Selectively replace multiple instances of a character within a string

Task

This is admittedly a trivial task but I thought it would be interesting to see how succinctly (or otherwise) different languages can handle it.

Given the string: "abracadabra", replace programatically:

the first 'a' with 'A'
the second 'a' with 'B'
the fourth 'a' with 'C'
the fifth 'a' with 'D'
the first 'b' with 'E'
the second 'r' with 'F'

Note that there is no replacement for the third 'a', second 'b' or first 'r'.

The answer should, of course, be : "AErBcadCbFD".

Other tasks related to string operations:

Metrics

Counting

Remove/replace

Anagrams/Derangements/shuffling

Find/Search/Determine

Formatting

Song lyrics/poems/Mad Libs/phrases

Tokenize

Sequences

11l

Translation of: Python

V rep = [‘a’ = [1 = ‘A’, 2 = ‘B’, 4 = ‘C’, 5 = ‘D’], ‘b’ = [1 = ‘E’], ‘r’ = [2 = ‘F’]]

F trstring(oldstring, repdict)
   DefaultDict[Char, Int] seen
   V newchars = ‘’
   L(c) oldstring
      V i = ++seen[c]
      newchars ‘’= I c C repdict & i C repdict[c] {repdict[c][i]} E c
   R newchars

print(‘abracadabra -> ’trstring(‘abracadabra’, rep))

Output:

abracadabra -> AErBcadCbFD

Ada

-- Selectively replace multiple instances of a character within a string
-- J. Carter     2024 Jun

with Ada.Strings.Fixed;
with Ada.Text_IO;

procedure Selectively_Replace is
   procedure Replace
      (Letter : in Character; Occurrence : in Positive; Within : in String; By : in Character; Into : in out String)
   with Pre => Into'First = Within'First and Into'Last = Within'Last;
   -- Finds the index of the Occurrence-th instance of Letter in Within and sets the character in Into at that index to By
   -- If there is no such index, has no effect
   -- Before the first call to replace, Within should be = Into

   procedure Replace
      (Letter : in Character; Occurrence : in Positive; Within : in String; By : in Character; Into : in out String)
   is
      Start : Natural := 0;
   begin -- Replace
      Find : for I in 1 .. Occurrence loop
         Start := Ada.Strings.Fixed.Index (Within, Letter & "", Start + 1);

         if Start = 0 then -- Within has fewer than Occurrence instances of Letter
            return;
         end if;
      end loop Find;

      Into (Start) := By;
   end Replace;

   Source : constant String := "abracadabra";

   Result : String := Source;
begin -- Selectively_Replace
   Replace (Letter => 'a', Occurrence => 1, Within => Source, By => 'A', Into => Result);
   Replace (Letter => 'a', Occurrence => 2, Within => Source, By => 'B', Into => Result);
   Replace (Letter => 'a', Occurrence => 4, Within => Source, By => 'C', Into => Result);
   Replace (Letter => 'a', Occurrence => 5, Within => Source, By => 'D', Into => Result);
   Replace (Letter => 'b', Occurrence => 1, Within => Source, By => 'E', Into => Result);
   Replace (Letter => 'r', Occurrence => 2, Within => Source, By => 'F', Into => Result);
   Ada.Text_IO.Put_Line (Item => Source & " => " & Result);
end Selectively_Replace;

Output:

abracadabra => AErBcadCbFD

ALGOL 68

CO in the string "abracadabra", replace the first  'a' with 'A', the second 'a' with 'B'
                                      , the fourth 'a' with 'C', the fifth  'a' with 'D'
                                        the first  'b' with 'E', the second 'r' with 'F'
CO
BEGIN
    [,]STRING replacements = ( ( "a", "ABaCD" ), ( "b", "E" ), ( "r", "rF" ) );
    [ 1 LWB replacements : 1 UPB replacements ]INT position;
    STRING input = "abracadabra";
    [ LWB input : UPB input ]CHAR output;
    FOR i FROM LWB position TO UPB position DO position[ i ] := LWB replacements[ i, 2 ] OD;
    FOR c pos FROM LWB input TO UPB input DO
        CHAR c           = input[ c pos ];
        output[ c pos ] := c;
        BOOL found      := FALSE;
        FOR r pos FROM 1 LWB replacements TO 1 UPB replacements WHILE NOT found DO
            STRING r = replacements[ r pos, 1 ];
            IF c = r[ LWB r ] THEN
                found := TRUE;
                IF position[ r pos ] <= UPB replacements[ r pos, 2 ] THEN
                    output[   c pos ]  := replacements[ r pos, 2 ][ position[ r pos ] ];
                    position[ r pos ] +:= 1;
                    found              := TRUE
                FI
            FI
        OD
    OD;
    print( ( """", input, """ -> """, output, """" ) );
    IF output /= "AErBcadCbFD" THEN print( ( " ** UNEXPECTED RESULT" ) ) FI;
    print( ( newline ) )
END

