Functions

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Here is the complete playlist for video lectures and tutorials for the absolute beginners. The language has been kept simple so that anybody can easily understand them. I have avoided complex jargon in these videos.

Function Syntax

A function consists of the following parts

1. Name
2. Return Type
3. Code Body
4. Parameters

void fx(int i, int j)
{
    int sum = i + j;
    
    cout << "Sum = " << sum << endl;
 
}
 

Execution of a Function

The code of a function can be executed only through a function call. It doesn't execute automatically. It is like a calculator where you have to give inputs and press the enter key to start a calculation and display the result. The following code shows how to call a function. The function is being called from the main function by passing 8 and 9 as the input arguments.

void showSum(int x, int y)
{
     // store sum in z
     int z = x + y;
 
     // display the answer
     cout << z << endl;
}
 
int main()
{
 
     // calling the function, pass arguments
     showSum(8, 9);
 
}

Steps in the Execution

I will explain them with the help of an analogy. Suppose you are in a car and driving along a highway. It can be thought of as a journey along the main function of a C program. You are executing statements one by one - like changing the gears, etc.,

After some time you spot an eating restaurant, and wish to have some snacks. For this you take your car out of the highway and move it to the parking of that restaurant. There you spend some time, and then again resume your journey back on the highway.

The highway can be thought of as the main function. When you stop at a restaurant, it is like jumping out of the main function and calling into another function, and when that function is completed you come back to main, and resume your journey towards the return 0 statement.

A function stops its execution at a return statement, or when all the statements have been executed and the closing brace reached.

Return from a function

A function can return data. It could return an int number, or a char, and even objects of classes and structs. The return is usually an end result of the work done by the code of a function. For example, it could be the result of a mathematical calculation.

The return statement is also used to return success or failure. In such cases the return value is a boolean type.

It can also be used to return detailed status codes, like the reason for a failure. For example if you are using a function to connect to a database, your function could return a different integer number, representing different reasons of failure, or success. Usually, zero is for success, and other codes can be given other custom meanings.

Maximum number of Arguments

According to the standard, a C++ compiler should support atleast 256 arguments to a function call. Similarly, the number of parameters that a function should support is 256.

Call by Value

When arguments are passed to a function, they are copied into the parameter variables. The function acts on it's parameters, and any changes it makes to it's parameters have no effect on the arguments passed to it. This is pass by value. In the code below, z is copied into y. The function fx makes changes to y, but not to z. So the output in the main would be "z = 9". This is the default behaviour, and it is called passing an argument by value.

int fx(int y)
{
    y += 10;
    cout << y << endl;
}
 
int main()
{
    int z = 9;
    
    fx(z);
     
    // 9
    cout << "z = " << z << endl;
 
    return 0;
}

Call by Reference

First of all you should be aware of a reference. A reference is an alternate name for the same identifier. It is also called an alias. A student has a certain name in the school, but at home he has another name. The same person can be referred through his home name also. His home name is an alias for him.

In C++ we can give alternate name to any variable. Consider the code below. Here y is a reference to x. It is not a separate variable. If you make changes to x, and later display the variable y, the output would be the latest value of x only, because y is just a second name for x.

int x = 9;
 
// y is a reference or alias to x
int& y = x;
 
x++;
 
// prints 10
cout << y << endl;

Function parameters can be expressed as references. In the code below, The parameter y is an alias name for the argument z. When z is passed to the function, it gets a new name called y. So y is basically z only. The function fx makes changes to y, but since y is a reference for z, the changes are actually taking place on z. When z is printed in main, the output is 19.

int fx(int& y)
{
    y += 10;
    cout << y << endl;
}
 
int main()
{
    int z = 9;
    
    fx(z);
 
    // 19
    cout << "z = " << z << endl;
 
    return 0;
}

Simple function no argument no return type

#include<iostream>
#include<cstdio>
 
using namespace std;
 
void show() // function definition 
{
 
    cout << "I am Hoven trained ";
    
}
 
int main()
{
    
    show(); // function call
        
}

A function with argument but no return type

#include<iostream>
#include<cstdio>
 
using namespace std;

void prod( int a,int b) // function definition
{ 
            
    int s; 
         
    s = a * b;
        
    printf("prod = %d", s); 
        
}
     
int main()
{
    
    prod (3, 4); // function call
     
    return 0; 
     
}
 
    
 

