[Guide for C] Type of User-defined Functions in C

There can be 4 different types of user-defined functions, they are:
- Function with no arguments and no return value
- Function with no arguments and a return value
- Function with arguments and no return value
- Function with arguments and a return value

Below, we will discuss about all these types, along with program examples.
Function with no arguments and no return value

Such functions can either be used to display information or they are completely dependent on user inputs.

Below is an example of a function, which takes 2 numbers as input from user, and display which is the greater number.

#include<stdio.h>

void greatNum();       // function declaration

int main()
{
    greatNum();        // function call
    return 0;
}

void greatNum()        // function definition
{
    int i, j;
    printf("Enter 2 numbers that you want to compare...");
    scanf("%d%d", &i, &j);
    if(i > j) {
        printf("The greater number is: %d", i);
    }
    else {
        printf("The greater number is: %d", j);
    }
}

---

Function with no arguments and a return value

We have modified the above example to make the function greatNum() return the number which is greater amongst the 2 input numbers.

#include<stdio.h>

int greatNum();       // function declaration

int main()
{
    int result;
    result = greatNum();        // function call
    printf("The greater number is: %d", result);
    return 0;
}

int greatNum()        // function definition
{
    int i, j, greaterNum;
    printf("Enter 2 numbers that you want to compare...");
    scanf("%d%d", &i, &j);
    if(i > j) {
        greaterNum = i;
    }
    else {
        greaterNum = j;
    }
    // returning the result
    return greaterNum;
}

---

Function with arguments and no return value

We are using the same function as example again and again, to demonstrate that to solve a problem there can be many different ways.

This time, we have modified the above example to make the function greatNum() take two intvalues as arguments, but it will not be returning anything.

#include<stdio.h>

void greatNum(int a, int b);       // function declaration

int main()
{
    int i, j;
    printf("Enter 2 numbers that you want to compare...");
    scanf("%d%d", &i, &j);
    greatNum(i, j);        // function call
    return 0;
}

void greatNum(int x, int y)        // function definition
{
    if(x > y) {
        printf("The greater number is: %d", x);
    }
    else {
        printf("The greater number is: %d", y);
    }
}

---

Function with arguments and a return value

This is the best type, as this makes the function completely independent of inputs and outputs, and only the logic is defined inside the function body.

#include<stdio.h>

int greatNum(int a, int b);       // function declaration

int main()
{
    int i, j, result;
    printf("Enter 2 numbers that you want to compare...");
    scanf("%d%d", &i, &j);
    result = greatNum(i, j); // function call
    printf("The greater number is: %d", result);
    return 0;
}

int greatNum(int x, int y)        // function definition
{
    if(x > y) {
        return x;
    }
    else {
        return y;
    }
}

---

Nesting of Functions

C language also allows nesting of functions i.e to use/call one function inside another function's body. We must be careful while using nested functions, because it may lead to infinite nesting.

function1()
{
    // function1 body here
    
    function2();
    
    // function1 body here
}

If function2() also has a call for function1() inside it, then in that case, it will lead to an infinite nesting. They will keep calling each other and the program will never terminate.

Not able to understand? Lets consider that inside the main() function, function1() is called and its execution starts, then inside function1(), we have a call for function2(), so the control of program will go to the function2(). But as function2() also has a call to function1() in its body, it will call function1(), which will again call function2(), and this will go on for infinite times, until you forcefully exit from program execution.

What is Recursion?

Recursion is a special way of nesting functions, where a function calls itself inside it. We must have certain conditions in the function to break out of the recursion, otherwise recursion will occur infinite times.

function1()
{   
    // function1 body
    function1();
    // function1 body
}

Example: Factorial of a number using Recursion

#include<stdio.h>

int factorial(int x);       //declaring the function

void main()
{
    int a, b;
    
    printf("Enter a number...");
    scanf("%d", &a);
    b = factorial(a);       //calling the function named factorial
    printf("%d", b);
}

int factorial(int x) //defining the function
{
    int r = 1;
    if(x == 1) 
        return 1;
    else 
        r = x*factorial(x-1);       //recursion, since the function calls itself
    
    return r;
}

Similarly, there are many more applications of recursion in C language. Go to the programs section, to find out more programs using recursion.