std::copy(data, new_end, std::ostream_iterator<int>(std::cout, " ");
std::cout << std::endl;
}

Using sort, unique, and erase on a vector.

Works with: C++11
#include <algorithm>
#include <iostream>
#include <vector>
 
int main() {
std::vector<int> data = {1, 2, 3, 2, 3, 4};
 
std::sort(data.begin(), data.end());
data.erase(std::unique(data.begin(), data.end()), data.end());
 
for(int& i: data) std::cout << i << " ";
std::cout << std::endl;
return 0;
}

[edit] C#

Works with: C# version 2+
int[] nums = { 1, 1, 2, 3, 4, 4 };
List<int> unique = new List<int>();
foreach (int n in nums)
if (!unique.Contains(n))
unique.Add(n);
Works with: C# version 3+
int[] nums = {1, 1, 2, 3, 4, 4};
int[] unique = nums.Distinct().ToArray();

[edit] CafeOBJ

 
-- The parametrized module NO-DUP-LIST(ELEMENTS :: TRIV) defines the signature of simple Haskell like list structure.
-- The removal of duplicates is handled by the equational properties listed after the signature in brackets {}
-- The binary operation _,_ is associative, commutative, and idempotent.
-- This list structure does not permit duplicates, they are removed during evaluation (called reduction in CafeOBJ)
-- Actual code is contained in module called NO-DUP-LIST.
-- The tests are performed after opening instantiated NO-DUP-LIST with various concrete types.
-- For further details see: http://www.ldl.jaist.ac.jp/cafeobj/
mod! NO-DUP-LIST(ELEMENTS :: TRIV) {
[ List < Elem < Elt] -- Sorts in Ordered Sorted Algebra
op [] : -> List { prec: 0 } -- Empty List
op _,_ : Elt Elt -> Elt { comm assoc idem prec: 80 l-assoc }
op [_] : Elt -> List { prec: 0 }
}
 
-- Test on lists of INT, CHARACTER, and STRING
open NO-DUP-LIST(INT)
reduce [ 1 , 2 , 1 , 1 ] . -- Gives [ 1 , 2 ]
open NO-DUP-LIST(CHARACTER)
reduce [ 'a' , 'b' , 'a' , 'a' ] . -- Gives [ 'a' , 'b' ]
open NO-DUP-LIST(STRING)
reduce [ "abc" , "def" , "abc" ] . -- Gives [ "def" , "abc" ]
 

[edit] Clojure

user=> (distinct [1 3 2 9 1 2 3 8 8 1 0 2])
(1 3 2 9 8 0)
user=>

[edit] Common Lisp

To remove duplicates non-destructively:

(remove-duplicates '(1 3 2 9 1 2 3 8 8 1 0 2))
> (9 3 8 1 0 2)

Or, to remove duplicates in-place:

(delete-duplicates '(1 3 2 9 1 2 3 8 8 1 0 2))
> (9 3 8 1 0 2)

[edit] D

void main() {
import std.stdio, std.algorithm;
 
[1, 3, 2, 9, 1, 2, 3, 8, 8, 1, 0, 2]
.sort()
.uniq
.writeln;
}
Output:
[0, 1, 2, 3, 8, 9]

Using an associative array:

void main() {
import std.stdio;
 
immutable data = [1, 3, 2, 9, 1, 2, 3, 8, 8, 1, 0, 2];
 
bool[int] hash;
foreach (el; data)
hash[el] = true;
hash.byKey.writeln;
}
Output:
[8, 0, 1, 9, 2, 3]

[edit] Delphi

Generics were added in Delphi2009.

program RemoveDuplicateElements;
 
{$APPTYPE CONSOLE}
 
uses Generics.Collections;
 
var
i: Integer;
lIntegerList: TList<Integer>;
const
INT_ARRAY: array[1..7] of Integer = (1, 2, 2, 3, 4, 5, 5);
begin
lIntegerList := TList<Integer>.Create;
try
for i in INT_ARRAY do
if not lIntegerList.Contains(i) then
lIntegerList.Add(i);
 
for i in lIntegerList do
Writeln(i);
finally
lIntegerList.Free;
end;
end.

Output:

1
2
3
4
5

[edit] Déjà Vu

}
for item in [ 1 10 1 :hi :hello :hi :hi ]:
@item
!. keys set{
Output:
[ 1 :hello 10 :hi ]

[edit] E

[1,2,3,2,3,4].asSet().getElements()

[edit] ECL

 
inNumbers  := DATASET([{1},{2},{3},{4},{1},{1},{7},{8},{9},{9},{0},{0},{3},{3},{3},{3},{3}], {INTEGER Field1});
DEDUP(SORT(inNumbers,Field1));
 

Output:

0
1
2
3
4
7
8
9

[edit] Erlang

List = [1, 2, 3, 2, 2, 4, 5, 5, 4, 6, 6, 5].
UniqueList = gb_sets:to_list(gb_sets:from_list(List)).
% Alternatively the builtin:
Unique_list = lists:usort( List ).
 

[edit] Euphoria

include sort.e
 
function uniq(sequence s)
sequence out
s = sort(s)
out = s[1..1]
for i = 2 to length(s) do
if not equal(s[i],out[$]) then
out = append(out, s[i])
end if
end for
return out
end function
 
constant s = {1, 2, 1, 4, 5, 2, 15, 1, 3, 4}
? s
? uniq(s)

Output:

{1,2,1,4,5,2,15,1,3,4}
{1,2,3,4,5,15}

[edit] F#

The simplest way is to build a set from the given array (this actually works for any enumerable input sequence type, not just arrays):

 
set [|1;2;3;2;3;4|]
 

gives:

 
val it : Set<int> = seq [1; 2; 3; 4]
 

[edit] Factor

USING: sets ;
V{ 1 2 1 3 2 4 5 } members .
 
V{ 1 2 3 4 5 }

[edit] Forth

Forth has no built-in hashtable facility, so the easiest way to achieve this goal is to take the "uniq" program as an example.

The word uniq, if given a sorted array of cells, will remove the duplicate entries and return the new length of the array. For simplicity, uniq has been written to process cells (which are to Forth what "int" is to C), but could easily be modified to handle a variety of data types through deferred procedures, etc.

The input data is assumed to be sorted.

\ Increments a2 until it no longer points to the same value as a1
\ a3 is the address beyond the data a2 is traversing.
: skip-dups ( a1 a2 a3 -- a1 a2+n )
dup rot ?do
over @ i @ <> if drop i leave then
cell +loop ;
 
\ Compress an array of cells by removing adjacent duplicates
\ Returns the new count
: uniq ( a n -- n2 )
over >r \ Original addr to return stack
cells over + >r \ "to" addr now on return stack, available as r@
dup begin ( write read )
dup r@ <
while
2dup @ swap ! \ copy one cell
cell+ r@ skip-dups
cell 0 d+ \ increment write ptr only
repeat r> 2drop r> - cell / ;

Here is another implementation of "uniq" that uses a popular parameters and local variables extension words. It is structurally the same as the above implementation, but uses less overt stack manipulation.

