Decrement operator and decrement assignment, different behaviour

Question

Hello we are doing this programming exercise: String doubles.

We would like to know with a detailed explanation (and more examples step by step if possible) why this code:

using System;
using System.Text;

public static class Kata
{
  public static string Doubles(string str)
  {  
    Console.WriteLine("str: "+str);
    for(int i=0; i<str.Length-1;i++){
      if(str[i]==str[i+1]){
        str=str.Remove(i--,2); //➖➖ operator does not store the modified i value inside i?
        Console.WriteLine("str: "+str);
      }
    }
    return str;
  }
}

Behaves differently than:

using System;
using System.Text;

public static class Kata
{
  public static string Doubles(string str)
  {  
    Console.WriteLine("str: "+str);
    for(int i=0; i<str.Length-1;i++){
      if(str[i]==str[i+1]){
        str=str.Remove(i,2); 
        Console.WriteLine("str: "+str);
        i-=1; //➖= operator DOES store the modified i value inside i?
      }
    }
    return str;
  }
}

We have read:

• How do Prefix (++x) and Postfix (x++) operations work?
• What is the difference between i++ and ++i?

EDIT: For example with the following string:

zpezyhgwqevkekipkxboraceiehzpvajbjalaymsgvqlepelorhjgznaqtaghusuiypywzthoxasudhterdcwzvgyflmcgbjgfwqvefgmimdnzfsrgcpdqewohehvqlphqyzjvwpnbmpthydaiszcgnealtybeydagjvkndocgibgmxmnadtyxcynidjsxluzxelamjmpaxlfkjvsghmslztxuvevfymnawxwrtmnqinksdiopjwedqlnfqfqjblyfnlzanvybclifrmqfwczavthkdnvimztxucqjirndaediodolnylxkueowazqcdzilusmuimstcgvyrtolwnaxbmirxawfuwulawaeclrabiabhworlvpgrqnamqprwxmzegtrljvzwtlgkametzgnuzrlwawbwbdwgbkelrutpfgoerdmtrmjhlryouvrxkjixpyrpcpqkmfhvnajhfptpvklwcwewyrptmxrpjurafthzirncdtdachdgmtgqhqgtompqfzyuerjrolicsenpqetgedhxupmqsocbmnoebhdlcusbldyrcksjmcrofsyzjgpeqjniahdjlgmqrkoblsjdblmbhogkveotmtetjkfcqlokxudwnqxmcewmbmatosohbxyvxrjyxzuamxrmgyfbhcrhwxjloyxhizwizvhyocjvzeuonpnmazfqyezmpwdfprpyzjtinoxasnculkgitgjvwvjdrnsupdzpqesrlrsymljulsygxioemyzkxkiyvjihroxcnlikloqebavjgqamrusbtiqlzdajiuvdvcmkihcoseiwpxifwapmaxzmalvxqohazilysuvhodnphzfmuhskhqohikfbohgmjyhvrsjwzozgprjwnaioavklbnoyxejvqheypzblrzhcuoaozrmonvjgqwhaqvwuacrokinpekdiqnvwyieribtjpedqjmgznpalerexkdrbpdbkchrflzzayilnbrcqzfwmelrnufdtobvanohrmeswmxbadvoqdesajsrlgtesvcdlyetpztoqpvbifxyrhnepxzstxcfwvsxjqdlcapdawkrwmeupgiezcuxfvmlszjkvhufenpladmhwhwabcgloeqbrfqpelilrugypcrxevpmbyzljqtxwtozskzbzkpcucmeurauledwueforkpzcmjpjwbtwqixrhdpdeqwhjvwaqfljovqzhswuxpgrmnwnykchkjgeaunvloldyvsyjogvfojyjrijdcgahgkgsdscgbesnusruyekehbwzivilerihkwslmvdodkrlpzrofyafogkhkrckwkcxafisxoasxliwfjtpmjlownxfvylmltngirymgnbhvcynxnymzobnhdwzdohexclhjyiespfvhiwkoyvbhuqkecrbdgzcewznpqdzeuvlhmwoctzwxacghgampdrkuyuwxmyecsgdzkgquhbfqpnwsxujwevlftzbnmzhvsxkzdhqfyidsekydzrpiyxvxfkfdhitjxrdwopedxwlcuqpizpgkznegcfavabpuvmhbojnclsweponyiecmspsinzdcvknhfijoupcvzxbljypmibskriurigwogafoyheyetbhnvogypkrutscqwosfotombgvzjxzvzdchgdqonicxbtmtqznuxemzdpcjkvhbcgmkvbdsdnupgzvoxuqderbhdrpjqaexqjncyvbuwfruptxseuyzcvjoxldugxixzwlspbtwhyzmsicvlywkzqhstkypqbyugrvsiedcacsdvojibyxiptzfgfaeganjngutkdnjghtpthtmdofxywvndvbzvbnhoaklwcruxylklygafybxvnbkafgshgmsclbsrdgezxctfspauwaqmazalrvzmrlsiaenzngxodhgaodhmqslztuahaihsimjlencbgqcavmvseqhwefvawalsygfpjxgqyabgfapldgyhckalodrioecjcaknrygfujuhvyzuxeuqdvoxjcypgrxrdepctfztnsptqpdwxdgxpmwoytodgejdfirseaclfvhtzcsdkzcribdaqwnjpmjmuirolotojliclulasjbdnderbintwtmuzrkdswgnhwsbovsjxotzlqvnjtmkbpopqrvmtwdnwqlgqdwhzmtyjwmrickxafqpgxwyufcgqguxthpeimpwojnhbeijtknhbcqkorunidcoxamfpnpuawkpxqwsmkajkmgnoxbabjfmghwnqsutgesbnxuydsdluzortzvajkjpwythprgbunkaedgmjvmwvzedzgjsrdoefcevcmesfehtzrtxbocadtcmrzscvhmzuxylkwqtqrsqpfhrzubaceyksqufadiozctinexikmblwlsbcbfmxpynstbktnispbfeyqgxctumjgtjnzaguoqkdfqtcmpwdjvufncmvijdvlelzsxumbxbkvbsanuqexialydawolmfnorbetjoukclumjlhntzxdepxplwxgjrwtkbjngoqotnmdeymnroxesurwlmaymrpndrqkwpjmwjlvlmprbqjmbtnrcpkunqvuxhshugyilqbnocnimnhdidbrjgyqhurpmicnwurlrzd

We see:

Answer 1

In the first case, the value of i changes, but after we pass the current value to the Remove method, if we want to pass the new value to the Remove method, we put the decrement operator in front: --i or do it on a separate line:

 i--;
 str = str.Remove(i,2);