String Comparision

[user34]

(Sorry if this gets posted twice)
Hi all. I am trying to learn C.As a simple exercise i tried to write a
code which would compare two strings using pointers.
However i am not getting the correct result.
On tracing the program i noticed that the strings "s" and "t" get
modified somehow in the comp function and as a result i am getting
incorrect results.
Can anyone please point out the error?
#include<stdio. h>

int comp(char *s,char *t)
{
for(;*s==*t;s++ ,t++);
if(*s=='\0')
return 0;
else
return (*s -*t);
}

int main(void)
{
char str[]="Hello";
char str2[]="Hello";

if(comp(str,str 2)!=0)
printf("strings are different");
else
printf("strings are same");
return 0;
}

[osmium]

"user34" writes:

Hi all. I am trying to learn C.As a simple exercise i tried to write a
code which would compare two strings using pointers.
However i am not getting the correct result.
On tracing the program i noticed that the strings "s" and "t" get modified
somehow in the comp function and as a result i am getting incorrect
results.
Can anyone please point out the error?
#include<stdio. h>

int comp(char *s,char *t)
{
for(;*s==*t;s++ ,t++);
if(*s=='\0')
return 0;
else
return (*s -*t);

You return after examining the first char, no matter how it turns out.
Rethink your logic.

}

int main(void)
{
char str[]="Hello";
char str2[]="Hello";

if(comp(str,str 2)!=0)
printf("strings are different");
else
printf("strings are same");
return 0;
}

[user34]

osmium wrote:

"user34" writes:

>Hi all. I am trying to learn C.As a simple exercise i tried to write a
code which would compare two strings using pointers.
However i am not getting the correct result.
On tracing the program i noticed that the strings "s" and "t" get modified
somehow in the comp function and as a result i am getting incorrect
results.
Can anyone please point out the error?
#include<stdio .h>

int comp(char *s,char *t)
{
for(;*s==*t;s++ ,t++);
if(*s=='\0')
return 0;
else
return (*s -*t);

You return after examining the first char, no matter how it turns out.
Rethink your logic.

Can you elaborate? The for loop should parse the string until the
characters in the two strings differ ,is'nt it?

[Flash Gordon]

user34 wrote, On 29/04/07 17:53:

(Sorry if this gets posted twice)
Hi all. I am trying to learn C.As a simple exercise i tried to write a
code which would compare two strings using pointers.
However i am not getting the correct result.
On tracing the program i noticed that the strings "s" and "t" get
modified somehow in the comp function and as a result i am getting
incorrect results.
Can anyone please point out the error?

First off, did you copy and paste or retype? I suspect you retyped and
in the process typed correctly what is wrong in the copy you ran.

#include<stdio. h>

They stopped charging for white space yesterday, so your could make the
above easier to read by doing
#include <stdio.h>

int comp(char *s,char *t)

Since this function is not meant to modify the strings it would be
better to make it explicit
int comp(const char *s,const char *t)

This will also mean the compiler will bitch at you if you try to modify
them.

{
for(;*s==*t;s++ ,t++);

It is easy to fail to spot the semicolon above, for the human reader
something like
for (; *s==*t; s++,t++) continue;
Or
for (; *s==*t; s++,t++) /* do nothing */ ;
Or
for (; *s==*t; s++,t++) {}

Also, what if both strings are entirely the same? In that case you will
not stop at the end of the strings but will continue going in to the
wild blue yonder.
for (; *s && *s==*t; s++,t++) continue;

if(*s=='\0')
return 0;
else
return (*s -*t);

You do not need the parenthesis
return *s - *t;

Also, on some embedded systems where a byte (in C terms) is 16 bits or
larger and the same size as an int this could fail.
return (*s *t) ? 1 : -1;

}

int main(void)
{
char str[]="Hello";
char str2[]="Hello";

if(comp(str,str 2)!=0)
printf("strings are different");
else
printf("strings are same");
return 0;
}

You should add a newline on to the end of the strings. One easy way as
you are not using any format specifiers is to call puts instead of printf.
--
Flash Gordon

[Flash Gordon]

user34 wrote, On 29/04/07 18:15:

osmium wrote:

>"user34" writes:

>>Hi all. I am trying to learn C.As a simple exercise i tried to write
a code which would compare two strings using pointers.
However i am not getting the correct result.
On tracing the program i noticed that the strings "s" and "t" get
modified somehow in the comp function and as a result i am getting
incorrect results.
Can anyone please point out the error?
#include<stdi o.h>

int comp(char *s,char *t)
{
for(;*s==*t;s++ ,t++);
if(*s=='\0')
return 0;
else
return (*s -*t);

You return after examining the first char, no matter how it turns out.
Rethink your logic.

