Write a Do while Loop That Continues to Prompt a User to Enter a Number Less Than 100

In computer programming, loops are used to repeat a block of code.

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.

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.

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C++ while Loop

The syntax of the while loop is:

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

Here,

• 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.

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

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Flowchart of while Loop

Flowchart of C++ while loop

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Example 1: Display Numbers from 1 to 5

          // C++ Program to print numbers from 1 to 5  #include <iostream>  using namespace std;  int main() {     int i = 1;       // while loop from 1 to 5     while (i <= 5) {         cout << i << " ";         ++i;     }          return 0; }        

Output

          1 2 3 4 5        

Here is how the program works.

Iteration	Variable	i <= 5	Action
1st	i = 1	true	1 is printed and i is increased to 2.
2nd	i = 2	true	2 is printed and i is increased to 3.
3rd	i = 3	true	3 is printed and i is increased to 4
4th	i = 4	true	4 is printed and i is increased to 5.
5th	i = 5	true	5 is printed and i is increased to 6.
6th	i = 6	false	The loop is terminated

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Example 2: Sum of Positive Numbers Only

          // 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 number;     int sum = 0;      // take input from the user     cout << "Enter a number: ";     cin >> number;      while (number >= 0) {         // add all positive numbers         sum += number;          // take input again if the number is positive         cout << "Enter a number: ";         cin >> number;     }      // display the sum     cout << "\nThe sum is " << sum << endl;          return 0; }        

Output

          Enter a number: 6 Enter a number: 12 Enter a number: 7 Enter a number: 0 Enter a number: -2  The sum is 25        

In this program, the user is prompted to enter 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.

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C++ do...while Loop

The do...while loop is a variant of the while loop with one important difference: the body of do...while loop is executed once before the condition is checked.

Its syntax is:

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

Here,

• 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 stops.

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Flowchart of do...while Loop

Flowchart of C++ do...while loop

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Example 3: Display Numbers from 1 to 5

          // C++ Program to print numbers from 1 to 5  #include <iostream>  using namespace std;  int main() {     int i = 1;       // do...while loop from 1 to 5     do {         cout << i << " ";         ++i;     }     while (i <= 5);          return 0; }        

Output

          1 2 3 4 5        

Here is how the program works.

Iteration	Variable	i <= 5	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	true	5 is printed and i is increased to 6
5th	i = 6	false	The loop is terminated

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Example 4: Sum of Positive Numbers Only

          // 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 number = 0;     int sum = 0;      do {         sum += number;          // take input from the user         cout << "Enter a number: ";         cin >> number;     }     while (number >= 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.

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Infinite while loop

If the condition of a loop is always true, the loop runs for infinite times (until the memory is full). For example,

          // infinite while loop while(true) {     // body of the loop }        

Here is an example of an infinite do...while loop.

          // infinite do...while loop  int count = 1;  do {    // body of loop }  while(count == 1);        

In the above programs, the condition is always true. Hence, the loop body will run for infinite times.

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for vs while loops

A for loop is usually used when the number of iterations is known. For example,

          // This loop is iterated 5 times for (int i = 1; i <=5; ++i) {    // body of the loop }        

Here, we know that the for-loop will be executed 5 times.

However, while and do...while loops are usually used when the number of iterations is unknown. For example,

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

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Check out these examples to learn more:

• C++ Program to Display Fibonacci Series
• C++ Program to Find GCD
• C++ Program to Find LCM

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Source: https://www.programiz.com/cpp-programming/do-while-loop

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