Output:

"abracadabra" -> "AErBcadCbFD"

Arturo

replacement: function [rule,ch,order][
    loop rule 'r ->
        if r\[0] = order -> return r\[1]
    return ch
]
multiReplace: function [str, rules][
    cntr: #.raw flatten couple keys rules repeat 0 size rules

    join map str 'ch [
        (key? cntr ch)? [
            cntr\[ch]: cntr\[ch] + 1
            replacement rules\[ch] ch dec cntr\[ch]
        ] -> ch
    ]
]

print multiReplace "abracadabra" #[
    a: [[0 `A`][1 `B`][3 `C`][4 `D`]]
    b: [[0 `E`]]
    r: [[1 `F`]]
]

Output:

AErBcadCbFD

AutoHotkey

str := "abracadabra"
steps := [[1, "a", "A"]
        , [2, "a", "B"]
        , [4, "a", "C"]
        , [5, "a", "D"]
        , [1, "b", "E"]
        , [2, "r", "F"]]

MsgBox % result := Selectively_replace(str, steps)
return

Selectively_replace(str, steps){
    Res := [], x := StrSplit(str)
    for i, step in steps {
        n := step.1, L := step.2, R := step.3, k := 0
        for j, v in x
            if (v=L) && (++k = n) {
                Res[j] := R
                break
            }
    }
    for j, v in x
        result .= Res[j] = "" ? x[j] : Res[j]
    return result
}

Output:

AErBcadCbFD

C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void) {
  const char string[] = "abracadabra";

  char *replaced = malloc(sizeof(string));
  strcpy(replaced, string);

  // Null terminated replacement character arrays
  const char *aRep = "ABaCD";
  const char *bRep = "E";
  const char *rRep = "rF";

  for (char *c = replaced; *c; ++c) {
    switch (*c) {
    case 'a':
      if (*aRep)
        *c = *aRep++;
      break;
    case 'b':
      if (*bRep)
        *c = *bRep++;
      break;
    case 'r':
      if (*rRep)
        *c = *rRep++;
      break;
    }
  }

  printf("%s\n", replaced);

  free(replaced);
  return 0;
}

Output:

AErBcadCbFD

C++

#include <map>
#include <iostream>
#include <string>

int main()
{
  std::map<char, std::string> rep = 
    {{'a', "DCaBA"}, // replacement string is reversed
     {'b', "E"},
     {'r', "Fr"}};

  std::string magic = "abracadabra";

  for(auto it = magic.begin(); it != magic.end(); ++it)
  {
    if(auto f = rep.find(*it); f != rep.end() && !f->second.empty())
    {
      *it = f->second.back();
      f->second.pop_back();
    }
  }

  std::cout << magic << "\n";
}

Output:

AErBcadCbFD

Delphi

Works with: Delphi version 6.0

Library: SysUtils,StdCtrls

var TestStr: string = 'abracadabra';


function FindNthChar(C: char; S: string; N: integer): integer;
{Routine to find the Nth version of C, string S}
begin
for Result:=1 to Length(S) do
 if S[Result]=C then
	begin
	Dec(N);
	if N<=0 then exit;
	end;
Result:=-1;
end;


procedure ReplaceNthChar(COld,CNew: char; var S: string; N: integer);
{Find and replace the Nth version COld with CNew}
var Inx: integer;
begin
Inx:=FindNthChar(COld,S,N);
if Inx<1 then exit;
S[Inx]:=CNew;
end;


procedure SelectivelyReplaceChars(Memo: TMemo);
var I: integer;
begin
Memo.Lines.Add('Before: '+TestStr);
{Do the replacement toward the end of string first}
ReplaceNthChar('a','D',TestStr,5);
ReplaceNthChar('a','C',TestStr,4);
ReplaceNthChar('a','B',TestStr,2);
ReplaceNthChar('r','F',TestStr,2);
ReplaceNthChar('a','A',TestStr,1);
ReplaceNthChar('b','E',TestStr,1);
Memo.Lines.Add('After: '+TestStr);
end;

Output:

Before: AErBcadCbFD
After: AErBcAdCEFD
Elapsed Time: 1.749 ms.