: uniqv { a n \ r e -- n }
a n cells+ to e
a dup to r
\ the write address lives on the stack
begin
r e <
while
r @ over !
r cell+ e skip-dups to r
cell+
repeat
a - cell / ;

To test this code, you can execute:

create test 1 , 2 , 3 , 2 , 6 , 4 , 5 , 3 , 6 ,
here test - cell / constant ntest
: .test ( n -- ) 0 ?do test i cells + ? loop ;
 
test ntest 2dup cell-sort uniq .test

output

1 2 3 4 5 6 ok

[edit] Fortran

Fortran has no built-in hash functions or sorting functions but the code below implements the compare all elements algorithm.

 
 
program remove_dups
implicit none
integer :: example(12) ! The input
integer :: res(size(example)) ! The output
integer :: k ! The number of unique elements
integer :: i, j
 
example = [1, 2, 3, 2, 2, 4, 5, 5, 4, 6, 6, 5]
k = 1
res(1) = example(1)
outer: do i=2,size(example)
do j=1,k
if (res(j) == example(i)) then
! Found a match so start looking again
cycle outer
end if
end do
! No match found so add it to the output
k = k + 1
res(k) = example(i)
end do outer
write(*,advance='no',fmt='(a,i0,a)') 'Unique list has ',k,' elements: '
write(*,*) res(1:k)
end program remove_dups
 
 

[edit] Frink

The following demonstrates two of the simplest ways of removing duplicates.

 
b = [1, 5, 2, 6, 6, 2, 2, 1, 9, 8, 6, 5]
 
// One way, using OrderedList. An OrderedList is a type of array that keeps
// its elements in order. The items must be comparable.
a = new OrderedList
println[a.insertAllUnique[b]]
 
// Another way, using the "set" datatype and back to an array.
println[toArray[toSet[b]]
 

Output. Note that sets are not guaranteed to be printed in any specific order.

[1, 2, 5, 6, 8, 9]
[9, 8, 6, 5, 2, 1]

[edit] GAP

# Built-in, using sets (which are also lists)
a := [ 1, 2, 3, 1, [ 4 ], 5, 5, [4], 6 ];
# [ 1, 2, 3, 1, [ 4 ], 5, 5, [ 4 ], 6 ]
b := Set(a);
# [ 1, 2, 3, 5, 6, [ 4 ] ]
IsSet(b);
# true
IsList(b);
# true

[edit] Go

[edit] Map solution

package main
 
import "fmt"
 
func uniq(list []int) []int {
unique_set := make(map[int]bool, len(list))
for _, x := range list {
unique_set[x] = true
}
result := make([]int, 0, len(unique_set))
for x := range unique_set {
result = append(result, x)
}
return result
}
 
func main() {
fmt.Println(uniq([]int{1, 2, 3, 2, 3, 4})) // prints: [3 4 1 2] (but in a semi-random order)
}

[edit] Map preserving order

It takes only small changes to the above code to preserver order. Just store the sequence in the map:

package main
 
import "fmt"
 
func uniq(list []int) []int {
unique_set := make(map[int]int, len(list))
i := 0
for _, x := range list {
if _, there := unique_set[x]; !there {
unique_set[x] = i
i++
}
}
result := make([]int, len(unique_set))
for x, i := range unique_set {
result[i] = x
}
return result
}
 
func main() {
fmt.Println(uniq([]int{1, 2, 3, 2, 3, 4})) // prints: [1 2 3 4]
}

[edit] Float64, removing duplicate NaNs

In solutions above, you just replace int with another type to use for a list of another type. (See Associative_arrays/Creation#Go for acceptable types.) Except a weird thing happens with NaNs. They (correctly) don't compare equal, so you have to special case them if you want to remove duplicate NaNs:

package main
 
import (
"fmt"
"math"
)
 
func uniq(list []float64) []float64 {
unique_set := map[float64]int{}
i := 0
nan := false
for _, x := range list {
if _, exists := unique_set[x]; exists {
continue
}
if math.IsNaN(x) {
if nan {
continue
} else {
nan = true
}
}
unique_set[x] = i
i++
}
result := make([]float64, len(unique_set))
for x, i := range unique_set {
result[i] = x
}
return result
}
 
func main() {
fmt.Println(uniq([]float64{1, 2, math.NaN(), 2, math.NaN(), 4})) // Prints [1 2 NaN 4]
}

[edit] Any type using reflection

Go doesn't have templates or generics, but it does have reflection. Using this it's possible to build a version that will work on almost any array or slice type. Using the reflect package for this does make the code less readable.

Normally in Go this type of solution is somewhat rare. Instead, for very short code (such as min, max, abs) it's common to cast or make a type specific function by hand as needed. For longer code, often an interface can be used instead (see the sort package for an example).

Note: due to details with how Go handles map keys that contain a NaN somewhere (including within a complex or even within a sub struct field) this version simply omits any NaN containing values it comes across and returns a bool to indicate if that happened. This version is otherwise a translation of the above order preserving map implementation, it does not for example call reflect.DeepEqual so elements with pointers to distinct but equal values will be treated as non-equal.

package main
 
import (
"fmt"
"math"
"reflect"
)
 
func uniq(x interface{}) (interface{}, bool) {
v := reflect.ValueOf(x)
if !v.IsValid() {
panic("uniq: invalid argument")
}
if k := v.Kind(); k != reflect.Array && k != reflect.Slice {
panic("uniq: argument must be an array or a slice")
}
elemType := v.Type().Elem()
intType := reflect.TypeOf(int(0))
mapType := reflect.MapOf(elemType, intType)
m := reflect.MakeMap(mapType)
i := 0
for j := 0; j < v.Len(); j++ {
x := v.Index(j)
if m.MapIndex(x).IsValid() {
continue
}
m.SetMapIndex(x, reflect.ValueOf(i))
if m.MapIndex(x).IsValid() {
i++
}
}
sliceType := reflect.SliceOf(elemType)
result := reflect.MakeSlice(sliceType, i, i)
hadNaN := false
for _, key := range m.MapKeys() {
ival := m.MapIndex(key)
if !ival.IsValid() {
hadNaN = true
} else {
result.Index(int(ival.Int())).Set(key)
}
}
 
return result.Interface(), hadNaN
}
 
type MyType struct {
name string
value float32
}
 
func main() {
intArray := [...]int{5, 1, 2, 3, 2, 3, 4}
intSlice := []int{5, 1, 2, 3, 2, 3, 4}
stringSlice := []string{"five", "one", "two", "three", "two", "three", "four"}
floats := []float64{1, 2, 2, 4,
math.NaN(), 2, math.NaN(),
math.Inf(1), math.Inf(1), math.Inf(-1), math.Inf(-1)}
complexes := []complex128{1, 1i, 1 + 1i, 1 + 1i,
complex(math.NaN(), 1), complex(1, math.NaN()),
complex(math.Inf(+1), 1), complex(1, math.Inf(1)),
complex(math.Inf(-1), 1), complex(1, math.Inf(1)),
}
structs := []MyType{
{"foo", 42},
{"foo", 2},
{"foo", 42},
{"bar", 42},
{"bar", 2},
{"fail", float32(math.NaN())},
}
 
