K&R2, exercise 5.3

[arnuld]

PURPOSE: write a pointer version of strcat function.

WHAT I DID: I have 2 implementations of this function. Both compile
and run but 1st one gives some strange results, 2nd is ok. I am unable
to figure out why the 1st implementation gives strange results:


/* A rudimentary strcat function
*
*/


#include <stdio.h>
#include <stdlib.h>


enum MAXSIZE { ARR_SIZE = 1000 };


void my_strcat( char s[], char t[] );


int main()
{
char s[1000] = "Saurabh";
char t[1000] = "Nirkhey\n";

printf("\ns --> %s\nt --> %s\n\n", s, t );
printf("--- concatenating strings -----\n\n");
my_strcat( s, t );
printf("\ns --> %s\nt --> %s\n\n", s, t );



return EXIT_SUCCESS;
}


void my_strcat( char s[], char t[] )
{
char *ps, *pt;

ps = s;
pt = t;

while( *ps++ != '\0' )
{
printf("*ps = %c\n", *ps );
}

while( *pt != '\0' )
{
*ps++ = *pt++;
}

}

=========== OUTPUT ===============

/home/arnuld/programs/C $ gcc -ansi -pedantic -Wall -Wextra 5-3.c
/home/arnuld/programs/C $ ./a.out

s --> Saurabh
t --> Nirkhey


--- concatenating strings -----

*ps = a
*ps = u
*ps = r
*ps = a
*ps = b
*ps = h
*ps = ^@



==> 2nd version's strcat function:

void my_strcat( char s[], char t[] )
{
char *ps, *pt;

ps = s;
pt = t;

while( *ps != '\0' )
{
printf("*ps = %c\n", *ps );
ps++;
}

while( *pt != '\0' )
{
*ps++ = *pt++;
}

}

============ OUPUT ============
/home/arnuld/programs/C $ gcc -ansi -pedantic -Wall -Wextra 5-3.c
/home/arnuld/programs/C $ ./a.out

s --> Saurabh
t --> Nirkhey


--- concatenating strings -----

*ps = S
*ps = a
*ps = u
*ps = r
*ps = a
*ps = b
*ps = h

s --> SaurabhNirkhey

t --> Nirkhey


/home/arnuld/programs/C $






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Please remove capital 'V's when you reply to me via e-mail.

 

[Irrwahn Grausewitz]

PURPOSE: write a pointer version of strcat function.

WHAT I DID: I have 2 implementations of this function. Both compile
and run but 1st one gives some strange results, 2nd is ok. I am unable
to figure out why the 1st implementation gives strange results:
[SNIP]

void my_strcat( char s[], char t[] )
{
char *ps, *pt;

ps = s;
pt = t;

Here the pointer ps gets incremented, regardless of the result of
the comparison. That way, in the loop you print always the character
*following* the one ps pointed to when the loop condition was last
checked; the output of your debug print statements should have been
a hint...

while( *ps++ != '\0' )
{
printf("*ps = %c\n", *ps );
}

At this point, ps points one character *beyond* the terminating null
character in s. Effectively, you are now going to copy the contents
of t to "dead"[1] space in the array s:

while( *pt != '\0' )
{
*ps++ = *pt++;
}
[SNIP]

==> 2nd version's strcat function:

[SNIP]

This time you got it right: increment ps only if the loop condition is
true (and debug-print the character ps points to *before* incrementing
the pointer).

while( *ps != '\0' )
{
printf("*ps = %c\n", *ps );
ps++;
}

[SNIP]

Alternatively, you could have used a for loop, of course:

for ( ps = s; *ps != '\0'; ++ps )
/* skip, or whatever */;



[1] In the sense of: "will be ignored, when s is treated as a string".


Best regards

 

[Philip Potter]

arnuld wrote:
[problem with strcat implementation]
[first version, buggy:]

void my_strcat( char s[], char t[] )
{
char *ps, *pt;

ps = s;
pt = t;

while( *ps++ != '\0' )
{
printf("*ps = %c\n", *ps );
}

while( *pt != '\0' )
{
*ps++ = *pt++;
}

}

==> 2nd version's strcat function: [working]

void my_strcat( char s[], char t[] )
{
char *ps, *pt;

ps = s;
pt = t;

while( *ps != '\0' )
{
printf("*ps = %c\n", *ps );
ps++;
}

while( *pt != '\0' )
{
*ps++ = *pt++;
}

}

You have correctly identified the location of the problem. Specifically,
the problem is the following while loop in the first version:

while (*ps++ != '\0')

This says "Check whether *ps equals '\0', and afterwards, increment ps".
Note that ps is incremented whether or not the equality test succeeds.

Look at the printf() output. By the time printf() prints *ps, ps has
already been incremented. This is why you never output the first
character of the string.

Similarly, when you detect a '\0' character, you then increment ps past
that character. As a result, the '\0' is never overwritten with a
character from pt; the contents of the char array after the call are as
follows:

"Saurabh\0Nirkhey\n"

The '\0' part-way through a char array means "end of string", so the
printf() call in main() won't print anything beyond that point.

I hope this helps you understand why your first version doesn't work but
your second version does.