EasyLang

rs$[] = [ "a" "b" "r" ]
rc[][] = [ [ 1 2 4 5 ] [ 1 ] [ 2 ] ]
rd$[][] = [ [ "A" "B" "C" "D" ] [ "E" ] [ "F" ] ]
s$ = "abracadabra"
# 
len cnt[] len rs$[]
for c$ in strchars s$
   for i to len rs$[]
      if c$ = rs$[i]
         cnt[i] += 1
         for j to len rc[i][]
            if rc[i][j] = cnt[i]
               c$ = rd$[i][j]
            .
         .
      .
   .
   r$ &= c$
.
print r$

Output:

AErBcadCbFD

ed

Port of #sed solution.

# by Artyom Bologov
H
s/a\([^a]*\)a\([^a]*a[^a]*\)a\([^a]*\)a/A\1B\2C\3D/
s/b/E/
s/\(r[^r]*\)r/\1F/
,p
Q

Output:

$ ed -s selective-replace.input < selective-replace.ed 
Newline appended
AErBcadCbFD

EMal

fun transmogrify  = text by text input, Map replacements
  Map indexes = text%int[]
  text result = ""
  for each text ch in input
    result.append(when(replacements.has(++indexes[ch] + ch), replacements[indexes[ch] + ch], ch))
  end
  return result
end
writeLine(transmogrify("abracadabra",
  text%text["1a" => "A", "2a" => "B", "4a" => "C", "5a" => "D", "1b" => "E", "2r" => "F"]))

Output:

AErBcadCbFD

Factor

Works with: Factor version 0.99 2022-04-03

USING: assocs formatting grouping kernel random sequences ;

CONSTANT: instrs {
    CHAR: a 1 CHAR: A
    CHAR: a 2 CHAR: B
    CHAR: a 4 CHAR: C
    CHAR: a 5 CHAR: D
    CHAR: b 1 CHAR: E
    CHAR: r 2 CHAR: F
}

: counts ( seq -- assoc )
    H{ } clone swap [ 2dup swap inc-at dupd of ] zip-with nip ;

: replace-nths ( seq instrs -- seq' )
    [ counts ] dip 3 group [ f suffix 2 group ] map substitute keys ;

: test ( str -- )
    dup instrs replace-nths "" like "%s -> %s\n" printf ;


"abracadabra" test
"abracadabra" randomize test

Output:

abracadabra -> AErBcadCbFD
caaarrbabad -> cABarFECbDd

FreeBASIC

Function replaceChar(Byref S As String) As String
    Dim As String A = "ABaCD", B = "Eb", R = "rF"
    Dim As Byte pA = 1, pB = 1, pR = 1
    For i As Byte = 0 To Len(S)
        Select Case Mid(S,i,1)
        Case "a"
            Mid(S,i,1) = Mid(A,pA,1)
            pA += 1
        Case "b"
            Mid(S,i,1) = Mid(B,pB,1)
            pB += 1
        Case "r"
            Mid(S,i,1) = Mid(R,pR,1)
            pR += 1
        End Select
    Next i
    Return S
End Function

Dim As String S
S = "abracadabra"
Print S; " -> "; replaceChar(S)
S = "caarabadrab"
Print S; " -> "; replaceChar(S)
Sleep

Output:

abracadabra -> AErBcadCbFD
caaarrbabad -> cABarFECbDd

FutureBasic

include "NSLog.incl"

void local fn DoIt
  long               a = 0, b = 0, r = 0, length, i
  CFMutableStringRef string = fn MutableStringWithString( @"abracadabra" )
  CFStringRef        s
  
  length = len(string)
  for i = 0 to length - 1
    s = NULL
    select ( mid(string,i,1) )
      case @"a"
        a++
        select ( a )
          case 1 : s = @"A"
          case 2 : s = @"B"
          case 4 : s = @"C"
          case 5 : s = @"D"
        end select
      case @"b"
        b++
        if ( b == 1 ) then s = @"E"
      case @"r"
        r++
        if ( r == 2 ) then s = @"F"
    end select
    if ( s ) then mid(string,i,1) = s
  next
  
  NSLog(@"%@",string)
end fn

fn DoIt

HandleEvents

Output:

AErBcadCbFD

Go

Translation of: Wren

package main

import (
    "fmt"
    "strings"
)

func main() {
    s := "abracadabra"
    ss := []byte(s)
    var ixs []int
    for ix, c := range s {
        if c == 'a' {
            ixs = append(ixs, ix)
        }
    }
    repl := "ABaCD"
    for i := 0; i < 5; i++ {
        ss[ixs[i]] = repl[i]
    }
    s = string(ss)
    s = strings.Replace(s, "b", "E", 1)
    s = strings.Replace(s, "r", "F", 2)
    s = strings.Replace(s, "F", "r", 1)
    fmt.Println(s)
}

