Comparing string through pointers

Question

I am new to the C language. Please, could someone tell me why I am always getting zero as output when comparing differing strings using my own implementation of strcmp?

I wrote the function xstrcmp to compare two strings: if they are equal, then it returns 0; otherwise, it returns the numeric difference between the ASCII values of the first non-matching pair of characters.

    #include<stdio.h>

    int xstrcmp(char*,char*);

    int main()
    {   
        int i;
        char string1[]="jerry";
        char string2[]="ferry";
        i=xstrcmp(string1,string2);
        printf("difference=%d\n",i);
        return 0;
    }

    int xstrcmp(char*p,char*q)
    {
        int m;
        while(*p!=*q)
        {
             if((*p=='\0')&&(*q=='\0'))
                break;
          p++;
          q++;
        }
        m=(*p)-(*q);
        return m;
    }

You have asked several questions but haven't accepted any answers, mate. — Jim Balter
– Jim Balter, Commented Jul 9, 2012 at 21:19

Jim Balter · Accepted Answer · 2012-07-09 08:49:57Z

3

You loop until you find equal chars, then you subtract them -- so of course the result is always 0.

Also, the condition inside the loop will always fail ... if the chars aren't equal, they can't both be NUL.

That should be enough for you to fix your code.

answered Jul 9, 2012 at 8:49

Jim Balter

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Comments

Dr. S · Accepted Answer · 2012-07-09 09:31:58Z

0

The reason why you get zero always is that your while loop while(*p!=*q) means that the loop will execute as long as the characters are NOT same.

The exit from loop will happen when *p and *q have the same value.

Hence the return value, which is m=(*p)-(*q); will always be zero.

while (*p == *q) /* as long as they have same value, loop; otherwise exit */
{
      p++; /* increment the pointers */
      q++;
}
return (*p)-(*q);

would be the way to go.

answered Jul 9, 2012 at 9:31

Dr. S

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1 Comment

Jim Balter Over a year ago

This has undefined behavior for equal strings.

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