Difference Between While and Do-While Loop

Loops are one of the basic building blocks for creating programs. There are times when we wish to execute the statements more than once, in which case loops are used. A loop is a sequence of instruction that iterates a statement based on certain conditions and then executes block or blocks of code repeatedly until the condition is satisfied. A loop allows us to write short pieces of code that runs multiple times rather than writing one long set of instructions to accomplish a task. Loop helps run a shorter piece of instruction a number of times, which gives your program greater versatility in working in the real world.

That being said, there are three ways by which we can repeat required instructions: for loop, while loop, and do-while loop. We are here to discuss while and do-while loops. The while and do-while loops are used when you do not know exactly how many times a loop should repeat. The difference lies in the place where the condition is tested. The while loop tests the condition before executing any of the statements within the while loop whereas the do-while loop tests the condition after the statements have been executed within the loop. Let’s discuss the differences between the two in detail.

What is While Loop?

The while loop is the most basic looping structure used in programming and is used where the number of iterations are unknown. This means the while loop will repeat executing a set of statements infinite number of times until a certain condition is met. The while loop is used to execute a block of code until the condition is true, meaning the loop keeps running until the required condition is met. That could happen on the first try or the twenty-fifth try. Since you do not know in advance when a loop should end, you have to let the loop keep running until a certain condition is satisfied. It iterates the loop until the condition is false and the condition can be any Boolean expression.

What is Do-While Loop?

The do-while loop is similar to the while loop except it checks the condition only after it runs through its instructions and the do-while loop always runs at least once. It performs the statements inside the loop exactly once before evaluating the condition of the loop. If the condition is met, the statements inside the loop execute again and if the condition is false, the control is transferred to the statement immediately following the while condition. The do-while loop guarantees that the body is always executed at least once, regardless of whether the condition is met, unlike the while loop, which can be skipped entirely if the condition is false the first time. It is ideal when you do not know the exact number of iterations.

Difference between While and Do-While Loop

Structure

– The while loop is the most basic looping structure used in programming and is used where the number of iterations are unknown. The while loop is used to execute a block of code until the condition is true, meaning the loop keeps running until the required condition is met. The do-while loop is very similar to the while loop except it performs the statements inside the loop exactly once before evaluating the condition of the loop and it runs at least once, regardless of whether the condition is met.

Condition

– The do-while loop is almost identical to the while loop except the condition is always executed after the body of the loop. In a while loop, the body is executed only if a certain condition is met and it terminates when the condition is false. That could happen on the first try or the twenty-fifth try. The do-while loop, on the other hand, guarantees that the body is always executed at least once, regardless of whether the condition is false during the first try. Unlike the while loop, the condition to stop the loop does not tested until after the statements in the loop have executed.

Syntax

– The basic format of a while loop is:

while (condition)

{     statement 1;

      statement 2;

      .

      .

      statement(n);

  }

The while loop evaluates the condition at the beginning of the loop before the statement(s) inside the block execute. The keyword while is followed by a condition, which is enclosed in parenthesis, followed by statements. The condition must evaluate to either True or False value.

The basic format of a do-while loop is:

do {

       .

       statements // body of loop;

       } while (condition);

Here, the while keyword and the condition aren’t coded until after the body of the loop. This means we do not have to check the while condition on the very first entry into the loop. It executes the statements first before evaluating the condition of the loop.

While vs. Do-While Loop: Comparison Chart

Summary of While vs. Do-While Loop

In a nutshell, the structure of a while loop is very similar to that of a do-while loop, but the main difference lies in the fact that the while loop evaluates the condition first before executing the statements whereas the do-while loop executes the statements first before evaluating the condition of the loop. Since the do-while loop checks the condition only after it runs through its instructions, the loop always runs at least once. The while loop, on the other hand, will not execute the statements if the condition of the loop turns out to be false during the first attempt.

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