C++ do while Loop

In C++, the do-while loop is an exit-controlled loop that repeatedly executes a block of code at least once and continues executing as long as a given condition remains true. Unlike the while loop, the do-while loop guarantees that the loop body will execute at least once, regardless of whether the condition is true or false.

Let’s take a look at an example:

#include <iostream> using namespace std; int main() {  // do-while loop to print "Hi" 5 times  int i = 0;  do {  cout << "Hi" << endl;  i++;  } while (i < 5);  return 0; } 

Hi Hi Hi Hi Hi 

Explanation: The above do-while loop prints the text "Hi" 5 times. It first executes the body of the loop and then checks the condition. Since the condition i < 5 is true for the first few iterations, the loop continues running. Once i reaches 5, the condition becomes false, and the loop exits.

Syntax of do-while Loop

do {
// Body of the loop
// Update expression
} while (condition);

We have to define the loop variable beforehand and update it manually in the body of the loop. Notice the semicolon (";") in the end of loop. It is compulsory to terminate the do while loop after the semicolon.

The various parts of the do-while loop are: 

1. Condition: The condition is checked after the loop body executes. If the condition evaluates to true, the loop continues. If false, the loop exits.
2. Update Expression: Update the loop variable bringing it closer to termination condition.
3. Body: Body: It is a group of statement that will be executed surely for the first time and then till the condition remains true.

Working of do while Loop in C++

Let's understand the working of the do while loop using the given image:

1. Control falls into the do-while loop.
2. The statements inside the body of the loop get executed.
3. Updation takes place.
4. The flow jumps to Condition
5. Condition is tested. 
   
   • If the Condition yields true, go to Step 6.
   • If the Condition yields false, the flow goes outside the loop
6. The flow goes back to Step 2.
7. The do-while loop has been ended and flow has gone outside the loop.

Flow Diagram of do-while loop

Examples of do while Loop in C++

The below examples demonstrate the use of do while loop in different cases and situations:

Print Numbers Less than 0

#include <iostream> using namespace std; int main() {  // do-while loop to print "Hi" 5 times  int i = 1;  do {  cout << i << endl;  i++;  } while (i < 0);  return 0; } 

1 

Explanation: As we can see, even though the condition is false from the start, the body is still executed once.

User Input Validation with do-while Loop

Let’s consider an example where a user is prompted to enter a positive number. The program will continue to prompt the user until they enter a valid number.

#include <iostream> using namespace std; int main() {  int n;  // Do-while loop to ensure user enters a positive number  do {  cout << "Enter a positive number: ";  cin >> n;  } while (n <= 0);  cout << "Entered number: " << n << endl;  return 0; } 

Output

Enter a positive number: -1
Enter a positive number: -999
Enter a positive number: 2
Entered number: 2

This is one of the primary applications of do while loop in C++.

Print a Square Pattern using Nested Loops

Just like other loops, we can also nest one do while loop into another do while loop.

#include <iostream> using namespace std; int main() {  int i = 0;  // Outer loop to print each row  do {  int j = 0;    // Inner loop to print each character  // in each row  do {  cout << "* ";  j++;  } while (j < 4);  cout << endl;  i++;  }while (i < 4);  return 0; } 

* * * * * * * * * * * * * * * * 

Infinite do while Loop

#include <iostream> using namespace std; int main() {    // Infinite loop  do {  cout << "gfg" << endl;  }while (true);  return 0; } 

Output

gfg
gfg
.
.
.
infinite times