Why does the output of this program change at 130?

Question

I have the following code and it seems to me that it should always enter the true part of the if statement but, beyond 120, it appears to start executing the else clause. Why is that happening?

char x;
for (i=0;i<256;i+=10) {
    x=i;
    if (x==i)
        printf("%d true\n",i);
    else 
        printf("%d false\n",i);
}

The output is:

0 true
10 true
: all numbers from 20 to 100 are true
110 true
120 true
130 false
140 false
: all numbers from 150 to 230 are false
240 false
250 false

Answer 1

Your compiler probably uses an signed char by default when you use the char keyword. The range of a signed char is usually -128 to 127, whereas the range of an int is far greater. More esoteric hardware beyond the typical desktop PC may use other values.

Answer 2

It's because you are using a signed char and presumably an integer.

The char is overflowing when it reaches 130 (it becomes 130 - 256) but the integer does not overflow. 130 - 256 != 130.

A simple change fixes it so that the result is always true - just add unsigned on the first line:

unsigned char x;
for (int i=0;i<256;i+=10)
{
    x=i;
    if (x==i)
        printf("%d true\n",i);
    else 
        printf("%d false\n",i);
}