fmt.Print("intArray: ", intArray, " → ")
fmt.Println(uniq(intArray))
fmt.Print("intSlice: ", intSlice, " → ")
fmt.Println(uniq(intSlice))
fmt.Print("stringSlice: ", stringSlice, " → ")
fmt.Println(uniq(stringSlice))
fmt.Print("floats: ", floats, " → ")
fmt.Println(uniq(floats))
fmt.Print("complexes: ", complexes, "\n → ")
fmt.Println(uniq(complexes))
fmt.Print("structs: ", structs, " → ")
fmt.Println(uniq(structs))
// Passing a non slice or array will compile put
// then produce a run time panic:
//a := 42
//uniq(a)
//uniq(nil)
}
Output:
intArray: [5 1 2 3 2 3 4] → [5 1 2 3 4] false
intSlice: [5 1 2 3 2 3 4] → [5 1 2 3 4] false
stringSlice: [five one two three two three four] → [five one two three four] false
floats: [1 2 2 4 NaN 2 NaN +Inf +Inf -Inf -Inf] → [1 2 4 +Inf -Inf] true
complexes: [(1+0i) (0+1i) (1+1i) (1+1i) (NaN+1i) (1+NaNi) (+Inf+1i) (1+Infi) (-Inf+1i) (1+Infi)]
 → [(1+0i) (0+1i) (1+1i) (+Inf+1i) (1+Infi) (-Inf+1i)] true
structs: [{foo 42} {foo 2} {foo 42} {bar 42} {bar 2} {fail NaN}] → [{foo 42} {foo 2} {bar 42} {bar 2}] true

[edit] Groovy

def list = [1, 2, 3, 'a', 'b', 'c', 2, 3, 4, 'b', 'c', 'd']
assert list.size() == 12
println " Original List: ${list}"
 
// Filtering the List
list.unique()
assert list.size() == 8
println " Filtered List: ${list}"
 
list = [1, 2, 3, 'a', 'b', 'c', 2, 3, 4, 'b', 'c', 'd']
assert list.size() == 12
 
// Converting to Set
def set = new HashSet(list)
assert set.size() == 8
println " Set: ${set}"
 
// Converting to Order-preserving Set
set = new LinkedHashSet(list)
assert set.size() == 8
println "List-ordered Set: ${set}"

Output:

   Original List: [1, 2, 3, a, b, c, 2, 3, 4, b, c, d]
   Filtered List: [1, 2, 3, a, b, c, 4, d]
             Set: [1, d, 2, 3, 4, b, c, a]
List-ordered Set: [1, 2, 3, a, b, c, 4, d]

[edit] Haskell

values = [1,2,3,2,3,4]
unique = List.nub values

[edit] HicEst

REAL ::      nums(12)
CHARACTER :: workspace*100
 
nums = (1, 3, 2, 9, 1, 2, 3, 8, 8, 1, 0, 2)
WRITE(Text=workspace) nums ! convert to string
EDIT(Text=workspace, SortDelDbls=workspace) ! do the job for a string
READ(Text=workspace, ItemS=individuals) nums ! convert to numeric
 
WRITE(ClipBoard) individuals, "individuals: ", nums ! 6 individuals: 0 1 2 3 8 9 0 0 0 0 0 0

[edit] Icon and Unicon

This solution preserves the original order of the elements.

procedure main(args)
every write(!noDups(args))
end
 
procedure noDups(L)
every put(newL := [], notDup(set(),!L))
return newL
end
 
procedure notDup(cache, a)
if not member(cache, a) then {
insert(cache, a)
return a
}
end

A sample run is:

->noDups a b c d c a b e
a
b
c
d
e
->

[edit] IDL

non_repeated_values = array[uniq(array, sort( array))]

[edit] Inform 7

To decide which list of Ks is (L - list of values of kind K) without duplicates:
let result be a list of Ks;
repeat with X running through L:
add X to result, if absent;
decide on result.

[edit] J

The verb ~. removes duplicate items from any array (numeric, character, or other; vector, matrix, rank-n array). For example:

   ~. 4 3 2 8 0 1 9 5 1 7 6 3 9 9 4 2 1 5 3 2  
4 3 2 8 0 1 9 5 7 6
~. 'chthonic eleemosynary paronomasiac'
chtoni elmsyarp

Or

   0 1 1 2 0 */0 1 2
0 0 0
0 1 2
0 1 2
0 2 4
0 0 0
~. 0 1 1 2 0 */0 1 2
0 0 0
0 1 2
0 2 4

[edit] Java

Works with: Java version 1.5
import java.util.Set;
import java.util.HashSet;
import java.util.Arrays;
 
Object[] data = {1, 2, 3, "a", "b", "c", 2, 3, 4, "b", "c", "d"};
Set<Object> uniqueSet = new HashSet<Object>(Arrays.asList(data));
Object[] unique = uniqueSet.toArray();

[edit] JavaScript

This uses the === "strict equality" operator, which does no type conversions (4 == "4" is true but 4 === "4" is false)

function unique(ary) {
// concat() with no args is a way to clone an array
var u = ary.concat().sort();
for (var i = 1; i < u.length; ) {
if (u[i-1] === u[i])
u.splice(i,1);
else
i++;
}
return u;
}
 
var ary = [1, 2, 3, "a", "b", "c", 2, 3, 4, "b", "c", "d", "4"];
var uniq = unique(ary);
for (var i = 0; i < uniq.length; i++)
print(uniq[i] + "\t" + typeof(uniq[i]));
1 - number
2 - number
3 - number
4 - number
4 - string
a - string
b - string
c - string
d - string

Or, extend the prototype for Array:

Array.prototype.unique = function() {
var u = this.concat().sort();
for (var i = 1; i < u.length; ) {
if (u[i-1] === u[i])
u.splice(i,1);
else
i++;
}
return u;
}
var uniq = [1, 2, 3, "a", "b", "c", 2, 3, 4, "b", "c", "d"].unique();

[edit] jq

If it is acceptable to alter the ordering of elements, then the builtin (fast) filter, unique, can be used. It can be used for arrays with elements of any JSON type and returns the distinct elements in sorted order.

