A **function** is a block of statements that can perform a particular task.
As we all know, there is always at least one function in C, and that is `main()`.


# Example
In the example below, the function's name is `sum` and the data type is `int`. This task of this function is to produce the sum of two numbers:
```c           
int sum(int a,int b)
{
   return(a+b);
}
```

Below, the function is declared in `main()`:
```c
void main()
{
     int sum(int,int); //function declaration
     int x=5,y=6;
     total = sum(x,y);
}
```


# Formal parameters and actual parameters

When we call a function in `main()` or anywhere else in the program, and the function we created needs parameters,  we would pass parameters to it while calling the function. In the example above, we passed variables `x` and `y` to obtain the sum of `x` and `y`. 

According to the example above, the *formal parameters* are `a` and `b`, and the *actual **parameters* are `x` and `y`.

> Essentially, the variables being passed in the function call are *actual* *parameters*, and the variables being initialized and used in the function are  *formal** **parameters*.

> Note: Number of formal parameters = Number of actual parameters 
Always!

## Advantages of a function

1. A function can be used any number of times after it is defined once.

2. Functions make programs manageable and easy to understand.

> Note: After termination of the function, the program control goes back to `main()` to execute any left out statements.

## Function categories
There are 4 types of functions:

### 1. Functions with arguments and return values

This function has arguments and returns a value:


#include <stdio.h>
void main()
{
  int sub(int,int); //function with return value and arguments
  int x=10,y=7;
  int res = sub(x,y);
  printf("x-y = %d",res);
}
int sub(int a,int b) //function with return value and arguments
{
  return(a-b);  // return value
}

### 2. Functions with arguments and without return values

This function has arguments, but it does not return a value:





#include <stdio.h>
int main()
{
  void sum(float,float); //function with arguments and no return value
  float x=10.56,y=7.22;
  sum(x,y);
}
void sum(float a,float b) //function with arguments and no return value
{
  float z = a+b;
  printf("x + y = %f",z);
}

### 3. Functions without arguments and with return values

This function has no arguments,  but it has a return value:

#include<stdio.h>
int main()
{
  int sum();
  int c = sum();
  printf("Sum = %d",c);
}
int sum() //function with no arguments and return data type
{
  int x=10,y=20,z=5;
  printf("x = %d ; y = %d ; z = %d \n",x,y,z);
  int sum = x+y+z;
  return(sum);
}

### 4. Functions without arguments and without return values

This function has no arguments and no return value:

#include<stdio.h>
int main()
{
  void sum();
  sum();
}
void sum() //function with no arguments and return data type
{
  int x=15,y=35,z=5;
  printf("x = %d ; y = %d ; z = %d \n",x,y,z);
  int sum = x+y+z;
  printf("Sum = %d",sum);
}

What are the different types of functions in C?

Example

Formal parameters and actual parameters

Advantages of a function

Function categories

1. Functions with arguments and return values

2. Functions with arguments and without return values

3. Functions without arguments and with return values

4. Functions without arguments and without return values