Practical Application for C Programming: Recursive Functions

Instructor: Martin Gibbs

Martin has 16 years experience in Human Resources Information Systems and has a PhD in Information Technology Management. He is an adjunct professor of computer science and computer programming.

In this practical application lesson, you will write working C code. You will write and edit functions that take advantage of recursion. You will then be able to develop, run, and test your code.

Lesson Overview & Knowledge Required

In order to successfully complete this lesson, you should be able to describe recursion in the C computer programming language. You should be able to write, compile and test programs in C, as well as declare, create and call functions using C.

Program Code

A recursive function is one that calls itself. In order to do this, however, you always need a way out of the function or you will be trapped and the program will most likely crash.

Fibonacci Series

For this program code, you will calculate a Fibonacci sequence. This sequence is defined as follows:

  • The sum of two numbers preceding the current number equal that number
  • The first numbers are 0, 1, 1, 2
  • In the above sequence: 1 = 0 + 1, 2 = 1+1, and so on

Let's look at the first 20 numbers in the Fibonacci sequence. To do so, we will write a recursive C program. Open your developer tool (e.g. Dev C++) and enter the following code:

#include <stdio.h>
//declare the function
int fibo(int num);
//main function
int main() {
 printf("*** Fibonacci Sequence ***\n");
 int num;
 for(num = 0; num < 10; num++) {
  //call the function
  printf("%d\t", fibo(num));
 return 0;
//Recursive function here:
int fibo(int num) {
 if(num == 0) {
  return 0;
 if(num == 1) {
  return 1;
 return fibo(num - 1) + fibo(num - 2);

The function fibo calls itself until it has found the proper sequence.

Code Application

Now it is your turn. Instead of finding a Fibonacci sequence, find the sum of natural numbers for a given input. This means if the user enters 10, you would sum 1 + 2 + 3 + 4 + 5 + 6 +7 + 8 + 9 + 10 = 55.

We'll get you started by creating the main function that collects input from the user and calls the function. You will need to complete the function declaration and the function definition.

#include <stdio.h>
int main() {
 int num, result;
 printf("Enter a positive value");
 scanf("%d", &num);
 result = sum(number);
 printf("sum = %d", result);
 return 0;

Follow-Up Questions

Now that you have completed the code application, answer the following questions.

Question 1

What happens if you enter a very large number for the input, say 1,000,000? How can you fix this?

Question 2

What would happen if the only code in the sum function was to return the value if i + sum(i-1)?

Question 3

The following recursive function isn't working. The intent is to find the factorial of a number. For example, the factorial of 3 is 3 * 2 * 1 = 6. Correct the code, compile and run.

#include <stdio.h>
long fact(long f_num);
int main(void) {
 long tester = 8;
 printf("Factorial of %d is %d\n", tester, fact(tester));
long fact(long f_num) { 
 long number;
 number = fact(f_num-1) * f_num;

Answer Key

Following are the answers to the code application and the follow-up questions.

Code Application

Below is the full code for the working C program:

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