Output:

AErBcadCbFD

Haskell

As a map-accumulation:

import Data.List (mapAccumL)
import qualified Data.Map.Strict as M
import Data.Maybe (fromMaybe)

---------- POSITIONAL CHARACTER-REPLACEMENT RULES --------

nthCharsReplaced :: M.Map Char [Maybe Char] -> String -> String
nthCharsReplaced ruleMap = snd . mapAccumL go ruleMap
  where
    go a c =
      case M.lookup c a of
        Nothing -> (a, c)
        Just [] -> (a, c)
        Just (d : ds) ->
          ( M.insert c ds a,
            fromMaybe c d
          )

--------------------------- TEST -------------------------
main :: IO ()
main = putStrLn $ nthCharsReplaced rules "abracadabra"

rules :: M.Map Char [Maybe Char]
rules =
  M.fromList
    [ ('a', (Just <$> "AB") <> [Nothing] <> (Just <$> "CD")),
      ('b', [Just 'E']),
      ('r', [Nothing, Just 'F'])
    ]

Output:

AErBcadCbFD

J

   upd=: {{ x (n{I.y=m)} y }}
   'ABCD' 'a' upd 0 1 3 4 'E' 'b' upd 0 'F' 'r' upd 1 'abracadabra'
AErBcadCbFD

upd here takes four arguments -- two on the left (replacement characters, original character) and two on the right(index values for which instances to replace, and the original string).

However, here's a more compact approach (the first item in the left argument is the target, and the rest of the left argument explicitly provides values for every instance of that item in the right argument):

   chg=: {{ (}.x) (I.y={.x)} y}}
   'aABaCD' chg 'bEb' chg 'rrF' chg 'abracadabra'
AErBcadCbFD

Java

Translation of: JavaScript

Here's an example translated from JavaScript.

int findNth(String s, char c, int n) {
    if (n == 1) return s.indexOf(c);
    return s.indexOf(c, findNth(s, c, n - 1) + 1);
}

String selectiveReplace(String s, Set... ops) {
    char[] chars = s.toCharArray();
    for (Set set : ops)
        chars[findNth(s, set.old, set.n)] = set.rep;
    return new String(chars);
}

record Set(int n, char old, char rep) { }

selectiveReplace("abracadabra",
    new Set(1, 'a', 'A'),
    new Set(2, 'a', 'B'),
    new Set(4, 'a', 'C'),
    new Set(5, 'a', 'D'),
    new Set(1, 'b', 'E'),
    new Set(2, 'r', 'F'));

Output:

AErBcadCbFD

JavaScript

function findNth(s, c, n) {
  if (n === 1) return s.indexOf(c);
  return s.indexOf(c, findNth(s, c, n - 1) + 1);
}

function selectiveReplace(s, ops) {
  const chars = Array.from(s);
  for ([n, old, rep] of ops) {
    chars[findNth(s, old, n)] = rep;
  }
  return chars.join("");
}

console.log(
  selectiveReplace("abracadabra", [
    [1, "a", "A"], // the first 'a' with 'A'
    [2, "a", "B"], // the second 'a' with 'B'
    [4, "a", "C"], // the fourth 'a' with 'C'
    [5, "a", "D"], // the fifth 'a' with 'D'
    [1, "b", "E"], // the first 'b' with 'E'
    [2, "r", "F"], // the second 'r' with 'F'
  ])
);

Output:

AErBcadCbFD

Or, expressed as a map-accumulation:

(() => {
    "use strict";

    // -- INSTANCE-SPECIFIC CHARACTER REPLACEMENT RULES --

    // # nthInstanceReplaced :: Dict Char [(Null | Char)] ->
    const nthInstanceReplaced = ruleMap =>
        // A string defined by replacements specified for
        // the nth instances of various characters.
        s => mapAccumL(
            (a, c) => c in a ? (() => {
                const ds = a[c];

                return Boolean(ds.length) ? [
                    Object.assign(a, {[c]: ds.slice(1)}),
                    ds[0] || c
                ] : [a, c];
            })() : [a, c]
        )(Object.assign({}, ruleMap))(
            [...s]
        )[1].join("");


    // ---------------------- TEST -----------------------
    const main = () =>
    // Instance-specific character replacement rules.
        nthInstanceReplaced({
            a: ["A", "B", null, "C", "D"],
            b: ["E"],
            r: [null, "F"]
        })(
            "abracadabra"
        );


