C++ return negative string

Question

Hi i have the following code to convert string integer into integer, these are the code:

#include <stdio.h>
int str_to_int(char* str) {
   int output = 0;
   char* p = str;
   for (int i=0;str[i]!='\0';i++) {
      char c = *p++;
      if (c < '0' || c > '9')
        continue;
      output *= 10;
      output += c - '0';
   }   
   return output;
}

int main(){

    printf("%d\n", str_to_int("456xy"));
    printf("%d\n", str_to_int("32"));
    printf("%d\n", str_to_int("-5"));
    return 0;
}

But some how printf("%d\\n", str_to_int("-5")); some how returning 5 instead -5, where am i wrong here? Thanks

Answer 1

In your conversion function, you skip over the - character, because it is not in your tested range:

      if (c < '0' || c > '9')
        continue;

To fix it, you need to test for the - character, and negate the value after you are done converting the value.

      if (c < '0' || c > '9') {
        if (c == '-' && no_other_digits_processed_yet) is_neg = true;
        continue;
      }
      //...

   if (is_neg) output = -output;
   return output;

Answer 2

it's the if (c < '0' || c > '9') continue; that causes this problem.

When variable c is '-', you just simply skip it. So there is no negative sign in your result.

Please just test if this number is negative (has '-') before your whole logic.

Answer 3

You don't really seem to account for "-" at all in your code.

You should insert some kind of hook to detect if it's a negative value:

#include <stdio.h>
int str_to_int(char* str) {
   int output = 0;
   char* p = str;
   bool isNeg = false;
   if (*p == '-')
   {
       isNeg = true;
       p++;
   }
   for (int i=0;str[i]!='\0';i++) {
      char c = *p++;
      if (c < '0' || c > '9')
        continue;
      output *= 10;
      output += c - '0';
   }   
   if (isNeg)
   {
       output *= -1;
   }
   return output;
}

int main(){

    printf("%d\n", str_to_int("456xy"));
    printf("%d\n", str_to_int("32"));
    printf("%d\n", str_to_int("-5"));
    return 0;
}