Question

I’m stuck at yet another C problem. How can I concatenate two strings with the second string being inserted before the first string?

This is what I came up with. Unfortunately I’m stuck at all these pointer to chars, char arrays et cetera.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char* argv[] )
{
  char* output;
  int i;
  for(i = 9; i > 0; i--)
  {
    unsigned int value = (unsigned int)i;
    char buffer[20];
    sprintf(buffer, "%u", value);
//    strcat(ouput, buffer);  // append before the string.
    // first loop: 9
    // second loop: 89
    // third loop: 789
  }

  printf("%s", output);
}

How must I correct my code to make it work? I guess I have to somehow set the output variable to empty. When do I need fixed widths for the char array or the pointer? 20 was just a random guess from me.

Answer 1

I’m very confused, as your posted code has absolutely nothing to do with the problem you state. (Well, they both use strings, but that’s about it)

  char* src  = "Hello, ";
  char* dest = "World!";

  char* temp;
  temp = malloc(strlen(src) +strlen(dest) + 1);
  strcpy(temp, src);
  strcat(temp, dest);

  dest = temp;

Unless dest is a fixed buffer of adequate size for the combined string. If so, then replace the last line with:

  strcpy(dest, temp);
  free(temp);

Now, if you want to specifically build the list of digits backwards, let’s try a different tack:

  char buffer[10];
  buffer[9] = '\0';  // null terminate our string.
  char* output;    
  int i;    
  for(i = 9; i > 0; i--)    
  {    
       // this is a fast way of saying, sprintf("%u", i); 
       // works only for single digits
       char d = (char)('0' + i); 

       buffer[i-1] = d;
       output = &buffer[i-1];
       printf("%s", output);
   }