    // --------------------- GENERIC ---------------------

    // mapAccumL :: (acc -> x -> (acc, y)) -> acc ->
    // [x] -> (acc, [y])
    const mapAccumL = f =>
    // A tuple of an accumulation and a list
    // obtained by a combined map and fold,
    // with accumulation from left to right.
        acc => xs => [...xs].reduce(
            ([a, bs], x) => second(
                v => [...bs, v]
            )(
                f(a, x)
            ),
            [acc, []]
        );


    // second :: (a -> b) -> ((c, a) -> (c, b))
    const second = f =>
    // A function over a simple value lifted
    // to a function over a tuple.
    // f (a, b) -> (a, f(b))
        ([x, y]) => [x, f(y)];


    // MAIN --
    return main();
})();

Output:

AErBcadCbFD

jq

In this section, array-indexing and occurrence-counting are both 0-based except for the transcription of the task in `steps`.

Generic functions

# Emit empty if the stream does not have an $n-th item
# Note: jq's nth/2 does not serve our purposes.
def n_th($n; stream):
  if $n < 0 then empty
  else foreach stream as $x (-1; .+1; if . == $n then $x else empty end)
  end;

def positions(stream; $v):
  foreach stream as $x (-1; .+1; if $v == $x then . else empty end);

# Input: an array or string.
# Output: the input with an occurrence of $old replaced by $new.
# . and $reference are assumed to be of the same type and length.
# The search occurs in $reference and the corresponding spot in . is modified.
def replace_nth($occurrence; $old; $new; $reference):
  if type == "array"
  then ($reference | n_th($occurrence; positions(.[]; $old)) // null) as $ix
  | if $ix then .[:$ix] + [$new] + .[$ix + 1:] else . end
  else explode
  | replace_nth($occurrence; $old|explode|first; $new|explode|first; $reference|explode)
  | implode
  end;

The task

def steps:
  [1, "a", "A"],
  [2, "a", "B"],
  [4, "a", "C"],
  [5, "a", "D"],
  [1, "b", "E"],
  [2, "r", "F"];

def task(steps):
  . as $reference
  | reduce steps as [$occurrence, $old, $new] (.;
      replace_nth($occurrence - 1; $old; $new; $reference ));
  
"abracadabra" | task(steps)

Output:

AErBcadCbFD

Julia

rep = Dict('a' => Dict(1 => 'A', 2 => 'B', 4 => 'C', 5 => 'D'), 'b' => Dict(1 => 'E'), 'r' => Dict(2 => 'F'))

function trstring(oldstring, repdict)
    seen, newchars = Dict{Char, Int}(), Char[]
    for c in oldstring
        i = get!(seen, c, 1)
        push!(newchars, haskey(repdict, c) && haskey(repdict[c], i) ? repdict[c][i] : c)
        seen[c] += 1
    end
    return String(newchars)
end

println("abracadabra -> ", trstring("abracadabra", rep))

Output:

Same as Perl.

Lambdatalk

1) first answer

We first translate the replacements program into a sequence of rules

the first  'a' with 'A'   -> aA1
...and so on

Then we add to the existing set of array functions a new one finding the indexes of some value in a given array.

{def A.findindexes

 {def A.findindexes.rec
  {lambda {:v :a :b :i}
   {if {A.empty? :a}
    then :b
    else {A.findindexes.rec :v {A.rest :a} 
                            {if {W.equal? {A.first :a} :v}
                             then {A.addlast! :i :b}
                             else :b} 
                            {+ :i 1}} }}}

 {lambda {:v :a}
  {A.findindexes.rec :v :a {A.new} 0} }}
-> A.findindexes
 
{A.findindexes a {A.split abracadabra}}
-> [0,3,5,7,10]
... and so on

Using findindexes we can translate the aA1 aB2 aC4 aD5 bE1 rF2 sequence into a new one where numbers are replaced by indexes in the given string, here abracadabra.

{def replacements.rules
  {lambda {:w :r}
   {A.new 
    {W.get 0 :r}
    {W.get 1 :r}
    {A.get {- {W.get 2 :r} 1}  // arrays begin at 0
           {A.findindexes {W.get 0 :r} {A.split :w}}}}}}
->  replacements.rules

{A.join {replacements.rules abracadabra aA1}}
-> aA0
... and so on

Finally the replacements function will apply this sequence of rules to the word.

