Home

Learning Hub

Articles

The Difference Between While and Do While Loop in C

Loops are a very important part of any programming language. They are the fundamentals of programming as they help execute a block of code for a given time or repeatedly until a specified condition is met. While the C language has 3 types of loops, including for, while, and do-while, we will learn the differences between while and do-while loops in this article.

While the naming might seem similar, they have different workings, which will be discussed in this article, too. In this blog, we will dive into the functionalities of while and do-while loops to see their key differences and provide practical code examples for a better understanding. Let’s learn more in detail about while and do-while loops in the C language.

What is the While Loop in C?

A while loop is a control flow statement in which the loop body is executed till a certain condition is met. While loop is also known as an entry–controlled loop or pre-condition loop, where a condition is checked before the code inside is executed. Once a condition is checked, the iteration continues till the condition remains true. Let’s see the while loop syntax first.

Syntax

Below is the syntax used for defining a while loop in C language:

How Does a While Loop Work?

A while loop is a controlled flow statement in which the given condition is checked before executing the code. See the complete step-by-step working of a while loop in C language below:

1. First, the condition is evaluated.
2. Then, If the condition == true, the code inside the loop is executed.
3. After executing the code, the condition is then re-evaluated.
4. Finally, the process continues until the condition == false.

Note that a while loop can also be nested, which means there can be more than 2 while loops executed one inside the other.

Single While Loop:

There is no single while loop; it’s just a way to describe the usage of a while loop. The working of a single while loop is very simple, wherein it first evaluates the condition. If true, the given code inside the loop is executed, and if not, then it returns. See the syntax and example of a single while loop below.

Syntax:

Below is the syntax used for defining a while loop in C language:

Code Example: The below example demonstrates a single while loop in C.

Explanation:

• In this example, we are printing the message “Welcome to HeroVired”.
• Here, a while loop is used, and the condition name_count > 5 is evaluated first.
• Inside the while loop, the program executes to print a message “Welcome to HeroVired” and then decrements the value of name_count by 1.
• The loop continues to execute as long as the condition name_count > 5 remains true.
• When name_count <= 5, the loop terminates.

Output:

Nested While Loop:

A while loop can be nested inside one another and executed. Here, there are two things: an outer loop that controls the overall repetitions and an inner loop that executes a specific set of statements within each iteration of the outer loop. See the syntax and example of a nested while loop below.

Syntax:

Below is the syntax used for defining a nested while loop in C language:

Code Example: The below example demonstrates the nested while loop.

Explanation:

• In this example, we are printing a message “HeroVired” in an 8 * 4 size after initialising two variables with value row_no = 10 and cols_no = 5, representing the number of rows and columns, respectively.
• Here, the outer while loop iterates as long as row_no > 2. Inside this loop, the program enters another nested do-while loop.
• Inside the outer loop, an integer variable col_no = 2 and the inner while loop iterates as long as col_no <= cols_no
• Inside this loop, the program prints the message, and after each iteration of the inner loop, col_no is incremented by 1.
• Once the inner loop completes its iterations for a row, the program prints a message, and the outer loop decrements row_no by 1 using row_no–.
• The outer loop continues to execute until row_no <= 2.

Output:

Also Read: Difference Between Java and Python

What is the Do-while Loop in C?

A do-while loop is a control flow statement in which the loop body is executed till a certain condition is met. A do-while loop is also known as an exit–controlled loop or post-condition loop, where a condition is checked after the code inside is executed once. The do-while loop guarantees the loop body will be executed at least once before checking the condition regardless of whether the condition is true or false. Let’s see the do-while loop syntax.

Syntax

A do-while loop is a controlled flow statement in which the code is executed at least once before checking the condition. See the complete step-by-step working of a do-while loop in C language below:

1. First, the code is executed once.
2. Then, the condition is checked/evaluated.
3. Now, if the condition == true, the loop body is again executed.
4. Finally, the process continues until the condition == false.

Note that a do-while loop can also be nested, which means there can be more than 2 do-while loops executed one inside the other.

Single do-while Loop:

In a single do-while loop, the loop body is executed at least once, and only then is the condition or test expression tested/evaluated. In this do-while loop, the working is very simple: first, the body is executed, and then the condition is checked. If the condition becomes true again, the body is again executed, and if false, then it returns. See the syntax and example of a single do-while loop below.

Syntax:

Below is the syntax used for defining a do-while loop in C language:

Code Example: The below example demonstrates the single do-while loop.

Explanation:

In this example, we are printing the message “Welcome to HeroVired” using a do-while loop that ensures the message is printed at least once. Here, the name_count starts at 10 and is decremented within the loop till the name_count becomes 4. The loop continues as long as name_count is greater than 4, resulting in six iterations and six printed messages.

Output:

Nested do-while Loop:

In a nested do-while loop, the working is similar to that of a nested while loop. The outer do-while loop controls the overall iterations, while the inner loop executes at least once before checking the test expression or condition. See the syntax and example of a nested do-while loop below.

Syntax:

Below is the syntax used for defining a nested do-while loop in C language:

Code Example: The below example demonstrates the nested do-while loop.

Explanation:

In this example, we have created a 6 * 3 rectangle using a nested do-while loop. Here, the two variables r_no and c_no are declared as row and column, respectively. The outer loop controls the number of rows printed, while the inner loop controls how many times the “HeroVired” message is printed in each row.

Output:

Difference Between While and Do-while Loops in C

Some of the key differences between the while and do-while loops are given below:

Conclusion

In this blog, we learned the differences between the while and do-while loops in C. Understanding the differences between both these loops can help you in determining where to use them and how to use them as they provide you with a complete understanding of the condition or test expression check and execution.

We saw how the while loop evaluates the condition before the loop body, making it suitable for scenarios where the loop might not execute at all if the condition is false initially. We also saw the working of a do-while loop on how it checks the condition after the loop body, ensuring that the loop executes at least once.