[4,3,2,1,1,2,3,4] | unique

If all but the first occurrence of each element should be deleted, then the following function could be used, at least in a pinch. It retains the advantage of imposing no restrictions on the types of elements in the array:

def removeAllButFirst:
unique as $u
| if length == ($u|length) then .
else (null | ( [][ $u|length - 1 ] ) = null) as $track
| reduce .[] as $i
# [ answer, $track ]
([ [], $track ];
($u|index($i)) as $ix
| if .[1][$ix] == null then
[ (.[0] + [$i]), (.[1] | (.[$ix] = true)) ]
else .
end )
| .[0]
end ;

[edit] Julia

a = [1,2,3,4,1,2,3,4]
unique(a)

[edit] K

(Inspired by the J version.)

   a:4 5#20?13   / create a random 4 x 5 matrix
(12 7 12 4 3
6 3 7 4 7
3 8 3 1 2
2 12 6 4 1)
 
,/a / flatten to array
12 7 12 4 3 6 3 7 4 7 3 8 3 1 2 2 12 6 4 1
 
 ?,/a / distinct elements
12 7 4 3 6 8 1 2
 
 ?"chthonic eleemosynary paronomasiac"
"chtoni elmsyarp"
 
 ?("this";"that";"was";"that";"was";"this")
("this"
"that"
"was")
 
0 1 1 2 0 *\: 0 1 2
(0 0 0
0 1 2
0 1 2
0 2 4
0 0 0)
 
 ?0 1 1 2 0 *\: 0 1 2
(0 0 0
0 1 2
0 2 4)

[edit] Lang5

: dip  swap '_ set execute _ ;
 
: remove-duplicates
[] swap do unique? length 0 == if break then loop drop ;
: unique?
0 extract swap "2dup in if drop else append then" dip ;
 
[1 2 6 3 6 4 5 6] remove-duplicates .

Built-in function:

[1 2 6 3 6 4 5 6] 's distinct
[1 2 6 3 6 4 5 6] 's dress dup union .

[edit] Lasso

local(
x = array(3,4,8,1,8,1,4,5,6,8,9,6),
y = array
)
with n in #x where #y !>> #n do => { #y->insert(#n) }
// result: array(3, 4, 8, 1, 5, 6, 9)

[edit] Liberty BASIC

LB has arrays, but here the elements are stored in a space-separated string.

 
a$ =" 1 $23.19 2 elbow 3 2 Bork 4 3 elbow 2 $23.19 "
print "Original set of elements = ["; a$; "]"
 
b$ =removeDuplicates$( a$)
print "With duplicates removed = ["; b$; "]"
 
end
 
function removeDuplicates$( in$)
o$ =" "
i =1
do
term$ =word$( in$, i, " ")
if instr( o$, " "; term$; " ") =0 and term$ <>" " then o$ =o$ +term$ +" "
i =i +1
loop until term$ =""
removeDuplicates$ =o$
end function
 
Original set of elements = [ 1 $23.19 2 elbow 3 2 Bork 4 3 elbow 2 $23.19 ]
With duplicates removed  = [ 1 $23.19 2 elbow 3 Bork 4  ]

[edit]

Works with: UCB Logo
show remdup [1 2 3 a b c 2 3 4 b c d]   ; [1 a 2 3 4 b c d]

[edit] Lua

items = {1,2,3,4,1,2,3,4,"bird","cat","dog","dog","bird"}
flags = {}
io.write('Unique items are:')
for i=1,#items do
if not flags[items[i]] then
io.write(' ' .. items[i])
flags[items[i]] = true
end
end
io.write('\n')

Output:

Unique items are: 1 2 3 4 bird cat dog

[edit] MAXScript

uniques = #(1, 2, 3, "a", "b", "c", 2, 3, 4, "b", "c", "d")
for i in uniques.count to 1 by -1 do
(
id = findItem uniques uniques[i]
if (id != i) do deleteItem uniques i
)

[edit] Maple

This is simplest with a list, which is an immutable array.

> L := [ 1, 2, 1, 2, 3, 3, 2, 1, "a", "b", "b", "a", "c", "b" ];
L := [1, 2, 1, 2, 3, 3, 2, 1, "a", "b", "b", "a", "c", "b"]
 
> [op]({op}(L));
[1, 2, 3, "a", "b", "c"]

That is idiomatic, but perhaps a bit cryptic; here is a more verbose equivalent:

> convert( convert( L, 'set' ), 'list' );
[1, 2, 3, "a", "b", "c"]

For an Array, which is mutable, the table solution works well in Maple.

> A := Array( L ):
> for u in A do T[u] := 1 end: Array( [indices]( T, 'nolist' ) );
[1, 2, 3, "c", "a", "b"]

Note that the output (due to the Array() constructor) is in fact an Array.

[edit] Mathematica

Built-in function:

DeleteDuplicates[{0, 2, 1, 4, 2, 0, 3, 1, 1, 1, 0, 3}]

gives back:

{0, 2, 1, 4, 3}

Delete duplicates and return sorted elements:

 
Union[{0, 2, 1, 4, 2, 0, 3, 1, 1, 1, 0, 3}]

gives back:

{0, 1, 2, 3, 4}

[edit] MATLAB

MATLAB has a built-in function, "unique(list)", which performs this task.
Sample Usage:

>> unique([1 2 6 3 6 4 5 6])
 
ans =
 
1 2 3 4 5 6

NOTE: The unique function only works for vectors and not for true arrays.

[edit] Maxima

unique([8, 9, 5, 2, 0, 7, 0, 0, 4, 2, 7, 3, 9, 6, 6, 2, 4, 7, 9, 8, 3, 8, 0, 3, 7, 0, 2, 7, 6, 0]);
[0, 2, 3, 4, 5, 6, 7, 8, 9]


[edit] ML

[edit] mLite

A bit like option 3, except copying each element as encountered, and checking to see if it has already been encountered

fun mem (x, []) = false
| (x eql a, a :: as) = true
| (x, _ :: as) = mem (x, as)
;
fun remdup
([], uniq) = rev uniq
| (h :: t, uniq) = if mem(h, uniq) then
remdup (t, uniq)
else
remdup (t, h :: uniq)
| L = remdup (L, [])
 
;
println ` implode ` remdup ` explode "the quick brown fox jumped over the lazy dog";
println ` remdup [1,2,3,4,4,3,2,1, "dog","cat","dog", 1.1, 2.2, 3.3, 1.1];
 

Output

the quickbrownfxjmpdvlazyg
[1, 2, 3, 4, dog, cat, 1.1, 2.2, 3.3]

[edit] MUMPS

We'll take advantage of the fact that an array can only have one index of any specific value. Sorting into canonical order is a side effect. If the indices are strings containing the separator string, they'll be split apart.

