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C++ Introduction
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C++ Variables & Constants
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C++Scope of Variable
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C++ Basic I/O
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C++ Comments
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C++ If-else
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C++ Ternary Operator
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C++ for Loop
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C++ Ranged for Loop
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C++ while/do-while Loop
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C++ break Statement
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C++ switch Statement
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C++ Pointers
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C++ Void Pointers
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C++ Dynamic Memory Allocation
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C++ OOPs Concepts
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C++ Objects and Class
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C++ Constructors
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C++ Destructors
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C++ Constructor Overloading
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C++ Enumeration
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C++ Friend Function
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C++ Multithreading
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C++ Standard Library
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C++ Programming Tutorials
About Lesson

C++ while/do-while Loop

In this tutorial, we will learn about the C++ loops, while loop, do-while, nested while/do-while loop difference, and infinite while/do-while loop and its working with the help of some examples.


C++ Loops

In computer programming, loops are used to repeat a block of code. Loops in Programming come into use when we need to repeatedly a block of statements.
For example, let’s say we want to show a message 100 times. Then instead of writing the print statement 100 times, we can use a loop.
That was just a simple example; we can achieve much more efficiency and sophistication in our programs by making effective use of loops.

In programming, a loop is a sequence of instructions that is repeated until a certain condition is reached.


Types of Loops

There are 3 types of loops in C++:
1) for loop
2) while loop
3) do...while loop

In the previous tutorial, we learned about the C++ for loop. Here, we are going to learn about while and do...while loops.


C++ while Loop

  • while is a Entry Controlled Loop or we can say repetition control structure.
  • The while loop is used in situations where we do not know the exact number of iterations of loop beforehand.
  • The loop execution is terminated on the basic of the test condition.

Working of while loop

  • while loop evaluates the condition.
  • If the condition evaluates to true, the code inside the while loop is executed.
  • The condition is evaluated again.
  • This process continues until the condition is false.
  • When the condition evaluates to false, the loop terminates.

The syntax of the while loop is:

while (condition) {
// body of the loop
}

To learn more about the conditions, visit C++ Relational and Logical Operators.


Flowchart of while Loop

C++ while loop flowchart
Flowchart of C++ while loop

Example 1: Program to print “Hello world!” 3 times

#include <iostream>     
using namespace std;
      
int main() {

    // Initialization expression
    int i = 1; 
                                    
    // test expression
    while (i < 4) {
        cout <<"Hello World\n ";

        // update expression
        i++;
    }
    return 0;
}

Output

Hello World
Hello world
Hello world

Let’s understand how the program works.

Iteration Variable i <= 3 Action
1st i = 1 true Hello World is printed and i is increased to 2.
2nd i = 2 true Hello World is printed 2nd time and i is increased to 3.
3rd i = 3 true Hello World is printed 3rd time and i is increased to 4
4th i = 4 false The loop is terminated

Example 2: C++ Sum of Positive Integers Only

#include <iostream>
using namespace std;
      
int main() {

    // declare and initialize vector  
    vector<int> num = {1, 2, 3, 4, 5, 6};
                                      
    // print vector elements 
    for (int i : num) {
        cout << i << " ";
    }

    return 0;
}

Output

1 2 3 4 5 6

Example 3: Declare Collection inside the Loop

// program to find the sum of positive Integers
// if the user enters a negative Integers, the loop ends
// the negative integer entered is not added to the sum
                                  
#include <iostream>
using namespace std;
int main() {
    int number;
    int sum = 0;
                                  
    // take input from the user
    cout << "Enter a number: ";
    cin >> number;
                                  
    while (number >= 0) {
    // add all positive integers
    sum += number;
                                  
    // take input again if the integer is positive
    cout << "Enter a number: ";
    cin >> number;
    }
                                  
    // display the sum of all the positive  integers
    cout << "\nThe sum of positive integers: " << sum << endl;

    return 0;
}

Output

Enter a number: 6
Enter a number: 6
Enter a number: 10
Enter a number: 0
Enter a number: -12
The sum of positive integers: 22

Working of this program:

  • In this program, the user entered a number, which is stored in the variable number.
  • In order to store the sum of the numbers, we declare a variable sum and initialize it to the value of 0.
  • The while loop continues until the user enters a negative number.
  • During each iteration, the number entered by the user is added to the sum variable.
  • When the user enters a negative number, the loop terminates.
  • Finally, the total sum is displayed.

C++ Nested while Loop

A while loop within another while loop is called Nested while loop.

The syntax of nested while loop is:

while (condition) {
        while (condition) {
                // body of inner while-loop 
        }
    // body of outer while-loop 
}

Example 4: Program to display a triangular pattern

// C++ program to display a triangular pattern 
#include <iostream>
using namespace std;
                                            
int main() {
                                            
   int rows, i = 1;
   cout << "Enter the number of rows: ";
   cin >> rows;

   while (i <= rows) {
        int j = 1;
      while(j <= i) {
         cout << "* ";
         j++;
      }
    cout << "\n";
    i++;
   }
    return 0;
}
Enter the number of rows: 4
*    
*  *   
*  *  *  
*  *  *  *

In this program, the outer loop iterates from 1 to rows.

