Thread: I need to improve a code to concatenate 2 strings using only pointers

You should not be using gets. fgets is a viable alternative, though you should keep in mind that it saves the newline read if there is space.

As for the concatenation: you're accessing the strings as arrays using array index notation, even though you're doing this via pointers. That's fine, but it makes the pointers completely pointless in this context: you might as well use the arrays themselves. Of course, if you wrote a function instead, e.g.,

Code:

void concatenate_strings(char *p_str1, char *p_str2, char *p_str3)
{
    /* ... */
}

Then this would happen by default, but really, it isn't really much different in syntax from using the arrays directly. I suspect that if you want to meet the spirit of the restriction, you should use pointers to traverse the arrays, i.e., you set a pointer to point to the start of an array, then you increment the pointer instead of incrementing an index, and then instead of using array index notation, you dereference the pointer.

EDIT:
Just as gets is problematic because it does not allow you to call it such that buffer overflow can be avoided, your code also needs to be fixed to account for the possibility of buffer overflow, especially since str3 is the same size as the source arrays.

You should either check for too large input to fit in str3[1024] or increase the size so you do NOT need to check.
Best to use fgets instead of gets.

Tim S.

If your teacher doesn't quickly move onto explaining why gets() is bad, and why you should be using fgets() all the time, then you maybe in the wrong class.

Originally Posted by JorgeChemE

Unfortunately, in our class, we didn't learn fgets so I don't know how to use it.

There are plenty of resources, e.g., online references and tutorials, that you can read to learn how to use fgets.

Originally Posted by JorgeChemE

This excercise is supposed to be easy to use gets, even scanf

The issue is that gets is inherently vulnerable to buffer overflow, so you should not develop the habit of using it. scanf can be used to read strings in a way that avoids buffer overflow.

Personally, I wouldn't even do any input until I have tested with hardcoded strings, e.g.,

Code:

char str1[] = "hello";
char str2[] = "world!";
char str3[1024];
/* ... concatenation code here */
printf("%s\n", str3); /* prints "hello world!" */

This makes it easier to quickly repeat the initial testing without pausing to enter input. Once your code appears to work for your fixed test input, you could vary it a little to be certain, and finally change the code to do input, and then test with more varied input.

Originally Posted by JorgeChemE

concatenate two single words with a space between them. Still, I am unable to generate a space between two words.

You did not write the code to add a space between the strings as you concatenate them, so of course you are unable to generate a space between them when running the program.

Originally Posted by JorgeChemE

Almost I forget to mention, for this excercise we are not allowed to use functions.

That has no bearing on my example: I was giving you an example of defining a function with pointer parameters corresponding to arrays, not an example of using a function. Furthermore, this restriction is poorly stated, e.g., you used printf and gets, which are functions.

Originally Posted by JorgeChemE

I am not able to use pointers instead of arrays in nested loop for for. The program does not work so I leave it with strings.

You're using array index notation without any use of sizeof or address-of on the arrays beyond initialising the pointers, so I don't see a problem. Just replace every instance of the array name with the corresponding pointer to the first element of the array, and it will work, because in the first place, in these contexts, the array is already converted to a pointer to its first element. Replacing this implicit pointer with an explicit pointer changes nothing (other than being extra work on your part).

Originally Posted by JorgeChemE

We should solve it with pointers but... I failed. I used temp as a temp variable to sort characters. Do you know how can I put pointers instead of arrays in this nested loop?

Consider this example of printing a string character by character, using pointers:

Code:

#include <stdio.h>

int main(void)
{
    char str[] = "hello world!";
    char *end = str + sizeof(str) - 1;
    char *ptr;
    for (ptr = str; ptr != end; ptr++)
    {
        putchar(*ptr);
    }
    putchar('\n');
    return 0;
}