Do-while loop execution model in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple do-while loop

What is the output of this PHP code?

PHP

<?php
$i = 1;
do {
    echo $i . ' ';
    $i++;
} while ($i <= 3);
?>

A No output

B 1 2

C 1 2 3

D 2 3

💡 Hint

Remember, do-while runs the loop body at least once before checking the condition.

The loop starts with i=1, prints it, increments i, then checks if i <= 3. It repeats until i becomes 4, so it prints 1 2 3.

❓ Predict Output

intermediate

Do-while loop with initial false condition

What will this PHP code output?

PHP

<?php
$j = 5;
do {
    echo $j . ' ';
    $j++;
} while ($j < 5);
?>

A 5

B No output

C 5 6

D Syntax error

💡 Hint

Do-while executes the block once before checking the condition.

Even though the condition is false initially, the loop body runs once printing 5, then condition fails and loop stops.

🔧 Debug

advanced

Identify the error in this do-while loop

What error does this PHP code produce?

PHP

<?php
$k = 0;
do {
    echo $k . ' ';
    $k++
} while ($k < 3);
?>

A SyntaxError: missing semicolon

B RuntimeError: infinite loop

C No error, outputs '0 1 2 '

D TypeError: invalid condition

💡 Hint

Check punctuation carefully inside the loop body.

The line '$k++' is missing a semicolon at the end, causing a syntax error.

❓ Predict Output

advanced

Counting iterations in a do-while loop

How many times will the loop body execute in this PHP code?

PHP

<?php
$count = 0;
$num = 10;
do {
    $count++;
    $num += 5;
} while ($num < 25);
echo $count;

A 5

B 2

C 4

D 3

💡 Hint

Trace the value of $num each iteration and count how many times the loop runs.

Initial $num=10, after 1st iteration 15, 2nd iteration 20, 3rd iteration 25. Loop stops after 3 iterations because condition fails when $num=25.

🧠 Conceptual

expert

Understanding do-while loop execution order

Which statement best describes the execution model of a do-while loop in PHP?

A The condition is checked before the loop body runs, so the body may not run at all.

B The loop body runs once before the condition is checked, guaranteeing at least one execution.

C The loop body and condition are evaluated simultaneously, so execution depends on both.

D The loop body runs only if the condition is true at the start and after each iteration.

💡 Hint

Think about what makes do-while different from while loops.

In a do-while loop, the body executes first, then the condition is checked. This ensures the loop runs at least once.

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()