Do-while loop execution model in PHP

Introduction

A do-while loop runs a block of code at least once, then repeats it while a condition is true.

When you want to run code first, then check if it should repeat.

When you need to ask a user for input and repeat until valid.

When you want to process something at least once before stopping.

When the condition depends on the code that runs inside the loop.

Syntax

PHP

do {
    // code to run
} while (condition);

The code inside do { } runs first, before the condition is checked.

The loop repeats only if the condition after while is true.

Examples

This prints numbers 1 to 3 because the loop runs first, then checks if count is less or equal to 3.

PHP

<?php
$count = 1;
do {
    echo "Count is $count\n";
    $count++;
} while ($count <= 3);
?>

This prints "Hello" once because the condition is false at start, but the loop runs once before checking.

PHP

<?php
$x = 5;
do {
    echo "Hello\n";
} while ($x < 3);
?>

Sample Program

This program prints numbers from 1 to 5. It runs the code first, then checks if the number is still 5 or less to continue.

PHP

<?php
$number = 1;
do {
    echo "Number: $number\n";
    $number++;
} while ($number <= 5);
?>

Important Notes

Unlike a while loop, a do-while loop always runs the code block at least once.

Remember to update variables inside the loop to avoid infinite loops.

Summary

A do-while loop runs code first, then checks the condition.

It is useful when the code must run at least once.

Use it when the condition depends on the code inside the loop.

Practice

1. What is the main difference between a do-while loop and a while loop in PHP?

easy

A. while loop always executes the code block at least once before checking the condition.

B. do-while loop executes the code block at least once before checking the condition.

C. do-while loop never checks the condition.

D. while loop executes the code block only if the condition is false.

Solution

1. Step 1: Understand the execution order of do-while loop

   A do-while loop runs the code block first, then checks the condition to decide if it should repeat.

2. Step 2: Compare with while loop behavior

   A while loop checks the condition first before running the code block, so it may not run at all if the condition is false initially.

3. Final Answer:

   do-while loop executes the code block at least once before checking the condition. -> Option B

4. Quick Check:

   do-while runs code once before condition check [OK]

Hint: Remember: do-while runs code first, then checks condition [OK]

Common Mistakes:

• Thinking while loop runs code before condition check
• Believing do-while skips condition check
• Confusing execution order of loops

2. Which of the following is the correct syntax for a do-while loop in PHP?

easy

A. do { /* code */ } while (condition);

B. do (condition) { /* code */ };

C. while (condition) { /* code */ } do;

D. do { /* code */ } until (condition);

Solution

1. Step 1: Recall PHP do-while syntax

   The correct syntax is: do { /* code */ } while (condition); with a semicolon after the while condition.

2. Step 2: Identify incorrect options

   Options A, C, and D use wrong keywords or misplaced parentheses, which are invalid in PHP.

3. Final Answer:

   do { /* code */ } while (condition); -> Option A

4. Quick Check:

   Correct do-while syntax ends with semicolon [OK]

Hint: Syntax: do { } while(condition); ends with semicolon [OK]

Common Mistakes:

• Omitting semicolon after while condition
• Using 'until' instead of 'while'
• Placing condition before code block

3. What will be the output of the following PHP code?

$i = 3;
do {
    echo $i . ' ';
    $i--;
} while ($i > 0);

medium

A. 3 2 1

B. 2 1 0

C. 3 2 1 0

D. 1 2 3

Solution

1. Step 1: Trace the loop iterations

   Initial value: $i = 3. Loop prints $i then decrements it. Iterations:
   1) print 3, $i=2
   2) print 2, $i=1
   3) print 1, $i=0

2. Step 2: Check loop condition after each iteration

   Loop continues while $i > 0. After printing 1, $i becomes 0, condition fails, loop stops.

3. Final Answer:

   3 2 1 -> Option A

4. Quick Check:

   Loop prints 3,2,1 then stops [OK]

Hint: do-while runs once before condition; count down from 3 [OK]

Common Mistakes:

• Including 0 in output
• Starting output from 1 instead of 3
• Confusing pre- and post-decrement

4. Identify the error in this PHP do-while loop:

$count = 1;
do {
    echo $count;
    $count++
} while ($count <= 3);

medium

A. Using do instead of while

B. Incorrect loop condition syntax

C. Missing semicolon after $count++

D. Variable $count not initialized

Solution

1. Step 1: Check each line for syntax errors

   The line $count++ is missing a semicolon at the end, which is required in PHP.

2. Step 2: Verify other parts of the loop

   The loop condition and variable initialization are correct. The use of do is proper for a do-while loop.

3. Final Answer:

   Missing semicolon after $count++ -> Option C

4. Quick Check:

   Every statement must end with semicolon [OK]

Hint: Check for missing semicolons inside loop [OK]

Common Mistakes:

• Forgetting semicolon after statements
• Misplacing loop condition parentheses
• Confusing do-while with while syntax

5. You want to read user input at least once and continue reading while the input is not empty. Which PHP do-while loop correctly implements this?

do {
    $input = trim(fgets(STDIN));
    // process input
} ???;

hard

A. while ($input !== '');

B. while ($input);

C. while ($input == '');

D. while (!empty($input));

Solution

1. Step 1: Understand the input condition

   We want to continue reading while input is not empty. The PHP function empty() returns true if input is empty.

2. Step 2: Choose the correct condition

   Using while (!empty($input)); means loop continues while input has some content, stopping when empty.

3. Final Answer:

   while (!empty($input)); -> Option D

4. Quick Check:

   Loop continues while input is not empty [OK]

Hint: Use while(!empty($input)) to loop until input is empty [OK]

Common Mistakes:

• Using equality to empty string incorrectly
• Using condition that stops loop too early
• Confusing empty() with isset()