编辑代码

#include <stdio.h>
int atoi(char *str);
//字符串转换成整数
int atoi(char *str)
{
    int i = 0;
    int num = 0;
    int flag = 1;//正负标志
    if(str[i] == '-')//有负号
    {
        flag = -1;
        i++;
    }
    while(str[i] != '\0')
    {
        num = num * 10 + str[i] - '0';
        i++;
    }
    return num * flag;
}
int main () {
    char a[]="512";
    int b=atoi(a);
    int c =b-12;
    printf("%d\n",c);
    return 0;
}