为什么我在输出后得到一些随机值？

Question

#include <stdio.h>
int main()
{
    int k;
    scanf("%d",&k);
    for(int i=0;i<k;i++)
    {
        char num[10];
        scanf("%s",&num);
        int x = num[0]-'0';
        int sum;
        int y;
        for(int i=0;i<10;i++)
        {
            if(num[i]=='\0')
            {
                y = num[i-1]-'0';
                sum = x+y;
                printf("%d\n",sum);
            }
        }   
    }
    return 0;
}

This program is to obtain the sum of first and last digit of a given number. I am getting some random values after the output. Can anyone please explain me this and how do I eliminate it? enter image description here

Answer 1

You have an inner loop that looks for the terminating null byte in the array. You find it, but then you keep looping through the array looking for more null bytes. If you happen to find one, it takes whatever the prior byte is and treats it as the last digit.

You can fix this by breaking out of the loop when you find the null terminator:

    for(int i=0;i<10;i++)
    {
        if(num[i]=='\0')
        {
            y = num[i-1]-'0';
            sum = x+y;
            printf("%d\n",sum);
            break;
        }
    }   

Or by simply using strlen to find the end of the string.

    y = num[strlen(num)-1]-'0';
    sum = x+y;
    printf("%d\n",sum);

The code also doesn't handle reading too many characters, which can overflow the buffer, or reading an empty string, in which case num[strlen(num)-1] is off the start of the array. You can address the former by putting a field width in the scanf format, and the latter with an explicit check:

    char num[10];
    scanf("%9s",num);
    int sum;
    if (num[0] == 0) {
        sum = 0;
    } else {
        int x = num[0]-'0';
        int y = num[strlen(num)-1]-'0';
        sum = x + y;
    }
    printf("%d\n",sum);

Answer 2

The problem is that you don't stop the inner loop once you print the sum. You continue and use the indeterminate elements in the array num beyond the string terminator. That leads to undefined behavior .

You could solve it either by not having the inner loop (by using index 0 and strlen(num) - 1 ), or by using break to break out of the loop once you printed the sum.