strcmp 给出非零 output 即使字符串匹配

Question

In the program below, I tried to compare the string in the i-th place in the string using strcmp . The test case I used is 1+2+2+1+3 . However, except the first iteration of the loop, strcmp shows non-zero output even when the strings match. Why is this happening?

#include <stdio.h>
#include <string.h>

int main() {
    char num[101];
    int n1 = 0, n2 = 0, n3 = 0;

    scanf(" %s", num);
    int len = strlen(num);

    for (int i = 0; i < len; i = i + 2) {
        char dig = num[i];
        printf("\ndig: %c", dig);
        int c1 = strcmp(&dig, "1");
        printf("\nc1: %d", c1);
        if (c1 == 0) {
            n1++;
            continue;
        }
        int c2 = strcmp(&dig, "2");
        printf("\nc2: %d", c2);
        if (c2 == 0) {
            n2++;
            continue;
        }
        int c3 = strcmp(&dig, "3");
        printf("\nc3: %d\n\n", c3);
        if (c3 == 0) {
            n3++;
            continue;
        }
    }

    printf("\nn1: %d n2: %d n3: %d", n1, n2, n3);

    for (int j = 0; j < len; j = j + 2) {
        if (n1 > 0) {
            num[j] = '1';
            n1--;
            continue;
        }
        if (n2 > 0) {
            num[j] = '2';
            n2--;
            continue;
        }
        if (n3 > 0) {
            num[j] = '3';
            n3--;
            continue;
        }
    }

    printf("\n%s", num);

    return 0;
}

Answer 1

The strcmp expects both arguments to be null-terminated strings .

The expression &dig is a pointer to a single character, it's not a null-terminated string. That means strcmp will go out of bounds to find the terminator, and you will have undefined behavior .

If you want to compare two characters, you can do it with the == operator:

if (dig == '3')
{
    ...
}

You can also use a switch statement:

switch (dig)
{
case '1':
    // Do something...
    break;
case '2':
    // Do something...
    break;
case '3':
    // Do something...
    break;

default:
    // No match...
    break;
}