For loop execution model in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple for loop with break

What is the output of this PHP code?

PHP

<?php
for ($i = 0; $i < 5; $i++) {
    if ($i == 3) {
        break;
    }
    echo $i . ' ';
}
?>

A 0 1 2

B 0 1 2 3

C 0 1 2 3 4

D 1 2 3

💡 Hint

The loop stops when the condition inside the if is true.

The loop starts at 0 and prints each number. When i equals 3, the break stops the loop before printing 3.

❓ Predict Output

intermediate

For loop with continue statement

What will this PHP code output?

PHP

<?php
for ($i = 0; $i < 5; $i++) {
    if ($i % 2 == 0) {
        continue;
    }
    echo $i . ' ';
}
?>

A 1 3

B 0 2 4

C 1 3 5

D 0 1 2 3 4

💡 Hint

The continue skips the current loop iteration when the number is even.

The loop prints only odd numbers because it skips even numbers using continue.

❓ Predict Output

advanced

For loop with multiple expressions in control

What is the output of this PHP code?

PHP

<?php
for ($i = 0, $j = 5; $i < 3; $i++, $j--) {
    echo $i . '-' . $j . ' ';
}
?>

A 0-5 1-4

B 0-5 1-4 2-3 3-2

C 0-5 1-4 2-3

D 5-0 4-1 3-2

💡 Hint

Both variables change each loop iteration.

The loop runs 3 times. Each time, i increases and j decreases, printing pairs.

❓ Predict Output

advanced

For loop with missing increment expression

What will this PHP code output?

PHP

<?php
for ($i = 0; $i < 3;) {
    echo $i . ' ';
    $i++;
}
?>

A 0 1 2 3

B 0 1 2

C Infinite loop (no output ends)

D Syntax error

💡 Hint

Increment can be inside the loop body.

The loop increments i inside the body, so it prints 0,1,2 and stops.

🧠 Conceptual

expert

Understanding for loop execution order

In PHP, what is the correct order of execution for the parts of a for loop?

A 3,1,2,4

B 2,1,3,4

C 1,3,2,4

D 1,2,3,4

💡 Hint

Initialization happens once, then condition is checked before each loop.

First, initialization runs once. Then condition is checked before each iteration. If true, loop body runs, then increment runs, then condition checked again.

Practice

1. What is the role of the condition part in a PHP for loop?

easy

A. It decides if the loop should continue running or stop.

B. It sets the starting value of the loop variable.

C. It changes the loop variable after each iteration.

D. It runs only after the loop finishes.

Solution

1. Step 1: Understand the for loop parts

   A PHP for loop has three parts: initialization, condition, and increment.

2. Step 2: Identify the condition's role

   The condition is checked before each loop run to decide if the loop continues or stops.

3. Final Answer:

   It decides if the loop should continue running or stop. -> Option A

4. Quick Check:

   Condition controls loop continuation = It decides if the loop should continue running or stop. [OK]

Hint: Condition controls loop stop or continue [OK]

Common Mistakes:

• Confusing initialization with condition
• Thinking increment runs before loop starts
• Believing condition runs after loop ends

2. Which of the following is the correct syntax for a PHP for loop that counts from 0 to 4?

easy

A. for $i = 0; $i <= 4; $i++ {}

B. for ($i = 0; $i < 4; $i--) {}

C. for ($i = 0; $i <= 4; ++i) {}

D. for ($i = 0; $i <= 4; $i++) {}

Solution

1. Step 1: Check for loop syntax rules

   PHP for loops require parentheses around initialization, condition, and increment, separated by semicolons.

2. Step 2: Validate each option

   for ($i = 0; $i <= 4; $i++) {} uses correct syntax with $i starting at 0, condition $i <= 4, and increment $i++. Others have syntax errors or wrong increments.

3. Final Answer:

   for ($i = 0; $i <= 4; $i++) {} -> Option D

4. Quick Check:

   Correct for loop syntax = for ($i = 0; $i <= 4; $i++) {} [OK]

Hint: Use parentheses and semicolons correctly in for loops [OK]

Common Mistakes:

• Missing parentheses around for loop parts
• Using ++i without $ sign
• Wrong increment direction (i-- instead of i++)

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

$sum = 0;
for ($i = 1; $i <= 3; $i++) {
    $sum += $i;
}
echo $sum;

medium

A. 6

B. 123

C. 3

D. 0

Solution

1. Step 1: Trace the loop iterations

   The loop runs with $i = 1, 2, 3. Each time, $sum adds $i: 0+1=1, 1+2=3, 3+3=6.

2. Step 2: Calculate final sum

   After the loop, $sum is 6, which is printed by echo.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Sum 1+2+3 = 6 [OK]

Hint: Add loop variable each iteration to sum [OK]

Common Mistakes:

• Concatenating numbers instead of adding
• Starting loop at 0 instead of 1
• Using wrong loop condition

4. Identify the error in this PHP for loop:

for ($i = 0; $i < 5; $i) {
    echo $i;
}

medium

A. Condition should be $i <= 5.

B. Missing increment operation in the loop header.

C. Initialization is incorrect.

D. Echo statement syntax is wrong.

Solution

1. Step 1: Examine the for loop header

   The loop header has initialization ($i = 0), condition ($i < 5), but increment is just $i without ++ or --.

2. Step 2: Identify the missing increment

   Without increment like $i++, the loop variable never changes, causing an infinite loop or no progress.

3. Final Answer:

   Missing increment operation in the loop header. -> Option B

4. Quick Check:

   Increment missing = Missing increment operation in the loop header. [OK]

Hint: Ensure increment changes loop variable each time [OK]

Common Mistakes:

• Forgetting ++ or -- in increment
• Changing condition to wrong comparison
• Misplacing semicolons in loop header

5. You want to print all even numbers from 2 to 10 using a PHP for loop. Which code correctly does this?

hard

A. for ($i = 2; $i < 10; $i += 2) { echo $i . ' '; }

B. for ($i = 0; $i <= 10; $i += 2) { echo $i . ' '; }

C. for ($i = 1; $i <= 10; $i++) { if ($i % 2 == 0) echo $i . ' '; }

D. for ($i = 2; $i < 10; $i++) { if ($i % 2 == 0) echo $i . ' '; }

Solution

1. Step 1: Understand the goal

   We want to print even numbers from 2 to 10 inclusive.

2. Step 2: Check each option

   for ($i = 2; $i < 10; $i += 2) { echo $i . ' '; } prints 2 4 6 8, missing 10. for ($i = 1; $i <= 10; $i++) { if ($i % 2 == 0) echo $i . ' '; } loops 1 to 10 and prints only even numbers 2 4 6 8 10 correctly. for ($i = 2; $i < 10; $i++) { if ($i % 2 == 0) echo $i . ' '; } stops at 9, missing 10. for ($i = 0; $i <= 10; $i += 2) { echo $i . ' '; } starts at 0, which is not in the range 2 to 10.

3. Step 3: Choose the best option

   for ($i = 1; $i <= 10; $i++) { if ($i % 2 == 0) echo $i . ' '; } is correct and flexible, printing even numbers by checking inside the loop.

4. Final Answer:

   for ($i = 1; $i <= 10; $i++) { if ($i % 2 == 0) echo $i . ' '; } -> Option C

5. Quick Check:

   Loop 1-10, print if even = for ($i = 1; $i <= 10; $i++) { if ($i % 2 == 0) echo $i . ' '; } [OK]

Hint: Use modulo % to check even numbers inside loop [OK]

Common Mistakes:

• Stopping loop before 10
• Starting loop at 0 when not needed
• Forgetting to check even condition