Using ascii for manipulating words

[Sudoku]

I am attempting to scramble a word using a key word that defines the positional shift of each letter(the difference between it and ![041]). If it reaches an end of its letter choices ![041] and ~[176] then it is moved to the other end and continues moving. To make it even more random the direction of the moving switches each turn.

for(n=0;n<sCode.length();n++,t++){
    if(sMaster.length()==t){
        t=0;
    }
    if(n % 2==0){
         for(w=0;w<sMaster.at(t)-041;w++){
              if(sCode.at(n)==176){
                  sCode.at(n)='!';
              }
              sCode.at(n)=sCode.at(n)+1;	
         }
    }
    else{
        for(w=0;w<sMaster.at(t)-041;w++){
            if(sCode.at(n)==041){
                sCode.at(n)='~';
            }
            sCode.at(n)=sCode.at(n)-1;	
        }
    }
}

This is my attempt at it and it crashes. I recieve the message "Unhandled exception in Directory.exe (KERNEL32.DLL): 0xE06D7363: Microsoft C++ Exception.".
I rarely play with ascii while programming so I am not sure if this is correct at all or if its one small thing.

 

[cpjust]

If you run it in Debug mode, can you see which line it crashes on?

 

[Sudoku]

oh its first crash is at:

for(w=0;w<sMaster.at(t)-041;w++)

 

[xwb]

Try changing this

    if(sMaster.length()==t){

to this

    if(sMaster.length()>=t){

It could still crash if sMaster.length() == 0

 

[cpjust]

Put this whole block of code in a try catch block. string::at() will throw an out_of_range exception if you try to access an invalid index.