{def replacements 

 {def replacements.rec
  {lambda {:word :rules}
   {if {A.empty? :rules}
    then {A.join :word}
    else {replacements.rec {A.set! {A.get 2 {A.first :rules}}
                                   {A.get 1 {A.first :rules}}
                                   :word} 
                           {A.rest :rules}} }}}

 {lambda {:word :rules}
  {replacements.rec
   {A.split :word} 
   {A.map {replacements.rules :word} {A.new :rules}} }}}
-> replacements

{replacements abracadabra aA1 aB2 aC4 aD5 bE1 rF2}
-> AErBcadCbFD
  (AErBcadCbFD)

{replacements caaarrbabad aA1 aB2 aC4 aD5 bE1 rF2} 
-> cABarFECbDd
  (cABarFECbDd)

2) second answer using regexps

Here is a quick & dirty answer using the S.replace_once primitive.

{def multrepl_rex
 {lambda {:word :rules}
  {if {A.empty? :rules}
   then :word
   else {multrepl_rex 
         {S.replace_once {W.first {A.first :rules}}
                     by {W.last {A.first :rules}}
                     in :word }
         {A.rest :rules}} }}}
-> multrepl_rex

{multrepl_rex
 abracadabra
  {A.new
   aA aB a3 aC aD 3a   // save third "a" as "3" and restore it
   bE                  // first "b"
   r1 rF 1r            // save first "r" as "1" and restore it
}}
-> AErBcadCbFD 
  (AErBcadCbFD)

Nim

import std/tables

type
  # Table of replacements for a character.
  Replacements = Table[int, char]
  # Table mapping characters to their replacement table.
  ReplacementTable = Table[char, Replacements]

const ReplTable = {'a': {1: 'A', 2: 'B', 4: 'C', 5: 'D'}.toTable,
                   'b': {1: 'E'}.toTable,
                   'r': {2: 'F'}.toTable
                  }.toTable

proc replace(text: string; replTable: ReplacementTable): string =
  var counts: Table[char, int]  # Follow count of characters.
  for c in text:
    if c in replTable:
      counts.mgetOrPut(c, 0).inc  # Update count for this char.
      let pos = counts[c]
      result.add replTable[c].getOrDefault(pos, c)
    else:
      result.add c

echo replace("abracadabra", ReplTable)

Output:

AErBcadCbFD

Perl

use strict;
use warnings;
use feature 'say';

sub transmogrify {
    my($str, %sub) = @_;
    for my $l (keys %sub) {
        $str =~ s/$l/$_/ for split '', $sub{$l};
        $str =~ s/_/$l/g;
    }
    $str
}

my $word = 'abracadabra';
say "$word -> " . transmogrify $word, 'a' => 'AB_CD', 'r' => '_F', 'b' => 'E';

Output:

abracadabra -> AErBcadCbFD

Phix

Couldn't really decide which I prefer so posted both.

with javascript_semantics
function replace_nth(string s, r)
    string res = s
    for i=1 to length(r) by 3 do
        res[find_all(r[i],s)[r[i+1]-'0']] = r[i+2]
    end for
    return res
end function
?replace_nth("abracadabra","a1Aa2Ba4Ca5Db1Er2F")

-- Alternative version
function replace_nths(string s, sequence r)
    for icr in r do
        {sequence idx, integer ch, string reps} = icr
        s = reinstate(s,extract(find_all(ch,s),idx),reps)
    end for
    return s
end function

constant r = {{{1,2,4,5},'a',"ABCD"},
              {{1},'b',"E"},
              {{2},'r',"F"}}
?replace_nths("abracadabra",r)

Output:

"AErBcadCbFD"
"AErBcadCbFD"

Phixmonti

/# Rosetta Code problem: https://rosettacode.org/wiki/Selectively_replace_multiple_instances_of_a_character_within_a_string
by Galileo, 11/2022 #/

include ..\Utilitys.pmt

"ABaCD" var A "Eb" var B "rF" var R

"abracadabra" len for >ps
    tps get tochar
    dup "a" == if drop A pop var A tps set else
    dup "b" == if drop B pop var B tps set else
    "r" == if R pop var R tps set
    endif endif endif
    ps> drop
endfor

pstack

Output:

["AErBcadCbFD"]

=== Press any key to exit ===

Python

Translation of: Julia

from collections import defaultdict

rep = {'a' : {1 : 'A', 2 : 'B', 4 : 'C', 5 : 'D'}, 'b' : {1 : 'E'}, 'r' : {2 : 'F'}}
 
def trstring(oldstring, repdict):
    seen, newchars = defaultdict(lambda:1, {}), []
    for c in oldstring:
        i = seen[c]
        newchars.append(repdict[c][i] if c in repdict and i in repdict[c] else c)
        seen[c] += 1
    return ''.join(newchars)

print('abracadabra ->', trstring('abracadabra', rep))

