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

• A 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

• A 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

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.

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

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.

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

while loop vs do-while loop