REMDUPE(L,S)
 ;L is the input listing
 ;S is the separator between entries
 ;R is the list to be returned
NEW Z,I,R
FOR I=1:1:$LENGTH(L,S) SET Z($PIECE(L,S,I))=""
 ;Repack for return
SET I="",R=""
FOR SET I=$O(Z(I)) QUIT:I="" SET R=$SELECT($L(R)=0:I,1:R_S_I)
KILL Z,I
QUIT R
Example:
USER>W $$REMDUPE^ROSETTA("1,2,3,4,5,2,5,""HELLO"",42,""WORLD""",",")
1,2,3,4,5,42,"HELLO","WORLD"

[edit] Nemerle

using System.Console;
 
module RemDups
{
Main() : void
{
def nums = array[1, 4, 6, 3, 6, 2, 7, 2, 5, 2, 6, 8];
def unique = $[n | n in nums].RemoveDuplicates();
WriteLine(unique);
}
}

[edit] NetRexx

This sample takes advantage of the NetRexx built-in Rexx object's indexed string capability (associative arrays). Rexx indexed strings act very like hash tables:

/* NetRexx */
options replace format comments java crossref symbols nobinary
 
-- Note: Task requirement is to process "arrays". The following converts arrays into simple lists of words:
-- Putting the resulting list back into an array is left as an exercise for the reader.
a1 = [2, 3, 5, 7, 11, 13, 17, 19, 'cats', 222, -100.2, +11, 1.1, +7, '7.', 7, 5, 5, 3, 2, 0, 4.4, 2]
a2 = [1, 2, 3, 'a', 'b', 'c', 2, 3, 4, 'b', 'c', 'd']
a3 = ['Now', 'is', 'the', 'time', 'for', 'all', 'good', 'men', 'to', 'come', 'to', 'the', 'aid', 'of', 'the', 'party.']
x = 0
lists = ''
x = x + 1; lists[0] = x; lists[x] = array2wordlist(a1)
x = x + 1; lists[0] = x; lists[x] = array2wordlist(a2)
x = x + 1; lists[0] = x; lists[x] = array2wordlist(a3)
 
loop ix = 1 to lists[0]
nodups_list = remove_dups(lists[ix])
say ix.right(4)':' lists[ix]
say ''.right(4)':' nodups_list
say
end ix
 
return
 
-- =============================================================================
method remove_dups(list) public static
 
newlist = ''
nodups = '0'
loop w_ = 1 to list.words()
ix = list.word(w_)
nodups[ix] = nodups[ix] + 1 -- we can even collect a count of dups if we want
end w_
loop k_ over nodups
newlist = newlist k_
end k_
 
return newlist.space
 
-- =============================================================================
method array2wordlist(ra = Rexx[]) public static
 
wordlist = ''
loop r_ over ra
wordlist = wordlist r_
end r_
 
return wordlist.space
 

Output:

   1: 2 3 5 7 11 13 17 19 cats 222 -100.2 11 1.1 7 7. 7 5 5 3 2 0 4.4 2
    : 13 2 3 17 19 7. 4.4 5 222 7 -100.2 1.1 cats 0 11

   2: 1 2 3 a b c 2 3 4 b c d
    : c 2 d 3 4 a b 1

   3: Now is the time for all good men to come to the aid of the party.
    : Now aid for men to the party. come time of is all good

[edit] NewLISP

(unique '(1 2 3 a b c 2 3 4 b c d))

[edit] Nial

uniques := [1, 2, 3, 'a', 'b', 'c', 2, 3, 4, 'b', 'c', 'd']
cull uniques
=+-+-+-+-+-+-+-+-+
=|1|2|3|a|b|c|4|d|
=+-+-+-+-+-+-+-+-+

Using strand form

cull 1 1 2 2 3 3
=1 2 3

[edit] Nimrod

import sequtils, algorithm, intsets
 
# Go through the list, and for each element, check the rest of the list to see
# if it appears again,
var items = @[1, 2, 3, 2, 3, 4, 5, 6, 7]
echo deduplicate(items) # O(n^2)
 
proc filterDup(xs): seq[int] =
result = @[xs[0]]
var last = xs[0]
for x in xs[1..xs.high]:
if x != last:
result.add(x)
last = x
 
# Put the elements into a hash table which does not allow duplicates.
var s = initIntSet()
for x in items:
s.incl(x)
echo s
 
# Sort the elements and remove consecutive duplicate elements.
sort(items, system.cmp[int]) # O(n log n)
echo filterDup(items) # O(n)

[edit] Objective-C

NSArray *items = [NSArray arrayWithObjects:@"A", @"B", @"C", @"B", @"A", nil];
 
NSSet *unique = [NSSet setWithArray:items];

[edit] Objeck

 
use Structure;
 
bundle Default {
class Unique {
function : Main(args : String[]) ~ Nil {
nums := [1, 1, 2, 3, 4, 4];
unique := IntVector->New();
 
each(i : nums) {
n := nums[i];
if(unique->Has(n) = false) {
unique->AddBack(n);
};
};
 
each(i : unique) {
unique->Get(i)->PrintLine();
};
}
}
}
 

[edit] OCaml

let uniq lst =
let unique_set = Hashtbl.create (List.length lst) in
List.iter (fun x -> Hashtbl.replace unique_set x ()) lst;
Hashtbl.fold (fun x () xs -> x :: xs) unique_set []
 
let _ =
uniq [1;2;3;2;3;4]

Another solution (preserves order of first occurrence):

let uniq lst =
let seen = Hashtbl.create (List.length lst) in
List.filter (fun x -> let tmp = not (Hashtbl.mem seen x) in
Hashtbl.replace seen x ();
tmp) lst
 
let _ =
uniq [1;2;3;2;3;4]

[edit] Octave

 
input=[1 2 6 4 2 32 5 5 4 3 3 5 1 2 32 4 4];
output=unique(input);
 

[edit] ooRexx

 
data = .array~of(1, 2, 3, "a", "b", "c", 2, 3, 4, "b", "c", "d")
uniqueData = .set~new~union(data)~makearray~sort
 
say "Unique elements are"
say
do item over uniqueData
say item
end
 

[edit] Oz

The following solutions only works if the value type is allowed as a key in a dictionary.

declare
 
fun {Nub Xs}
D = {Dictionary.new}
in
for X in Xs do D.X := unit end
{Dictionary.keys D}
end
 
in
 
{Show {Nub [1 2 1 3 5 4 3 4 4]}}

[edit] PARI/GP

Sort and remove duplicates. Other methods should be implemented as well.

rd(v)={
vecsort(v,,8)
};

[edit] Pascal

Program RemoveDuplicates;
 
const
iArray: array[1..7] of integer = (1, 2, 2, 3, 4, 5, 5);
 
var
rArray: array[1..7] of integer;
i, pos, last: integer;
newNumber: boolean;
 
begin
rArray[1] := iArray[1];
last := 1;
pos := 1;
while pos < high(iArray) do
begin
inc(pos);
newNumber := true;
for i := low(rArray) to last do
if iArray[pos] = rArray[i] then
begin
newNumber := false;
break;
end;
if newNumber then
begin
inc(last);
rArray[last] := iArray[pos];
end;
end;
for i := low(rArray) to last do
writeln (rArray[i]);
end.

Output:

% ./RemoveDuplicates
1
2
3
4
5

[edit] Perl

(this version even preserves the order of first appearance of each element)

use List::MoreUtils qw(uniq);
 
my @uniq = uniq qw(1 2 3 a b c 2 3 4 b c d);

It is implemented like this:

my %seen;
my @uniq = grep {!$seen{$_}++} qw(1 2 3 a b c 2 3 4 b c d);

Note: the following two solutions convert elements to strings in the result, so if you give it references they will lose the ability to be dereferenced.

