Debug a String Code in C

Question

int main(void)
{
    char in[20];
    gets(in);
    char in2[20];
    gets(in2);
    char out[39];
    strcat(out,in);
    int index = strlen(in);
    while(index <39){
      out[index] = in2[index-strlen(in)];
        index = index + 1;
    
    printf(out);
    
    }
    
    return 0;
}

Answer 1

If u don't mine can u say what is the problem with the code.

Answer 2

I am trying to get it to consternate in and in2 but there seem to be a problem with this line - strcat(out,in); and may be this line - out[index] = in2[index-strlen(in)]; any assistance would be hugely appreciated.