C++ for loop doesnt exit [closed]

Question

I have a for loop that initializes an array with dynamic data
the code looks like this

for (char i=0; i<256; i++){
  data[i] = i; // some random dynamic data
}

I figured out that its always smaller than 256 cause the max positive number it can store is 127. So i made this an unsinged char. But it still runs forever.
I fixed this by lowering to 254

for (unsigned char i=0; i<254; i++){
  // do the things
}

and it worked.
My question is: why wont it work with 255?

edit: fixed small "typo" on the first loop plz note that those 2 loops dont run together im just showing you the different iterations of the code

Answer 1

The following loop will do what you'd expect: run 255 times

for (unsigned char i = 0; i < 255; i++) {
    //stuff...
}

If, however, you want it to run 256 times like you expected in the first case, then the type of i is the problem. You can't use an unsigned char to run a loop 256 times because you exhaust all possible values. An 8bit unsigned integer can only go from 0 to 255, so conditions like i < 256 or i <= 255 will always be true. Consider using int or size_t (since you're working with arrays).

for (size_t i = 0; i < 256; i++) {
    //stuff...
}

If you insist on using unsigned char, the only option would be a do-while where the comparison occurs at the end of each loop. The following will run 256 times.

unsigned char i = 0;
do {
    //stuff...
} while (i++ != 255); // First check that i isn't 255 and then increment i

Answer 2

It actually works but after reaching i=255 and doing the stuff, it resets to 0 as 255+=1=256 which goes to 0 as the range of unsigned char is [0, 255].

So instead of

for (unsigned char i=0;i<256;i++){
    // do stuff
}

you should try

for (unsigned char i=0;;i++){
    //do stuff
    if(i==255)break;
}