Just like there exists a function for strings that returns its length (strlen()), the string library contains other useful string functions. In this exercise, we will explore how to concatenate two strings together using the strcat() function.

The strcat() function takes one string, src, and appends it to the end of another string, dst. Using this function does not create a separate third string, but modifies the dst string to include the src string at the end. The syntax for this is:

strcat(dst, src);

At first glance, it may appear that some magic has taken place here because it is known that strings (because they are arrays) are immutable! What actually happens is the function takes the two strings and creates a char array of size strlen(src) + strlen(dst) + 1. It then populates the array with the characters of both strings starting with dst. Finally, it discards the memory occupied by the original dst string and associates the dst string variable with the new char array created. This is all done for you “under the hood” using the concept of pointers, a topic that will be discussed in a future lesson.

Here it is in action:

#include<stdio.h>
#include<string.h> // Don't forget to include this!

int main() {
  char s1[] = "abcd";
  char s2[] = "efgh";
  strcat(s1, s2);
  printf("%s", s1); 
  // Output will be "abcdefgh"
}