C - Adding the numbers in 2 strings together if a different length

Question

If I had two strings:

a = \"1234\"
b = \"4321\"

I could add the two numbers together like this:

for(i=0; i

        

Answer 1

cited from C - Adding the numbers in 2 strings together if a different length answer, I write a more readable code：

void str_reverse(char *beg, char *end){
    if(!beg || !end)return;
    char cTmp;
    while(beg < end){
        cTmp = *beg;
        *beg++ = *end;
        *end-- = cTmp;
    }
}

#define c2d(c) (c - '0')
#define d2c(d) (d + '0')
void str_add(const char* s1, const char* s2, char* s_ret){
    int s1_len = strlen(s1);
    int s2_len = strlen(s2);

    int max_len = s1_len;
    int min_len = s2_len;
    const char *ps_max = s1;
    const char *ps_min = s2;

    if(s2_len > s1_len){
        ps_min = s1;min_len = s1_len;
        ps_max = s2;max_len = s2_len;
    }

    int carry = 0;
    int i, j = 0;
    for (i = max_len - 1; i >= 0; --i) {
        // this wrong-prone
        int idx = (i - max_len + min_len) >=0 ? (i - max_len + min_len) : -1;
        int sum = c2d(ps_max[i]) + (idx >=0  ? c2d(ps_min[idx]) : 0) + carry;

        carry = sum / 10;
        sum = sum % 10;

        s_ret[j++] = d2c(sum);
    }

    if(carry)s_ret[j] = '1';
    str_reverse(s_ret, s_ret + strlen(s_ret) - 1);
}

test code as below:

void test_str_str_add(){
    char s1[] = "123";
    char s2[] = "456";
    char s3[10] = {'\0'};

    str_add(s1, s2, s3);
    std::cout<

output:

579

456912

100020