printf返回有什么区别？

Question

Compute the sum of the two given integer values. If the two values are the same, then return triple their sum.

int main() {
    printf("%d\n", test(1, 2));
    printf("%d", test(2, 2));
}

int test(int x, int y) {
    int sum;

    if (x == y) {
        sum = (x + y) * 3;
        printf("sum = %d\n", sum);
    } else  {
        sum = (x + y);
        printf("sum = %d", sum);
    }
}

When I run this code, I get the wrong output, but when I use return statements instead of the printf , I get the right input. Why?

Answer 1

Since test does not return a value (there is no return statement), the attempt to reference the return value results in undefined behavior. It might be easier to understand what is happening in your program if you make a few minor modifications:

#include <stdio.h>
int
test(int x, int y)
{
        int sum = x + y;
        if( x==y ){
                sum *= 3;
        }
        printf("sum = %d XXX ", sum);
        return sum;
}

int
main(void)
{
        printf("** %d **\n", test(1, 2));
        printf("** %d **\n", test(2, 2));
        return 0;
}

The output of the above program is:

sum = 3 XXX ** 3 **
sum = 12 XXX ** 12 **

The test in the argument list to printf in main is called before the printf in main can be called. The printf inside the test function executes and prints sum =... XXX . The function returns a value which is passed as an argument to the printf in main which prints **... ** .

Answer 2

Your program is just a little bit wrong. Keeping with the general structure of your code, here are two ways that you can rewrite it.

#1: Instead of returning a value, the test function prints a value. Since test does not return a value, you must put the keyword void before the function name. To get the values to print, the main function must call the test function.

int main() {

    /** calls the test function,
        then the test function prints
        the sum value **/
    test(1,2); 
    test(2,2);

    return 0;
}

void test(int x, int y) {

    int sum;

    if (x == y) {
        sum = (x + y) * 3;
        printf("sum = %d\n", sum); /** prints the sum value**/
    } else  {
        sum = (x + y);
        printf("sum = %d", sum); /** prints the sum value**/
    }
}

#2 The test function returns the sum value. Since sum is an int , you must put the keyword int before the function name. The main function gets the value of sum by calling the test function. Then main prints this value.

int main() {

    /** prints the value returned 
        by the test function **/
    printf("%d\n", test(1,2)); 
    printf("%d", test(2,2));

    return 0;
}

int test(int x, int y) {

    int sum;

    if (x == y) {
        sum = (x + y) * 3;
    } else  {
        sum = (x + y);    
    }

    /** returns the value of sum **/
    return sum;
}