Now that we know how to call functions, use their arguments and capture their return values, let’s take a look at some functions provided by the C programming language.

C gives us access to many useful libraries that include many different functions for you to use. In order to have access to these functions you need to include header files like <stdio.h> and <stdlib.h>.

You have already used printf() to output text to the console and learned about abs() to find the absolute value of an integer. Here are a few more functions we might use in your programs:

• ceil(): takes any number as an argument, rounds it up to the nearest integer, and returns the integer.
• log(): takes an integer as an argument and returns the natural logarithm of the given number.
• toupper(): takes a char as an argument and returns the uppercase of any alphabetic character.
• isupper(): takes a char as an argument and returns 1 if the character is an uppercase letter, 0 otherwise.

Let’s use these functions in some code:

#include <stdio.h>
#include <math.h>
#include <ctype.h>

int main() {
  float number = 4.5;
  char letter = 'a';
  printf("%f\n", ceil(number)); // 5.000000
  printf("%f\n", log(number)); // 1.504077
  printf("%d\n", isupper(letter)); // 0
  letter = toupper(letter);
  printf("%d\n", isupper(letter)); // 1
}

In the above example:

• The math.h and ctype.h libraries are used in this code.
• The number variable is passed to ceil() and output. The result is 5.000000.
• The number variable is passed to log() and output. The result is 1.504077.
• The letter variable is passed to isupper() and output. The result is 0 which represents False.
• The letter variable is passed to toupper() and assigned to letter.
• The new value in the letter variable is passed to isupper() and output. The result is 1 which represents True.

There are many useful functions contained in the C standard libraries. Here is a list of some of them. It is worth looking through them to see how they can help your code.