Thread: appending a string onto end of another string

I have been reviewing this stuff on strings and I am confused.

Function needs to append contents of string s2 to the end of string 1 and return a pointer to resulting string.

Would the following work using the function declaration as shown???

Code:

char * catstr(char * s1, const char * s2)
{
     char *pc;
     pc = s1;
    
     while (*pc != '\0')
         pc++;

     while (*s2 != '\0')
         {
              *pc = *s1;
              pc++;
              s1++;
          }

       *pc = '\0';
       
       return s1;
}

I am still confused with functions with pointers as function types and as function parameters (arguments).

This is for review for my final. It's been a while since working with simplified code like this. I feel I am either missing something or making this more complicated than it should be.

Also, if not using strcat or strcpy commands, is there other ways to append strings to other strings???

EDIT: double post

It all looks good except

Code:

     while (*s2 != '\0')
         {
              *pc = *s1;
              pc++;
              s1++;
          }

Should be

Code:

     while (*s2 != '\0')
         {
              *pc = *s2;
              pc++;
              s2++;
          }

And, while the details of this function may change, the basic idea isn't going to change. This is pretty much 'the' way to concatenate strings. Any other way to do it is really gonna be the same way.

There are two important features about this function that you need to note. First, off, this function changes s1, and second, it assumes that there is enough memory allocated to s1. These still make it a useful function, but to make it work otherwise, you would have to do this...

Code:

// const stops us from changing the strings
char * catstr(const char * s1, const char * s2)
{
     char *pc, *result;

     result = pc = malloc (strlen(s1) + strlen(s2) + 1);
     // I know, we aren't supposed to use string functions, but I
     //  think this one is okay.
     // We have to use two pointers so we have one to walk the 
     //  string, and one to stay at the beginning.  This becomes
     //  important when we have to return something.
    
     // This loop puts the values of s1 into pc.
     while (s1 != '\0')
      {
         *pc = *s1;
         pc++
         s1++;
      }

     // This loop puts the values of s2 into pc
     while (*s2 != '\0')
         {
              *pc = *s2;
              pc++;
              s2++;
          }

       // Terminate the string.
       *pc = '\0';
       
       // pc now points at the end of the string, so we have to use
       //  result, which still points at the beginning of the string.
       return result;
}

What's a good main() function to test this function ??

I tried the following but am getting a type mismatch and such.
Please explain what compilation error message means.

Code:

#include <stdio.h>


main()
{
  char str1;
  char str2;
  char s1, s2;

  printf("Input a string of characters");
  s1 =  getchar();
  printf("Input another string, it will be added to the 1st");
  s2 = getchar();

  catstr(s1, s2);

  printf("Here is your str1 string: ");
  putchar(str1);
  printf("\n Here is your str2 string : ");
  printf("\n");
  putchar (str2);

  return 0;
}

ERROR message:


17 /accounts/student2/srbkpk/cs225/final gcc cat.c
cat.c:27: warning: type mismatch with previous implicit declaration
cat.c:15: warning: previous implicit declaration of `catstr'
cat.c:27: warning: `catstr' was previously implicitly declared to return `int'

17 /accounts/student2/srbkpk/cs225/final gcc cat.c
cat.c:27: warning: type mismatch with previous implicit declaration
cat.c:15: warning: previous implicit declaration of `catstr'
cat.c:27: warning: `catstr' was previously implicitly declared to return `int'

The problem is that you have not declared the function 'catstr' before using it. Since this is the case, it assumes that it returns an integer. When it actually gets to the function itself, it finds that the function is set to return a character pointer. Thus, it gives you the errors.

To fix this, tell the compiler what the function does before hand. You can do this by:

1) declaring it inside or before main:

char *catstr( char *str1, const char *str2 );

2) writing the whole function before 'main' so that 'main' is the last function in your program.

Either one will do the trick.

Quzah.

After you declare your prototype, you'll also have to get strings instead of chars...

Code:

#include <stdio.h>


main()
{
  char str1;
  char str2;
  char s1 [80], s2[80], s3[80];

  printf("Input a string of characters");
  scanf ("%s", s1);
  printf("Input another string, it will be added to the 1st");
  scanf ("%s", s2);

  s3 = catstr(s1, s2);

  printf("Here is your str1 string: ");
  printf ("%s", s1);
  printf("\n Here is your str2 string : ");
  printf("%s", s2);
  printf("\n Here is your str3 string : ");
  printf("%s", s3);
  printf ("\n");

  return 0;
}

s3 is an array. You cannot assign to it in this manner. This is incorrect. If s3 were a character pointer, your usage would be correct. Perhaps you should instead assign the return value to a character pointer first, then use strncpy into s3?

Quzah.

Oh, my bad... change this:
char s1[80], s2[80], s3[80];

to this...
char s1[80], s2[80], *s3;

can't change the value of an array.

Still getting a Seg Fault
and are we sure the catstr function is okay.
What causes a Seg Fault anyways??

Code:

#include <stdio.h>

char * catstr(char * s1, const char * s2);

main()
{
  char s1[80], s2[80], *s3;   /*  changed to a pointer */

  printf("Input a string of characters");
  scanf("%s",s1);
  printf("Input another string, it will be added to the 1st");
  scanf("%s",s2);

  s3 =  catstr(s1, s2);          /* is this call okay ?? */

  printf("Here is your s1 string: %s\n",s1);
  printf("Here is your s2 string : %s\n",s2);
   printf("Here is your s3 string : %s\n",s3);
  printf("\n");

  return 0;
}

char * catstr(char * s1, const char * s2)
{
     char *pc;
     pc = s1;

     while (*pc != '\0')
         pc++;

     while (*s2 != '\0')
         {
              *pc = *s1;
              pc++;
              s1++;
          }

       *pc = '\0';

       return s1;
}

A seg fault means that you are writing to, or reading from, an illegal memory address. 90% of the time, this means you're trying to use a pointer that is pointing to NULL.

The rest of the time, it's because you're doing something like this...

Code:

while (*s2 != '\0')
         {
              *pc = *s1;
              pc++;
              s1++;
          }

Now, the value of s2 never actually changes in this loop, so *s2 is going to always be the first character of the second string. So, unless it is an empty string, *s2 != '\0' will always be true. Therefore, the pointers, s1 and pc will continue to increment all the way into the nether-regeions of your computer's memory, untill they reach the point where your line accessing that memory (*pc = *s1) ends up trying to access an illegal memory location, and viola, segmentation fault.