circular buffer resets memory?

Question

When this cycles back around dd[0] is set to 0 as apposed to 256.1 ?? It seems like 'dd' memory is resetting with 'aa' back to 0.

unsigned char aa = 0;
double *dd = new double[256];

//circular buffer
dd[aa] = 0.1;
for(int i = 0; i < 600; i++){
    qstr += QString::number(aa,'d',0) + "  " + QString::number(dd[aa],'f',1) + "         ";
    aa++;//onces 'aa' reaches 255, the next increment resets back to 0 for 'aa'
    dd[aa] = dd[aa - 1] + 1;
}

Answer 1

You have aa declared as unsigned char . So when you hit 255 and increment, it goes back to 0. You should probably be using an int since this variable is used as an array index variable.

Answer 2

It is because when aa increments by one from 255 it goes to 0 because of unsigned char , So it becomes dd[0] = dd[-1] + 1 now any junk can be present at dd[-1] and here it seems you have -1 .

Also, you're accessing the array out of its bounds it is an undefined behavior. You should try avoiding when aa becomes 0 .

Answer 3

Unsigned char is 8 bit long. The maximum number that can be held by unsigned char is 255. (in binary 1111111). If you increment it by 1, it will become 0

Answer 4

I did casting to avoid the aa = -1:

    dd[aa] = 0.1;
    for(int i = 0; i < 600; i++){
        qstr += QString::number((unsigned char)((unsigned char)aa-(unsigned char)1),'d',0) + "  " + QString::number(dd[aa],'f',1) + "         ";
        aa++;
        dd[aa] = dd[(unsigned char)((unsigned char)aa-(unsigned char)1)] + 1;  // casting
    }