Can you elaborate? The for loop should parse the string until the
characters in the two strings differ ,is'nt it?

Also it won't stop at the end of the strings if they are the same.
--
Flash Gordon

[jg]

On Apr 29, 9:53 am, user34 <use...@invalid .comwrote:

for(;*s==*t;s++ ,t++);

Need to check both s and t are not pointing to somewhere
out of the string.
JG

[user34]

Flash Gordon wrote:

user34 wrote, On 29/04/07 17:53:

>(Sorry if this gets posted twice)
Hi all. I am trying to learn C.As a simple exercise i tried to write a
code which would compare two strings using pointers.
However i am not getting the correct result.
On tracing the program i noticed that the strings "s" and "t" get
modified somehow in the comp function and as a result i am getting
incorrect results.
Can anyone please point out the error?

First off, did you copy and paste or retype? I suspect you retyped and
in the process typed correctly what is wrong in the copy you ran.

I did not retype the code !
Here it is once again (without any of the above suggested
improvements)an d it still shows me the same error!

#include<stdio. h>

int comp(char *s,char *t)
{
for(;*s==*t;s++ ,t++);
if(*s=='\0')
return 0;
else
return (*s -*t);
}

int main(void)
{
char str[]="Hello";
char str2[]="Hello";

if(comp(str,str 2)!=0)
printf("strings are different");
else
printf("strings are same");
return 0;
}

[Flash Gordon]

user34 wrote, On 29/04/07 18:29:

Flash Gordon wrote:

>user34 wrote, On 29/04/07 17:53:

>>(Sorry if this gets posted twice)
Hi all. I am trying to learn C.As a simple exercise i tried to write
a code which would compare two strings using pointers.
However i am not getting the correct result.
On tracing the program i noticed that the strings "s" and "t" get
modified somehow in the comp function and as a result i am getting
incorrect results.
Can anyone please point out the error?

First off, did you copy and paste or retype? I suspect you retyped and
in the process typed correctly what is wrong in the copy you ran.

I did not retype the code !
Here it is once again (without any of the above suggested
improvements)an d it still shows me the same error!

<snip>

Then your description of the problem is inaccurate, the code does not
modify the strings. I suggest you examine the improvements and comments
by myself and the others and see if it then works.
--
Flash Gordon

[osmium]

"user34" writes:

Hi all. I am trying to learn C.As a simple exercise i tried to write a
code which would compare two strings using pointers.
However i am not getting the correct result.
On tracing the program i noticed that the strings "s" and "t" get modified
somehow in the comp function and as a result i am getting incorrect
results.
Can anyone please point out the error?
#include<stdio. h>

int comp(char *s,char *t)
{
for(;*s==*t;s++ ,t++);
if(*s=='\0')
return 0;
else
return (*s -*t);
}

int main(void)
{
char str[]="Hello";
char str2[]="Hello";

if(comp(str,str 2)!=0)
printf("strings are different");
else
printf("strings are same");
return 0;
}

The first problem I note is that the terminating condition on the for loop
should be an '\0', signifying end of string. This suggests the middle
expression should be something like

*s!='\0' && *t'='\0'

But this doesn't work because we don't know if both strings are the same
length. So now we precede your code with something to find the length of
the strings. But that uses stuff in <string.h>, the very thing you are
trying to avoid! But then note that you are barely using a for loop, it is
a force fit. I suggest you rewrite using a while loop. I don't like
suggesting new and different ways to attack a problem, but even more, I
don't like the code I foresee if the for loop approach is continued.

I consider the primary good attribute of a for loop the index variable. But
you don't *use* an index variable.

[osmium]

"user34" wrote:

osmium wrote:

>You return after examining the first char, no matter how it turns out.
Rethink your logic.

Can you elaborate? The for loop should parse the string until the
characters in the two strings differ ,is'nt it?

If you really prefer a for loop based solution, I would do it something like
this. As far as I know it works.
--------------------------
#include <stdio.h>

// return 1 iff they are equal (inverted sense from what you had)
int comp(char a[], char b[])
{
for(int i=0; ; i++)
{
if(a[i] != b[i])
return 0;
/* assert: a[i] == b[i], we just tested it
but it it the end of the string? */
if(a[i] == '\0')
return 1;
}
}
/*=============*/
int main()
{
char s[] = "test";
char t[] = "test";
if(comp(s, t))
printf("Same\n" );
else
printf("Differe nt");
return 0;
}