Alternately:

my %hash = map { $_ => 1 } qw(1 2 3 a b c 2 3 4 b c d);
my @uniq = keys %hash;

Alternately:

my %seen;
@seen{qw(1 2 3 a b c 2 3 4 b c d)} = ();
my @uniq = keys %seen;

[edit] Perl 6

my @unique = [1, 2, 3, 5, 2, 4, 3, -3, 7, 5, 6].uniq;

Or just make a set of it.

set(1,2,3,5,2,4,3,-3,7,5,6).list

[edit] PHP

$list = array(1, 2, 3, 'a', 'b', 'c', 2, 3, 4, 'b', 'c', 'd');
$unique_list = array_unique($list);

[edit] PicoLisp

There is a built-in function

(uniq (2 4 6 1 2 3 4 5 6 1 3 5))

Output:

-> (2 4 6 1 3 5)

[edit] PL/I

*process mar(1,72);
remdup: Proc options(main);
declare t(20) fixed initial (6, 6, 1, 5, 6, 2, 1, 7,
5, 22, 4, 19, 1, 1, 6, 8, 9, 10, 11, 12);
declare (i, j, k, n, e) fixed;
 
put skip list ('Input:');
put edit ((t(k) do k = 1 to hbound(t))) (skip,20(f(3)));
n = hbound(t,1);
i = 0;
outer:
do k = 1 to n;
e = t(k);
do j = k-1 to 1 by -1;
if e = t(j) then iterate outer;
end;
i = i + 1;
t(i) = e;
end;
 
put skip list ('Unique elements are:');
put edit ((t(k) do k = 1 to i)) (skip,20(f(3)));
end;
Output:
Input:
  6  6  1  5  6  2  1  7  5 22  4 19  1  1  6  8  9 10 11 12
Unique elements are:
  6  1  5  2  7 22  4 19  8  9 10 11 12   

[edit] Pop11

;;; Initial array
lvars ar = {1 2 3 2 3 4};
;;; Create a hash table
lvars ht= newmapping([], 50, 0, true);
;;; Put all array as keys into the hash table
lvars i;
for i from 1 to length(ar) do
1 -> ht(ar(i))
endfor;
 
;;; Collect keys into a list
lvars ls = [];
appdata(ht, procedure(x); cons(front(x), ls) -> ls; endprocedure);

[edit] PowerShell

The common array for both approaches:

$data = 1,2,3,1,2,3,4,1

Using a hash table to remove duplicates:

$h = @{}
foreach ($x in $data) {
$h[$x] = 1
}
$h.Keys

Sorting and removing duplicates along the way can be done with the Sort-Object cmdlet.

$data | Sort-Object -Unique

Removing duplicates without sorting can be done with the Select-Object cmdlet.

$data | Select-Object -Unique

[edit] PostScript

Library: initlib
 
[10 8 8 98 32 2 4 5 10 ] dup length dict begin aload let* currentdict {pop} map end
 

[edit] Prolog

uniq(Data,Uniques) :- sort(Data,Uniques).

Example usage:

?- uniq([1, 2, 3, 2, 3, 4],Xs).
Xs = [1, 2, 3, 4]


Because sort/2 is GNU prolog and not ISO here is an ISO compliant version:

member1(X,[H|_]) :- X==H,!.
member1(X,[_|T]) :- member1_(X,T).
 
distinct([],[]).
distinct([H|T],C) :- member1(H,T),!, distinct(T,C).
distinct([H|T],[H|C]) :- distinct(T,C).

Example usage:

?- distinct([A, A, 1, 2, 3, 2, 3, 4],Xs).
Xs = [A, 1, 2, 3, 4]

[edit] PureBasic

Task solved with the built in Hash Table which are called Maps in PureBasic

NewMap MyElements.s()
 
For i=0 To 9 ;Mark 10 items at random, causing high risk of duplication items.
x=Random(9)
t$="Number "+str(x)+" is marked"
MyElements(str(x))=t$ ; Add element 'X' to the hash list or overwrite if already included.
Next
 
ForEach MyElements()
Debug MyElements()
Next

Output may look like this, e.g. duplicated items are automatically removed as they have the same hash value.

Number 0 is marked
Number 2 is marked
Number 5 is marked
Number 6 is marked

[edit] Python

If all the elements are hashable (this excludes list, dict, set, and other mutable types), we can use a set:

items = [1, 2, 3, 'a', 'b', 'c', 2, 3, 4, 'b', 'c', 'd']
unique = list(set(items))

or if we want to keep the order of the elements

items = [1, 2, 3, 'a', 'b', 'c', 2, 3, 4, 'b', 'c', 'd']
unique = []
helperset = set()
for x in items:
if x not in helperset:
unique.append(x)
helperset.add(x)


If all the elements are comparable (i.e. <, >=, etc. operators; this works for list, dict, etc. but not for complex and many other types, including most user-defined types), we can sort and group:

import itertools
items = [1, 2, 3, 'a', 'b', 'c', 2, 3, 4, 'b', 'c', 'd']
unique = [k for k,g in itertools.groupby(sorted(items))]

If both of the above fails, we have to use the brute-force method, which is inefficient:

items = [1, 2, 3, 'a', 'b', 'c', 2, 3, 4, 'b', 'c', 'd']
unique = []
for x in items:
if x not in unique:
unique.append(x)


See also http://www.peterbe.com/plog/uniqifiers-benchmark and http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/52560

[edit] Qi

 
(define remove-duplicates
[] -> []
[A|R] -> (remove-duplicates R) where (element? A R)
[A|R] -> [A|(remove-duplicates R)])
 
(remove-duplicates [a b a a b b c d e])
 

[edit] R

items <- c(1,2,3,2,4,3,2)
unique (items)

[edit] Racket

Using the built-in function

 
-> (remove-duplicates '(2 1 3 2.0 a 4 5 b 4 3 a 7 1 3 x 2))
'(2 1 3 2.0 a 4 5 b 7 x)
 

Using a hash-table:

 
(define (unique/hash lst)
(hash-keys (for/hash ([x (in-list lst)]) (values x #t))))
 

Using a set:

 
(define unique/set (compose1 set->list list->set))
 

A definition that works with arbitrary sequences and allows specification of an equality predicate.

 
(define (unique seq #:same-test [same? equal?])
(for/fold ([res '()])
([x seq] #:unless (memf (curry same? x) res))
(cons x res)))
 
-> (unique '(2 1 3 2.0 a 4 5 b 4 3 a 7 1 3 x 2))
'(1 2 3 a b x 4 5 7 2.0)
-> (unique '(2 1 3 2.0 4 5 4.0 3 7 1 3 2) #:same-test =)
'(7 5 4 3 1 2)
-> (unique #(2 1 3 2.0 4 5 4.0 3 7 1 3 2))
'(7 5 4 3 1 2)
-> (apply string (unique "absbabsbdbfbd"))
"fdsba"

[edit] REBOL

print mold unique [1 $23.19 2 elbow 3 2 Bork 4 3 elbow 2 $23.19]

Output:

[1 $23.19 2 elbow 3 Bork 4]

[edit] Raven

[ 1 2 3 'a' 'b' 'c' 2 3 4 'b' 'c' 'd' ] as items
items copy unique print
 
list (8 items)
0 => 1
1 => 2
2 => 3
3 => "a"
4 => "b"
5 => "c"
6 => 4
7 => "d"

[edit] REXX

Note that in REXX, strings are quite literal.

