Difference Between While and Do-While Loop

In programming languages like C, C++, and Java, loops are used to execute a block of code repeatedly. The while loop and do-while loop perform similar functions but have one fundamental difference in how they evaluate their conditions.

Question (Click to Flip)

What is the main difference between a while and do-while loop?

Answer

The main difference is that a while loop checks the condition before executing the code (may run 0 times), whereas a do-while loop executes the code first and checks the condition after (always runs at least 1 time).

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Key Facts

While Loop: Entry-controlled. Condition checked first.

Do-While Loop: Exit-controlled. Code runs first, condition checked later.

Crucial Difference: A do-while loop always executes at least ONE time, even if the condition is false.

1. While Loop

Type: It is an entry-controlled loop.
Mechanism: The condition is checked before the code block is executed.
Execution: If the condition is false initially, the loop body will not execute even once.
Syntax:

while (condition) {
    // block of code
}

2. Do-While Loop

Type: It is an exit-controlled loop.
Mechanism: The condition is checked after the code block is executed.
Execution: The loop body will execute at least once, regardless of whether the condition is true or false.
Syntax:

do {
    // block of code
} while (condition);

Summary Comparison

Feature	While Loop	Do-While Loop
Condition Check	Top of the loop (Entry)	Bottom of the loop (Exit)
Minimum Execution	0 times (if condition is false)	1 time (always)
Semicolon	No semicolon at the end.	Semicolon is required at the end: `while(condition);`

Questions and Answers

What is the main difference between a while and do-while loop?+

Which loop is known as an exit-controlled loop?+

The do-while loop is known as an exit-controlled loop because the condition is evaluated at the exit (bottom) of the loop.

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