Value retention principle

The ASNI C standard rule takes the value preserving principle, that is, when several integer operands are mixed, the result type may be either a conforming number or an unsigned number, depending on the relative size of the operand's type.

#include <stdio.h>

#define Size_of_array (arr) (sizeof (ARR)/sizeof (arr[0))

int main () {

int d =-1;

int a[] = {1, 2, 3, 4, 5, 6, 7};

if (D<size_of_array (a)) {

printf ("1\n");

} else {

printf (" -1\n");

}

return 0;

}

The execution results are as follows: 1

The type of the value defined by Size_of_array is the unsigned int type (the type that sizeof returns is an unsigned type), and the IF statement tests equality between signed int and unsigned int, so d is promoted to unsigned int Type. 1 is a large value when converted to unsigned int type, so the expression is false.

If you cast the return result of Size_of_array to int, the result will be as follows because of the same type:

if (d< (int) (Size_of_array (a)))

The execution results are as follows: 1

Value retention principle