  • +7   is different from   7   (but compares numerically equal).
  • 00   is different from   0   (but compares numerically equal).
  • -0   is different from   0   (but compares numerically equal).
  • 7.   is different from   7   (but compares numerically equal).
  • Ab   is different from   AB   (but can compare equal if made case insensitive).



Note that all three REXX examples below don't care what type of element is used, integer, floating point, character, binary, ...

[edit] version 1, using a modified method 3

Instead of discard an element if it's a duplicated, it just doesn't add it to the new list.
This method is faster than method 3.

/*REXX program to  remove  duplicate  elements  (items)  in a  list.    */
$= '2 3 5 7 11 13 17 19 cats 222 -100.2 +11 1.1 +7 7. 7 5 5 3 2 0 4.4 2'
say 'original list:' $
say right(words($),13) ' words in the original list.'; say
 
do j=words($) by -1 to 1; y=word($,j) /*process words in the list, */
_=wordpos(y, $, j+1); if _\==0 then $=delword($, _, 1) /*del if dup.*/
end /*j*/
 
say 'modified list:' space($)
say right(words($),13) ' words in the modified list.'
/*stick a fork in it, we're done.*/

output

original list: 2 3 5 7 11 13 17 19 cats 222 -100.2 +11 1.1 +7 7. 7 5 5 3 2 0 4.4 2
           23  words in the original list.

modified list: 2 3 5 7 11 13 17 19 cats 222 -100.2 +11 1.1 +7 7. 0 4.4
           17  words in the modified list.

[edit] version 2, using method 3

/*REXX program to  remove  duplicate  elements  (items)  in a  list.    */
old= '2 3 5 7 11 13 17 19 cats 222 -100.2 +11 1.1 +7 7. 7 5 5 3 2 0 4.4 2'
say 'original list:' old
say right(words(old),13) ' words in the original list.'; say
new= /*start with a clean slate. */
 
do j=1 for words(old); _=word(old,j) /*process the words in old list. */
if wordpos(_,new)==0 then new=new _ /*Doesn't exist? Then add to list*/
end /*j*/
 
say 'modified list:' space(new)
say right(words(new),13) ' words in the modified list.'
/*stick a fork in it, we're done.*/

output is identical to the 1st version.

[edit] version 3, using method 1 (hash table) via REXX stems

/* REXX ************************************************************ 
* 26.11.2012 Walter Pachl
* added: show multiple occurrences
**********************************************************************/

old='2 3 5 7 11 13 17 19 cats 222 -100.2 +11 1.1 +7 7. 7 5 5',
'3 2 0 4.4 2'
say 'old list='old
say 'words in the old list=' words(old)
new=''
found.=0
count.=0
Do While old<>''
Parse Var old w old
If found.w=0 Then Do
new=new w
found.w=1
End
count.w=count.w+1
End
say 'new list='strip(new)
say 'words in the new list=' words(new)
Say 'Multiple occurrences:'
Say 'occ word'
Do While new<>''
Parse Var new w new
If count.w>1 Then
Say right(count.w,3) w
End

Output:

old list=2 3 5 7 11 13 17 19 cats 222 -100.2 +11 1.1 +7 7. 7 5 5 3 2 0 4.4 2 
words in the old list= 23                                                    
new list=2 3 5 7 11 13 17 19 cats 222 -100.2 +11 1.1 +7 7. 0 4.4             
words in the new list= 17                                                    
Multiple occurrences:                                                        
occ word                                                                     
  3 2                                                                        
  2 3                                                                        
  3 5                                                                        
  2 7  

[edit] Ruby

Ruby has an Array#uniq built-in method, which returns a new array by removing duplicate values in self.

ary = [1,1,2,1,'redundant',[1,2,3],[1,2,3],'redundant']
p ary.uniq # => [1, 2, "redundant", [1, 2, 3]]

You can also write your own uniq method.

class Array
# used Hash
def uniq1
each_with_object({}) {|elem, h| h[elem] = true}.keys
end
# sort (requires comparable)
def uniq2
sorted = sort
pre = sorted.first
sorted.each_with_object([pre]){|elem, uniq| uniq << (pre = elem) if elem != pre}
end
# go through the list
def uniq3
each_with_object([]) {|elem, uniq| uniq << elem unless uniq.include?(elem)}
end
end
 
ary = [1,1,2,1,'redundant',[1,2,3],[1,2,3],'redundant']
p ary.uniq1 #=> [1, 2, "redundant", [1, 2, 3]]
p ary.uniq2 rescue nil # Error (not comparable)
p ary.uniq3 #=> [1, 2, "redundant", [1, 2, 3]]
 
ary = [1,2,3,7,6,5,2,3,4,5,6,1,1,1]
p ary.uniq1 #=> [1, 2, 3, 7, 6, 5, 4]
p ary.uniq2 #=> [1, 2, 3, 4, 5, 6, 7]
p ary.uniq3 #=> [1, 2, 3, 7, 6, 5, 4]

A version without implementing class declarations:

 
def unique(array)
pure = Array.new
for i in array
flag = false
for j in pure
flag = true if j==i
end
pure << i unless flag
end
return pure
end
 
 
unique ["hi","hey","hello","hi","hey","heyo"] # => ["hi", "hey", "hello", "heyo"]
unique [1,2,3,4,1,2,3,5,1,2,3,4,5] # => [1,2,3,4,5]
 

[edit] Scala

val list = List(1,2,3,4,2,3,4,99)
val l2 = list.distinct
// l2: scala.List[scala.Int] = List(1,2,3,4,99)
 
val arr = Array(1,2,3,4,2,3,4,99)
val arr2 = arr.distinct
// arr2: Array[Int] = Array(1, 2, 3, 4, 99)
 

[edit] Scheme

(define (remove-duplicates l)
(cond ((null? l)
'())
((member (car l) (cdr l))
(remove-duplicates (cdr l)))
(else
(cons (car l) (remove-duplicates (cdr l))))))
 
(remove-duplicates '(1 2 1 3 2 4 5))
(1 3 2 4 5)

Alternative approach:

(define (remove-duplicates l)
(do ((a '() (if (member (car l) a) a (cons (car l) a)))
(l l (cdr l)))
((null? l) (reverse a))))
 
(remove-duplicates '(1 2 1 3 2 4 5))
(1 2 3 4 5)

The function 'delete-duplicates' is also available in srfi-1.

