Do-while loop execution model in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a do-while loop in PHP?

A do-while loop is a control structure that executes a block of code once, and then repeats the loop as long as a specified condition is true.

beginner

How does the execution order of a do-while loop differ from a while loop?

A do-while loop executes the code block first, then checks the condition. A while loop checks the condition first before executing the code block.

beginner

What will happen if the condition in a do-while loop is false on the first check?

The code block will still run once before the condition is checked and the loop stops.

beginner

Write the basic syntax of a do-while loop in PHP.

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

beginner

Why might you choose a do-while loop over a while loop?

You use a do-while loop when you want the code inside the loop to run at least once, regardless of the condition.

In a do-while loop, when is the condition checked?

A After executing the loop body once

B Before executing the loop body

C At the same time as executing the loop body

D Only if the loop body has executed more than once

The do-while loop always executes the loop body first, then checks the condition.

What will this PHP code output? $i = 5; do { echo $i . ' '; $i++; } while ($i < 5);

A No output

B 5 6

C 5

D Infinite loop

The loop runs once printing 5, then checks condition ($i < 5) which is false, so it stops.

Which of these is true about do-while loops?

A They are identical to while loops

B They never run if the condition is false initially

C They check the condition before running the loop body

D They always run the loop body at least once

Do-while loops execute the loop body first, so they always run at least once.

What keyword starts a do-while loop in PHP?

A while

B do

C loop

D repeat

The do-while loop starts with the keyword 'do'.

If you want to ensure a block of code runs at least once, which loop should you use?

A do-while loop

B while loop

C for loop

D foreach loop

The do-while loop guarantees the code runs once before checking the condition.

Explain how the do-while loop executes in PHP and why it might be useful.

Think about when you want code to run before checking a condition.

Write a simple PHP do-while loop that prints numbers from 1 to 3.

Start with a variable at 1 and increase it 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()