Alternative

import functools

from typing import Iterable
from typing import Tuple


@functools.cache
def find_nth(s: str, sub: str, n: int) -> int:
    assert n >= 1
    if n == 1:
        return s.find(sub)
    return s.find(sub, find_nth(s, sub, n - 1) + 1)


def selective_replace(s: str, ops: Iterable[Tuple[int, str, str]]) -> str:
    chars = list(s)
    for n, old, new in ops:
        chars[find_nth(s, old, n)] = new
    return "".join(chars)


print(
    selective_replace(
        "abracadabra",
        [
            (1, "a", "A"),  # the first 'a' with 'A'
            (2, "a", "B"),  # the second 'a' with 'B'
            (4, "a", "C"),  # the fourth 'a' with 'C'
            (5, "a", "D"),  # the fifth 'a' with 'D'
            (1, "b", "E"),  # the first 'b' with 'E'
            (2, "r", "F"),  # the second 'r' with 'F'
        ],
    )
)

Output:

AErBcadCbFD

Or, as a map-accumulation:

'''Instance-specific character replacement rules'''

from functools import reduce


# nthInstanceReplaced :: Dict Char [(None | Char)] ->
# String -> String
def nthInstanceReplaced(ruleMap):
    def go(a, c):
        ds = a.get(c, None)
        return (
            dict(a, **{c: ds[1:]}),
            ds[0] or c
        ) if ds else (a, c)

    return lambda s: ''.join(
        mapAccumL(go)(ruleMap)(s)[1]
    )


# ------------------------- TEST -------------------------
def main():
    '''Rule-set applied to a given string.'''

    print(
        nthInstanceReplaced({
            'a': ['A', 'B', None, 'C', 'D'],
            'b': ['E'],
            'r': [None, 'F']
        })(
            "abracadabra"
        )
    )


# ----------------------- GENERIC ------------------------

# mapAccumL :: (acc -> x -> (acc, y)) ->
# acc -> [x] -> (acc, [y])
def mapAccumL(f):
    '''A tuple of an accumulation and a map
       with accumulation from left to right.
    '''
    def go(a, x):
        return second(lambda v: a[1] + [v])(
            f(a[0], x)
        )
    return lambda acc: lambda xs: reduce(
        go, xs, (acc, [])
    )


# second :: (a -> b) -> ((c, a) -> (c, b))
def second(f):
    '''A simple function lifted to a function over a tuple,
       with f applied only to the second of two values.
    '''
    return lambda xy: (xy[0], f(xy[1]))


# MAIN ---
if __name__ == '__main__':
    main()

Output:

AErBcadCbFD

Quackery

If there is no nth instance of a particular character in the string, selectivereplace will disregard it. For example, if the nest of replacements in the example included [ 1 char z char ! ] the output would be the same, as there is no z in "abracadabra".

 [ dup size temp put
   witheach
     [ over = if
        [ swap 1 -
          tuck 0 = if
            [ i^ temp replace
              conclude ] ] ]
    2drop
    temp take ]               is nfind            ( n c $ --> n )

  [ temp put
    [] [] rot witheach
      [ 2 pluck
        dip rot join unrot
        nested join ]
    [] swap witheach
      [ do
        temp share nfind
        join ]
    temp take 0 join unrot
    witheach
      [ dip [ behead rot ]
        poke swap ]
    drop
    -1 split drop ]           is selectivereplace (   [ $ --> $ )


  ' [ [ 1 char a char A ]
      [ 2 char a char B ]
      [ 4 char a char C ]
      [ 5 char a char D ]
      [ 1 char b char E ]
      [ 2 char r char F ] ]
  $ "abracadabra"
  selectivereplace echo$

Output:

AErBcadCbFD

Raku

Set up to not particularly rely on absolute structure of the word. Demonstrate with both the original 'abracadabra' and with a random shuffled instance.

sub mangle ($str is copy) {
    $str.match(:ex, 'a')».from.map: { $str.substr-rw($_, 1) = 'ABaCD'.comb[$++] };
    $str.=subst('b', 'E');
    $str.substr-rw($_, 1) = 'F' given $str.match(:ex, 'r')».from[1];
    $str
}

say $_, ' -> ', .&mangle given 'abracadabra';

say $_, ' -> ', .&mangle given 'abracadabra'.comb.pick(*).join;

Output:

abracadabra -> AErBcadCbFD
caarabadrab -> cABraECdFDb

RPL

The character "-" in the rule string means that no replacement should be made for the occurrence concerned. Any other character can be chosen by modifying the code appropriately.

