#include <stdio.h>

void concat(char result[], char s1[], char s2[]) {
  int i=0;
  
  while (s1[i] != '\0')
    {
      result[i] = s1[i];
      i++;
    }

  int j=0;
  while (s2[j] != '\0')
    {
      result[i++] = s2[j];
      j++;
    }

  result[i] = '\0';
}

int main(void) {
  char string1[] = "This is ";
  char string2[] = "a test.";
  char string3[20];

  concat(string3, string1, string2);

  printf("%s\n", string3);

  return 0;
}