Go4Expert - strpos error

Go4Expert (http://www.go4expert.com/)

- C (http://www.go4expert.com/forumdisplay.php?f=20)

- - strpos error (http://www.go4expert.com/showthread.php?t=17754)

CrazyGal

30May2009 22:27

strpos error

Hi there,

Could someone tell me why this aint working? I understand this could be done using strstr() function. I just gave it a try and I cant quite figure out why this piece of code
returns NULL though the search string is present in the main string.

Please let me know.

Thanks,
Tina

Code:

#include <stdio.h>



char * strpos(char *, char *);



int main()

{

    char s[] = "I love my job";

    char t[] = "job";

    

    char *pos = strpos(s, t);



    printf("%s", pos);



    return 0;

}



char * strpos(char *a, char *b)

{

    char *pa;

    char *pb;



    for(; *a; a++ )

       for(pa = a, pb = b; *pb && (*pa == *pb); pa++, pb++ )

           if(*pb == '\0')

               return a;

    return NULL;



}

xpi0t0s

30May2009 23:11

Re: strpos error

If *pb is NULL, do you think "*pb && (*pa == *pb)" will evaluate TRUE or FALSE?

BTW, please use code blocks when posting code. It preserves the formatting and makes it much more readable.

CrazyGal

31May2009 06:49

Re: strpos error

It will evaluate to False. Yea, I figured out my mistake. I changed *pb == '\0' to pb = (b + strlen(b) -1) and it worked.

I'll start using code blocks from my next post. Sorry about that!

Thanks!

SaswatPadhi

31May2009 07:47

Re: strpos error

Quote:

Originally Posted by CrazyGal (Post 48653)

It will evaluate to False. Yea, I figured out my mistake. I changed *pb == '\0' to pb = (b + strlen(b) -1) and it worked.

I don't believe it ! How can it work ?
You should change it to pb == (b + strlen(b) -1) not pb = (b + strlen(b) -1)

xpi0t0s

31May2009 16:46

Re: strpos error

In my fix I just removed pb from the test, i.e.

Code:

for(pa = a, pb = b; (*pa == *pb); pa++, pb++ )

Edit: Oops! This only works if t is at the end of s. This works; tested with s="abcjobxyz" and t="job"

Code:

                while (*pb && (*pa == *pb))

                {

                        if(pb == (b + strlen(b) -1))

                                return a;

Edit2: SaswatPadhi's right, you ALWAYS need to be suspicious of a single = in an expression of any type. = and == are NOT the same. if (a=b) is valid; it assigns b to a then evaluates TRUE or FALSE depending on the value of both, so it's useful in stuff like while (x=somefunc()) where somefunc returns zero when it's finished.

CrazyGal

31May2009 23:39

Re: strpos error

Oops! That's a typo. I meant '==' and that's precisely I've in my code :)

Thanks,
Tina

All times are GMT +5.5. The time now is 18:09.