Due to the use of INCR and REPL instructions, this program will only work directly on HP48 compatible RPL versions. HP28 users must have programmed their own version of these instructions.

RPL code

Comment

 ≪ 0 → car rule occ
  ≪ 1 OVER SIZE FOR j
      IF DUP j DUP SUB car == THEN
        rule 'occ' INCR DUP SUB
        IF DUP "-" == THEN DROP ELSE j SWAP REPL END
     END NEXT
≫ ≫ 'REPLR' STO

REPLR ( "string" "character" "rule" -- "string" ) 
loop for j = 1 to string length
   if string[j] == character
      get todo = rule[++occ]
      replace if todo is different from "-"
   end if end loop
return string

Input:

"abracadabra"
≪ "a" "AB-CD" REPLR "b" "E" REPLR "r" "-F" REPLR ≫ EVAL

Output:

1: "AErBcadCbFD"

Ruby

str = "abracadabra"
rules = [
  ["a", 1, "A"],
  ["a", 2, "B"],
  ["a", 4, "C"],
  ["a", 5, "D"], 
  ["b", 1, "E"], 
  ["r", 2, "F"]]

indices = Hash.new{[]}
str.each_char.with_index{|c, i| indices[c] <<= i}

rules.each{|char, i, to| str[indices[char][i-1]] = to}

p str

Output:

"AErBcadCbFD"

sed

s/a\([^a]*\)a\([^a]*a[^a]*\)a\([^a]*\)a/A\1B\2C\3D/
s/b/E/
s/\(r[^r]*\)r/\1F/

V (Vlang)

A similar approach to the C++ entry.

fn selectively_replace_chars(s string, char_map map[string]string) string {
    mut bytes := s.bytes()
    mut counts := {
        'a': 0
        'b': 0
        'r': 0
    }
    for i := s.len - 1; i >= 0; i-- {
        c := s[i].ascii_str()
        if c in ['a', 'b', 'r'] {
            bytes[i] = char_map[c][counts[c]]
            counts[c]++
        }
    }
    return bytes.bytestr()
}

fn main() {
    char_map := {
        'a': 'DCaBA'
        'b': 'bE'
        'r': 'Fr'
    }
    for old in ['abracadabra', 'caaarrbabad'] {
        new := selectively_replace_chars(old, char_map)
        println('$old -> $new')
    }
}

Output:

abracadabra -> AErBcadCbFD
caaarrbabad -> cABarFECbDd

Wren

Library: Wren-seq

Library: Wren-str

Library: Wren-regex

Not particularly succinct but, thanks to a recently added library method, better than it would have been :)

import "./seq" for Lst
import "./str" for Str

var s = "abracadabra"
var sl = s.toList
var ixs = Lst.indicesOf(sl, "a")[2]
var repl = "ABaCD"
for (i in 0..4) sl[ixs[i]] = repl[i]
s = sl.join()
s = Str.replace(s, "b", "E", 1)
s = Str.replace(s, "r", "F", 2, 1)
System.print(s)

Output:

AErBcadCbFD

Alternatively, using regular expressions (embedded script) producing output as before.

import "./regex" for Regex

var s = "abracadabra"
var split = Regex.compile("a").split(s)
var repl = "ABaCD"
var res = ""
for (i in 0...split.count-1) res = res + split[i] + repl[i]
s = res + split[-1]
s = Regex.compile("b").replace(s, "E")
s = Regex.compile("r").replaceAll(s, "F", 2, 1)
System.print(s)

XPL0

string 0;
proc Mangle(S);
char S, A, B, R;
[A:= "ABaCD";  B:= "Eb";  R:= "rF";
while S(0) do
    [case S(0) of
      ^a: [S(0):= A(0);  A:= A+1];
      ^b: [S(0):= B(0);  B:= B+1];
      ^r: [S(0):= R(0);  R:= R+1]
    other [];
    S:= S+1;
    ];
];

char S;
[S:= "abracadabra"; 
Text(0, S);  Text(0, " -> ");  Mangle(S);  Text(0, S);  CrLf(0);
S:= "caarabadrab";
Text(0, S);  Text(0, " -> ");  Mangle(S);  Text(0, S);  CrLf(0);
]

Output:

abracadabra -> AErBcadCbFD
caarabadrab -> cABraECdFDb