Do-while Loop in C++

February 3, 2022

Do while loop

Here, in this section, we will discuss the do-while loop in C++. Do while is a looping structure in C++, these types of loops are called Exit control or Post-test loops.

Note In Computer Science do-while loop executes the loop body at least once irrespective of the condition and then executes body based on the condition

Note Its an exit controlled loop. In a do-while first the loop body is executed and then the test condition is checked as a result one time execution of loop body is assured even if the condition is false for the first time

Looping in C++

We have the following loops in C++ :

While loop
For loop
Do while
Nested loops

We will discuss do-while loop in this post –

Do while

Very similar to the while loop. To do repetitive work control structure has been created.

In do while we execute some statements first and then check a condition. If the condition is met we execute it again. We keep doing this until the condition keeps meeting.

Syntax

do{
    // execute statement(s)
}while(condition(s))

Along with increment and initialization logic it may look like 0

initialization;

do{
    // execute statement(s)
    incrementation;
    
}while(condition(s))

Let us learn do-while looping with a few examples –

Example 1

#include<iostream>
using namespace std;
 
int main()
{
    int i = 1; // initialization
    
    do{
        cout << i << ". Hello World\n";
        i++; // incrementation
    }while(i <= 5); 
    // condition(s) checking
    
    return 0;
}

Output

1. Hello World
2. Hello World
3. Hello World
4. Hello World
5. Hello World

Note Do-while is an exit controlled loop. Which means that it will be implemented atleast once.

The reason to this is that we first do i.e. execute statements and then after it we do condition checking to check if we need to implement the statements again or not.

The following examples show how it will be implemented at least once –

Example 1.1

#include<iostream>
using namespace std;
 
int main()
{
    int i = 1000;
    
    // i is set as 1000 which is definitely greater than 5
    // the condition set below is (i <= 5) which will surely be never met
    // as the loop is incrementing (i++)
    // but still the loop is executed once anyways
    
    do{
        cout << i << ". Hello World\n";
        i++; 
    }while(i <= 5); 

    return 0;
}

Output

1000. Hello World

Example 2

Program to print first n numbers.

#include<iostream>
using namespace std;
 
int main()
{
    int n = 25;
    
    int i = 1;
    do{
        cout << i << " ";
        i++; 
    }while(i <= n); 

    return 0;
}

Output

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Example 3

Program to print numbers divisible by 7 up to 100.

#include<iostream>
using namespace std;
 
int main()
{
    int n = 100;
    
    int i = 1;
    do{
        // if divisible by 7 must leave 0 remainder
        // on applying modulo operator
        if(i % 7 == 0){
            cout << i << " ";
        }
        i++; 
    }while(i <= n); 

    return 0;
}

Output

7 14 21 28 35 42 49 56 63 70 77 84 91 98

Example 4

Program to print items in a character array

#include<iostream>
using namespace std;
 
int main()
{
    char a[] = {'P', 'o', 'k', 'e', 'm', 'o', 'n'};
    
    int i = 0; // array index starts from 0
    do{
        cout << a[i] << " ";
        i++; 
    }while(i < 7); 

    return 0;
}

Output

P o k e m o n

Example 5

Program to print Ascii Values

#include
using namespace std;
 
int main()
{
    char i='A';//inialisation
    do
    {
        cout << i << " : " << (int) i << " ";
        i++; // ascii codes are incremented
        
    } while(i<='Z'); // internally ascicodes are compared 
}

Output

A : 65 B : 66 C : 67 D : 68 E : 69 
F : 70 G : 71 H : 72 I : 73 J : 74 
K : 75 L : 76 M : 77 N : 78 O : 79 
P : 80 Q : 81 R : 82 S : 83 T : 84 
U : 85 V : 86 W : 87 X : 88 Y : 89 Z : 90