Thread: Merge Strings Alternatively with C

I'm working on a code that takes two strings and outputs one string with the following output:

s1 = abc
s2 = def
output s3 = adbecf

if string 1 has more than 3 characters, those characters are placed in the back. Same thing with string 2:

s1 = abcwxy
s2 = deflmn
output s3 = adbecfwxylmn

I'm pretty sure I have the answer but I keep getting an error in the compiler. What's going on? Can someone help me run it and tell me what error you get too please?

Code:

include<math.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <assert.h>
#include <limits.h>
#include <stdbool.h>


/*
 * Complete the function below.
 */
char* mergeStrings(char* a, char* b) {
    
    //char* arr;
    int alen = strlen(a);
    int blen = strlen(b);
    int len = alen + blen;
    char arr[len];
    int i = 0, j = 0, k = 0;

    while(i < alen && j < blen){

        arr[k++] = a[i++];
        arr[k++] = b[j++];

    }

    while(i < alen){

        arr[k++] = a[i++];
    }

    while(j < blen){

        arr[k++] = b[j++];
    }

    return arr;

}
int main() {
    FILE *f = fopen(getenv("OUTPUT_PATH"), "w");
    char* res;
    char* _a;
    _a = (char *)malloc(512000 * sizeof(char));
    scanf("\n%[^\n]",_a);
    
    char* _b;
    _b = (char *)malloc(512000 * sizeof(char));
    scanf("\n%[^\n]",_b);
    
    res = mergeStrings(_a, _b);
    fprintf(f, "%s\n", res);
    
    fclose(f);
    return 0;

}

It compiles successfully but I still get null as my answer

This declares a variable length array:

Code:

char arr[len];

Now, the variable length array language feature was introduced in the 1999 edition of the C standard, but later became optional, so it might not be supported by your compiler... although it looks like your compiler, or at least the one involved in the automated testing, does support it. The thing is that this variable length array is local to the function, so this:

Code:

return arr;

ends up returning a pointer to a (non-static) local variable, which is a Bad Thing. You could use dynamic memory allocation instead:

Code:

size_t alen = strlen(a);
size_t blen = strlen(b);
size_t len = alen + blen;
char *arr = malloc(len + 1);

I used size_t instead of int because that is the return type of strlen (and the type of the result of sizeof). You need the +1 to account for the terminating null character. You don't need to use sizeof(char) because that is always 1, and you don't need to cast the return value of malloc. Now, your function should work, except that in the caller (i.e., the main function), you should have a call to the free function to match the malloc in mergeStrings. You should also check that malloc does not return a null pointer.

By the way, since you do not modify the source strings in mergeStrings, it should really be:

Code:

char* mergeStrings(const char* a, const char* b) {

This way, the compiler would warn you if you do accidentally attempt to modify them, and so you can safely test with string literals, e.g., mergeStrings("abcwxy", "deflmn").

Also, I recommend that you use descriptive names, and avoid coming up with names that begin with an underscore (in some situations, although admittedly not how you used them here, these may be reserved).

It worked! Thank you so much!