What is the difference in the way these two examples use the return value of scanf()?

Question

Why does the code use

input_is_good = (scanf("%ld",&num) == 1);

instead of

input_is_good = scanf("%ld",&num);

Both of them are success, are there any difference between them?

#include <stdio.h>
#include <stdbool.h>

int main(int argc, const char * argv[]) {
    long  num;
    long sum =0L;
    bool input_is_good;
    
    printf("please enter an integer to be summed");
    printf("(q to quit):");
    input_is_good = (scanf("%ld",&num) == 1);
    while (input_is_good) {
        sum+= num;
        printf("please enter next integer (q to quit):");
        input_is_good = (scanf("%ld",&num) == 1);
    }
    printf("sum to %ld\n",sum);
    
    
    return 0;
}

What is the distinction between the former and the latter?

Answer 1

scanf returns the number of successful conversions that occur, not a boolean value indicating success.

%ld is one conversion specifier, so on success scanf returns 1.

If you were performing two conversions, as in scanf("%d %d", &a, &b), you would expect the function to return 2 if both conversions were successful.

On failure, scanf returns EOF or a short conversion count (e.g., scanf("%d %d", &a, &b) == 1 indicates a was converted but not b).

Simply writing

input_is_good = scanf("%ld",&num);

would cause

while (input_is_good) {

to proceed even when scanf fails and returns EOF, because EOF is a negative value (usually -1) which means it is truthy (not zero) and will be converted to the bool value of true.

On the other hand, the comparison of the return value against 1 creates a boolean result ((scanf("%ld",&num) == 1) will always be 0 or 1).

Answer 2

• If you use input_is_good = scanf("%ld",&num);, input_is_good would be 1, 0 or EOF depending on the input. and they would break the while loop even if the user entered a valid long integer. For example, if the user entered 10, scanf would return 1 and input_is_good would be true. But if the user entered 20, scanf would return 0 because it already read the previous input and there is nothing left to read. This would make input_is_good false and break the while loop, even though the user entered a valid long integer.
• If you use input_is_good = (scanf("%ld",&num) == 1);, If the user entered something else, such as a letter or a symbol, or reached the end of input, scanf will return 0 or EOF, which will make input_is_good false and break the while loop. If the user entered a valid long integer, scanf will return 1, which will make input_is_good true and continue the while loop. The code then adds the entered number to the sum variable and prompts the user for the next input.