C Functions

A function is a group of code that performs a task together. Function in C language can be classified into two categories. They are:

1. Library functions
2. User-defined functions

The main is an example of a library function. There are some other library functions such as printf, scanf, sqrt etc. which are used in the program.
The main difference between library and user-defined is that you don't need to write library functions but you have to develop user defined functions at the time of writing a program.

There are three elements of a user-defined function. They are following:

1. Function definition
2. Function call
3. Function declaration

Function Definition

A function definition is also known as function implementation. It is divided into two parts. They are function header and function body. The function header includes the following statements:

1. Function name
2. Function type
3. Parameter list

The function body includes the following statements:

1. Local variable declarations
2. Function statements
3. A return statement

The general form of function definition is given below:

function_type function_name(parameter list) {
  local variable declaration;
  statement 1;
  statement 2;
  ............
  return statement;
}

An example of function definition is given below:

int add (int a, int b) {
  int sum;
  sum = a+b;
  return (sum);
}

Function Call

You can call a function simply using the function name which is followed by a list of actual parameters. When a program calls a function the control is transferred to the called function.

The example of calling a function is given below:

main() {
  int  x;
  statement 1;
  x = add(7, 5);    /* Function call */
  printf("%d\n", x);    
}

Function declaration

In C programming all functions must be declared before they are invoked. A function declaration contains four parts. They are following:

1. Function type
2. Function name
3. Parameter list
4. A semicolon at the end

The format of function declaration is given below:

function_type function_name(parameter_list);

Some acceptable forms of function declaration are given below:

int add(int, int);
int add(int a, int b);
add(int, int);

An example of a program using user-defined function:

#include <iostream>
#include <cmath>
using namespace std;
/* function declaration */
long long convert(int n);
int main() {
    int n;
    cout << "Enter a decimal number: ";
    cin >> n;
    /* function call */
    cout << n << " in decimal = " << convert(n) << " in binary " << endl;
    return 0;
}
/* function definition */
long long convert(int n) {
    long long bin = 0;
    /* local variable declaration */
    int rem, i = 1, step = 1;
    while (n != 0) {
        rem = n % 2;
        cout << "Step " << step++ << ":" << n << "/2, Remainder = " << rem << "Quotient = " << n/2 << endl;
        n /= 2;
        bin += rem * i;
        i *= 10;
    }
    return bin;
}

Enter a decimal number: 19
Step 1: 19/2, Remainder = 1, Quotient = 9
Step 2: 9/2, Remainder = 1, Quotient = 4
Step 3: 4/2, Remainder = 0, Quotient = 2
Step 4: 2/2, Remainder = 0, Quotient = 1
Step 5: 1/2, Remainder = 1, Quotient = 0
19 in decimal = 10011 in binary