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Rooted by: String Index Replacer
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    Description

    Obligatory Python golf solution. Don't really like the final version I ended up on. I don't know APL, but this problem almost felt designed for it. The shortest solution I could think of was {s r c←⍵⋄k←+/q←c⍷s⋄(({⍵'0'[' '=⍵]}¨(-k)↑⍕r)@(k↑⍒q))s}, I'm sure it can be greatly improved by somebody who knows what they're doing though. (index origin is 0 btw

     Code 
    # exponential time - fast enough when first testcase is commented out
f=lambda s,r,c:s and(f(z:=s[:-1],r,c)+s[-1],f(z,r//10,c)+str(r%10))[c==s[-1]]
# linear recursive
f=lambda s,r,c:s and(c!=s[-1]and f(s[:-1],r,c)+s[-1]or f(s[:-1],r//10,c)+str(r%10))

f=lambda s,r,c:"".join(x==c and[str(r%10),r:=r//10][0]or x for x in s[::-1])[::-1]
    Test Cases 
    import codewars_test as test
# TODO Write tests
import solution # or from solution import example

# test.assert_equals(actual, expected, [optional] message)
@test.describe("Example")
def test_group():
    @test.it("test case")
    def test_case():
        test.assert_equals(f("0000##81#059671####=1811", 3, '#'), "0000008100596710003=1811")
        test.assert_equals(f("9182 7391238 91$$$$", 3, '$'), "9182 7391238 910003")
        test.assert_equals(f("The counter is ---", 11, '-'), "The counter is 011")
    View Run Output
  • Code
    • using System;
    • using System.Linq;
    • using System.Collections.Generic;
    • # exponential time - fast enough when first testcase is commented out
    • f=lambda s,r,c:s and(f(z:=s[:-1],r,c)+s[-1],f(z,r//10,c)+str(r%10))[c==s[-1]]
    • # linear recursive
    • f=lambda s,r,c:s and(c!=s[-1]and f(s[:-1],r,c)+s[-1]or f(s[:-1],r//10,c)+str(r%10))
    • public class Test{
    • static void Main(){
    • Console.WriteLine(Replacer.sValueReplacer("0000##81#059671####=1811", 3, '#'));
    • }
    • }
    • public class Replacer{
    • public static string sValueReplacer(string source, int index, char char_to_replace)
    • {
    • string res = source;
    • try
    • {
    • if (!string.IsNullOrEmpty(char_to_replace.ToString()))
    • {
    • if (res.Contains(char_to_replace.ToString()))
    • {
    • // Get ALL Indexes position of character
    • var Indexes = GetIndexes(res, char_to_replace.ToString());
    • int max = GetMaxValue(Indexes.Count);
    • while (index >= max)
    • {
    • index -= max;
    • }
    • var new_value = index.ToString().PadLeft(Indexes.Count, '0');
    • for (int i = 0; i < Indexes.Count; i++)
    • {
    • res = res.Remove(Indexes[i], 1).Insert(Indexes[i], new_value[i].ToString());
    • }
    • }
    • }
    • }
    • catch (Exception)
    • {
    • res = source;
    • }
    • return res;
    • }
    • private static List<int> GetIndexes(string mainString, string toFind)
    • {
    • var Indexes = new List<int>();
    • for (int i = mainString.IndexOf(toFind); i > -1; i = mainString.IndexOf(toFind, i + 1))
    • {
    • // for loop end when i=-1 (line.counter not found)
    • Indexes.Add(i);
    • }
    • return Indexes;
    • }
    • private static int GetMaxValue(int numIndexes)
    • {
    • int max = 0;
    • for (int i = 0; i < numIndexes; i++)
    • {
    • if (i == 0)
    • max = 9;
    • else
    • max = max * 10 + 9;
    • }
    • return max;
    • }
    • }
    • f=lambda s,r,c:"".join(x==c and[str(r%10),r:=r//10][0]or x for x in s[::-1])[::-1]
     Test Cases
    • import codewars_test as test
    • # TODO Write tests
    • import solution # or from solution import example
    • # test.assert_equals(actual, expected, [optional] message)
    • @test.describe("Example")
    • def test_group():
    • @test.it("test case")
    • def test_case():
    • test.assert_equals(f("0000##81#059671####=1811", 3, '#'), "0000008100596710003=1811")
    • test.assert_equals(f("9182 7391238 91$$$$", 3, '$'), "9182 7391238 910003")
    • test.assert_equals(f("The counter is ---", 11, '-'), "The counter is 011")