Many functions

#include<iostream>
#include<cstdio>
 
using namespace std;
 
void addition(int a, int b) // function defination
{
    
    int i;
    
    i = a + b ;
    
    cout <<"After addition = " << i << endl;
    
}
 
void product(int a, int b)
{
    
    int i;
    
    i = a * b ;
    
    cout <<"After product = " << i << endl;
 
}
 
 
void division(int a, int b)
{
    
    int i;
    
    i = a / b ;
    
    cout <<"After division = " << i << endl;
    
}
 
void subtraction(int a, int b)
{
    
    int i;
    
     i = a - b ;
    
    cout <<"After subtraction = " << i<< endl ;
    
}
 
 
 
int main()
{
    
    addition(6, 7); 
    
    subtraction(5, 9);
    
    product(23, 98);
    
    division(145, 5);
    
    return 0;
    
}

Function, no argument with return type

#include<iostream>
#include<cstdio>
 
using namespace std;
 
float Circumference()
{

    int radius = 5;
    
    float f = 2 * 3.14 * radius;
    
    return f ;
        
}
 

int main()
{

    cout <<"Circumference of circle = " << Circumference() ;
    
    return 0;
    
}

Function with argument and return type

#include<iostream>
#include<cstdio>
 
using namespace std;
 
int sum (int num1, int num2) 
{ 
 
    int num3; 
    
    num3 = num1 + num2 ; 
    
    return num3 ; 
 
}
 
int main() 
{
    
    int a, b; 
    
    printf("\nEnter the two numbers one by one: ");
    
    cin >> a; 
    
    cin >> b;
    
    int c = sum( a, b );
    
    printf("\nSum is: %d",c); 
    
    return 0;
 
}
 

Function with return type and parameters

#include<iostream>
#include<cstdio>
 
using namespace std;
 
int cube (int num1) 
{ 
 
    int num3 = num1 * num1 * num1; 
    
    return num3 ; 
 
}
 
int square (int num1) 
{ 
 
    int num3 = num1 * num1 ; 
    
    return num3 ; 
 
}
 
int main() 
{
    
    int a; 
    
    printf("Enter a number: ");
    
    cin >> a; 
        
    int c = cube(a);
    
    printf("\n Cube of number is : %d", c); 
    
    int d = square(a);
    
    printf("\nSquare of number is : %d", d); 
    
    return 0;
 
}
 

Call by value

#include<iostream>
#include<cstdio>
 
using namespace std;
 
void call_by_value(int x) 
{
    
    printf("Inside call_by_value x = %d before adding 10.\n", x);
    
    x += 10;
    
    printf("Inside call_by_value x = %d after adding 10.\n", x);
    
}
 
int main() 
{
    
    int a = 10;
    
    printf("a = %d before function call_by_value.\n", a);
    
    call_by_value(a);
    
    printf("a = %d after function call_by_value.\n", a);
    
    return 0;
 
}
 

Use of global variable with function

#include<iostream>
#include<cstdio>
 
using namespace std;
 
// global variables
int A;
 
int B;
 
int Add()
{
 
    return A + B;
 
}
 
int main()
{
    
    int answer; // Local variable
        
    A = 5;
        
    B = 7;
        
    answer = Add();
        
    printf("%d\n",answer);
        
    return 0;
        
}

Calling a function from another

#include<iostream>
#include<cstdio> 
 
using namespace std;
 
void funct4()
{
 
    cout << "function 4 is called "<< endl;
 
} 
 
void funct3()
{
 
    printf("function 3 is called \n");
    
    funct4(); 
 
}
 
void funct2()
{
 
    printf("function 2 is called \n");
    
    funct3();
 
}
 
void funct1()
{
 
    printf("function 1 is called \n");
    
    funct2();
 
}
 
void func()
{
    printf("function is called \n");

    funct1();
    
    funct2();

    funct3();

    funct4();
}

int main()
{ 
 
    func();
    
    return 0;
    
}

Function overloading

#include<iostream>
#include<cstdio> 
 
using namespace std;
 

void funct(int a, int b, char c)
{
 
    printf("function with three params: %d %d %c\n",a,b,c);    
 
}
 
void funct(int a, int b)
{
 
    printf("function with two params:%d %d\n",a,b);

}
 
void funct(int a) 
{
 
    printf("function with one param: %d\n", a);
 
}
 
 
int main()
{ 
 
    funct(11); 

    funct(88, 9);

    funct('G', 'A', 'M');
    
    return 0;
    
}

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