[edit] Seed7

$ include "seed7_05.s7i";
 
const proc: main is func
local
const array integer: data is [] (1, 3, 2, 9, 1, 2, 3, 8, 8, 1, 0, 2);
var set of integer: dataSet is (set of integer).value;
var integer: number is 0;
begin
for number range data do
incl(dataSet, number);
end for;
writeln(dataSet);
end func;

Output:

{0, 1, 2, 3, 8, 9}

[edit] SETL

items := [0,7,6,6,4,9,7,1,2,3,2];
print(unique(items));

Output in arbitrary order (convert tuple->set then set->tuple):

proc unique(items);
return [item: item in {item: item in items}];
end proc;

Preserving source order

proc unique(items);
seen := {};
return [item: item in items, nps in {#seen} | #(seen with:= item) > nps];
end proc;

[edit] Sidef

var ary = [1,1,2,1,'redundant',[1,2,3],[1,2,3],'redundant'];
say ary.uniq.dump;
say ary.last_uniq.dump;
Output:
[1, 2, 'redundant', [1, 2, 3]]
[2, 1, [1, 2, 3], 'redundant']

[edit] Slate

[|:s| #(1 2 3 4 1 2 3 4) >> s] writingAs: Set. 
 
"==> {"Set traitsWindow" 1. 2. 3. 4}"

[edit] Smalltalk

"Example of creating a collection"
|a|
a := #( 1 1 2 'hello' 'world' #symbol #another 2 'hello' #symbol ).
a asSet.

Output:

Set (1 2 #symbol 'world' #another 'hello' )

the above has the disadvantage of loosing the original order (because Sets are unordered, and the hashing shuffles elements into an arbitrary order). When tried, I got:

Set('world' 1 #another 'hello' #symbol 2)

on my system. This can be avoided by using an ordered set (which has also O(n) complexity) as below:

Works with: Smalltalk/X
|a|
a := #( 1 1 2 'hello' 'world' #symbol #another 2 'hello' #symbol ).
a asOrderedSet.

Output:

OrderedSet(1 2 'hello' 'world' #symbol #another)

[edit] Sparkling

function undupe(arr) {
var t = {};
foreach(arr, function(key, val) {
t[val] = true;
});
 
var r = {};
foreach(t, function(key) {
r[sizeof r] = key;
});
 
return r;
}

[edit] Swift

Requires elements to be hashable:

func uniq<T: Hashable>(lst: [T]) -> [T] {
var uniqueSet = [T : Void](minimumCapacity: lst.count)
for x in lst {
uniqueSet[x] = ()
}
return Array(uniqueSet.keys)
}
 
println(uniq([3,2,1,2,3,4]))
Output:
[1, 2, 3, 4]

Another solution (preserves order of first occurrence). Also requires elements to be hashable:

func uniq<T: Hashable>(lst: [T]) -> [T] {
var seen = [T : Void](minimumCapacity: lst.count)
return lst.filter { x in
let unseen = seen[x] == nil
seen[x] = ()
return unseen
}
}
 
println(uniq([3,2,1,2,3,4]))
Output:
[3, 2, 1, 4]

Only requires elements to be equatable, but runs in O(n^2):

func uniq<T: Equatable>(lst: [T]) -> [T] {
var seen = [T]()
return lst.filter { x in
let unseen = find(seen, x) == nil
if (unseen) {
seen.append(x)
}
return unseen
}
}
 
println(uniq([3,2,1,2,3,4]))
Output:
[3, 2, 1, 4]

[edit] Tcl

The concept of an "array" in Tcl is strictly associative - and since there cannot be duplicate keys, there cannot be a redundant element in an array. What is called "array" in many other languages is probably better represented by the "list" in Tcl (as in LISP). With the correct option, the lsort command will remove duplicates.

set result [lsort -unique $listname]

[edit] TUSCRIPT

 
$$ MODE TUSCRIPT
list_old="b'A'A'5'1'2'3'2'3'4"
list_sort=MIXED_SORT (list_old)
list_new=REDUCE (list_sort)
PRINT list_old
PRINT list_new
 

Output (sorted)

b'A'A'5'1'2'3'2'3'4
1'2'3'4'5'A'b

or

 
$$ MODE TUSCRIPT
list_old="b'A'A'5'1'2'3'2'3'4"
DICT list CREATE
LOOP l=list_old
DICT list LOOKUP l,num
IF (num==0) DICT list ADD l
ENDLOOP
DICT list unload list
list_new=JOIN (list)
PRINT list_old
PRINT list_new
 

Output:

b'A'A'5'1'2'3'2'3'4
b'A'5'1'2'3'4

[edit] UnixPipes

Assuming a sequence is represented by lines in a file.

bash$ # original list
bash$ printf '6\n2\n3\n6\n4\n2\n'
6
2
3
6
4
2
bash$ # made uniq
bash$ printf '6\n2\n3\n6\n4\n2\n'|sort -n|uniq
2
3
4
6
bash$

or

bash$ # made uniq
bash$ printf '6\n2\n3\n6\n4\n2\n'|sort -nu
2
3
4
6
bash$

[edit] Ursala

The algorithm is to partition the list by equality and take one representative from each class, which can be done by letting the built in partition operator, |=, use its default comparison relation. This works on lists of any type including character strings but the comparison is based only on structural equivalence. It's up to the programmer to decide whether that's a relevant criterion for equivalence or else specify a better one.

#cast %s
 
example = |=hS& 'mississippi'

output:

'mspi'


[edit] Vedit macro language

The input "array" is an edit buffer where each line is one element.

Sort(0, File_Size)                                          // sort the data
While(Replace("^(.*)\N\1$", "\1", REGEXP+BEGIN+NOERR)){} // remove duplicates

[edit] Vim Script

call filter(list, 'count(list, v:val) == 1')

[edit] Wart

def (dedup l)
let exists (table)
collect+each x l
unless exists.x
yield x
exists.x <- 1

Output:

dedup '(1 3 2 9 1 2 3 8 8 1 0 2)
=> (1 3 2 9 8 0)

[edit] Wortel

@uniq [1 2 3 2 1 2 3] ; returns [1 2 3]

[edit] XPL0

code Text=12;           \built-in routine to display a string of characters
string 0; \use zero-terminated strings (not MSb terminated)
 
func StrLen(S); \Return number of characters in an ASCIIZ string
char S;
int I;
for I:= 0, -1>>1-1 do \(limit = 2,147,483,646 if 32 bit, or 32766 if 16 bit)
if S(I) = 0 then return I;
 
func Unique(S); \Remove duplicate bytes from string
char S;
int I, J, K, L;
[L:= StrLen(S); \string length
for I:= 0 to L-1 do \for all characters in string...
for J:= I+1 to L-1 do \scan rest of string for duplicates
if S(I) = S(J) then \if duplicate then
[for K:= J+1 to L do \ shift rest of string down (including
S(K-1):= S(K); \ terminating zero)
L:= L-1 \ string is now one character shorter
];
return S; \return pointer to string
];
 
Text(0, Unique("Pack my box with five dozen liquor jugs."))
Output:
Pack myboxwithfvedznlqurjgs.

[edit] zkl

Using built ins:

zkl: Utils.Helpers.listUnique(T(1,3,2,9,1,2,3,8,8,"8",1,0,2,"8"))
L(1,3,2,9,8,"8",0)
zkl: "1,3,2,9,1,2,3,8,8,1,0,2".unique()
,012389

Where listUnique is brute force:

fcn listUnique(xs){
xs.reduce(fcn(us,s){us.holds(s) and us or us.append(s)},L()) }
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