Question

#include<stdio.h>

int main()
{
    unsigned int a=6;
    int b=-20;
    (a+b>6)?puts(">6"):puts("<=6");
    return 0;
}

The above code outputs >6. But I got a doubt. b=-20 will hold a negative value (-18)
after doing the 2’s complement as it’s a signed integer. So it should output <=6 but its
giving an output as >6.

Answer 1

From the C99 standard, section 6.3.1.8 (“Usual arithmetic conversions”):

if the operand that has unsigned integer type has rank greater or
equal to the rank of the type of the other operand, then the operand
with signed integer type is converted to the type of the operand with
unsigned integer type.

Basically, in the case of your example code, unsigned int and int have the same rank, so it’s equivalent to:

(a + (unsigned int)b > 6)

To fix it, you need to explicitly cast in the other direction, i.e.:

((int)a + b > 6)