The inner loop iterates from 1 to i. Inside the inner loop, we print the character '*'.

  • Note: There is no rule that a loop must be nested inside its own type. In fact, there can be any type of loop nested inside any type and to any level.
  • If you don’t understand above example, We recommended you to know little more about Nested Loop.

C++ do…while Loop

  • Like while loop the do-while loop execution is also terminated on the basic of a test condition.
  • The do-while loop is exit-controlled loop.
  • in do-while loop the loop body will execute at least once irrespective of test condition.

Working of do-while loop

  • The body of the loop is executed at first. Then the condition is evaluated.
  • If the condition evaluates to true, the body of the loop inside the do statement is executed again.
  • The condition is evaluated once again.
  • If the condition evaluates to true, the body of the loop inside the do statement is executed again.
  • This process continues until the condition evaluates to false. Then the loop terminates.

The syntax of do-while loop is:

do {
// body of loop;
}
while (condition);

Flowchart of do…while Loop

C++ do...while loop flowchart
Flowchart of C++ do…while loop

Example 5: Display Numbers from 1 to 4

// C++ Program to print numbers from 1 to 4
#include <iostream>
using namespace std;
                                  
int main() {
    // Initialization expression
    int i = 1; 
                                  
    // do...while loop from 1 to 4
    do {
        cout << i << " ";
        ++i;
    }
    // test expression
    while (i <= 4);

    return 0;
}

Output

1 2 3 4

Here is how the program works.

Iteration Variable i <= 4 Action
  i = 1 not checked 1 is printed and i is increased to 2
1st i = 2 true 2 is printed and i is increased to 3
2nd i = 3 true 3 is printed and i is increased to 4
3rd i = 4 true 4 is printed and i is increased to 5
4th i = 5 false The loop is terminated

Example 6: Sum of Positive Numbers Only using do-while loop

// program to find the sum of positive numbers
// If the user enters a negative number, the loop ends
// the negative number entered is not added to the sum
                                  
#include <iostream>
using namespace std;
                                  
int main() {
    int num = 0, sum = 0;
                                  
    do {
        sum += number;
    // take input from the user
        cout << "Enter a number: ";
        cin >> num;
    }
    while (num >= 0);
    // display the sum
    cout << "\nThe sum is " << sum << endl;
                                                      
    return 0;
}

Output 1

Enter a number: 6
Enter a number: 12
Enter a number: 7
Enter a number: 0
Enter a number: -2

The sum is 25

Here, the do...while loop continues until the user enters a negative number. When the number is negative, the loop terminates; the negative number is not added to the sum variable.

Output 2

Enter a number: -6
The sum is 0.

The body of the do...while loop runs only once if the user enters a negative number.


Nested do-while Loop

A do-while loop within another do-while loop is called Nested do-while loop.

The syntax of nested do-while loop is:

do {

    do{
        // body of inner do-while-loop 
    }while (condition);

    // body of outer do-while-loop 
}while (condition);

Example 7: program to display a triangular pattern using nested do-while

// C++ program to display a triangular pattern 

#include <iostream>
using namespace std;
    
int main() {
    
int rows, i,j;
i = 1;
cout << "Enter the number of rows: ";
cin >> rows;
    do {
        j = 1;
        do{
            cout << "* ";
            j++;
        }while(j <= i);
        cout << "\n";
        i++;
    }while(i <= rows);
                                                     
    return 0;
}

Output

Enter the number of rows: 4
*    
*  *   
*  *  *  
*  *  *  *

for loop vs while loop

for loop while loop
Initialization may be either in loop statement or outside the loop. Initialization is always outside the loop.
Once the statements is executed then after increment is done. Increment can be done before or after the execution of the statement.
It is normanlly used when the number of iteration is known. It is normally used when the number of iterations is unknown.
Condition is a relational expression. Condition may be expression or non-zero value.
It is used when initialization and increment is simple. It is used for complex initialization.
For is entry Controlled loop. while is also entry controlled loop.
for (initialization; condition; iteration) {statement(s);} while(condition){statement(s);}
for loop used only when we already knew the number of iterations. while loop used only when the number of iteration are not exactly known.

while loop vs do-while loop

while loop do-while loop
Condition is checked first then statement(s) is executed Statement(s) is executed atleast once, thereafter condition is checked.
It might occur statement(s) is executed zero times, if condition is false. Atleast once the statement(s) is executed.
No semicolon at the end of while. while(condition) Brackets are always rewuired.
Variable in condition is initialized before the execution of loop. Variable may be initialized before or within the loop.
while loop is entry controlled loop. do-while loop is exit controlled loop.
while(condition){statement(s);} do{statement(s)}while(condition);
The iteration do not occur if, the condition at the first iteration, appears false. The iteration occurs atleast once even if the condition is false at the first iteration.

 

Exercise Files
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