Nested loop execution in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested for loops with break

What is the output of this PHP code?

PHP

<?php
for ($i = 1; $i <= 3; $i++) {
    for ($j = 1; $j <= 3; $j++) {
        if ($j == 2) {
            break;
        }
        echo "$i$j ";
    }
}
?>

A 12 22 32

B 11 12 21 22 31 32

C 11 12 13 21 22 23 31 32 33

D 11 21 31

💡 Hint

Remember that break exits the inner loop immediately when condition is met.

The inner loop breaks when $j == 2, so only $j == 1 is printed for each $i. So output is 11 21 31.

❓ Predict Output

intermediate

Output of nested while loops with continue

What is the output of this PHP code?

PHP

<?php
$i = 1;
while ($i <= 2) {
    $j = 1;
    while ($j <= 3) {
        if ($j == 2) {
            $j++;
            continue;
        }
        echo "$i$j ";
        $j++;
    }
    $i++;
}
?>

A 11 13 12 21 23 22

B 11 13 21 23

C 11 12 13 21 22 23

D 12 22

💡 Hint

The continue skips printing when $j == 2 but increments $j before continuing.

When $j == 2, it increments $j and continues, skipping echo. So 12 and 22 are skipped.

🧠 Conceptual

advanced

Number of iterations in nested loops

How many times will the innermost statement execute in this PHP code?

PHP

<?php
for ($a = 0; $a < 4; $a++) {
    for ($b = 0; $b < 3; $b++) {
        for ($c = 0; $c < 2; $c++) {
            // innermost statement
        }
    }
}
?>

A 24

B 9

C 12

D 6

💡 Hint

Multiply the number of iterations of each loop.

4 * 3 * 2 = 24 times the innermost statement runs.

🔧 Debug

advanced

Identify the error in nested loops

What error will this PHP code produce?

PHP

<?php
for ($x = 0; $x < 3; $x++) {
    for ($y = 0; $y < 3; $y++)
        echo "$x$y ";
}
?>

A No error, outputs '000102...222'

B Parse error: syntax error, unexpected '}'

C Fatal error: Maximum execution time exceeded

D Warning: Undefined variable

💡 Hint

Check the braces carefully for matching pairs.

The inner for loop lacks braces but the closing brace after echo is extra, causing syntax error.

🚀 Application

expert

Output of nested loops with conditional break and continue

What is the output of this PHP code?

PHP

<?php
for ($i = 1; $i <= 3; $i++) {
    for ($j = 1; $j <= 3; $j++) {
        if ($i == $j) {
            continue;
        }
        if ($j == 3) {
            break;
        }
        echo "$i$j ";
    }
}
?>

A 12 13 21 31 32

B 12 21 23 31 32

C 12 21 31 32

D 12 13 21 23 31

💡 Hint

Remember continue skips current iteration, break exits inner loop.

When $i == $j, continue skips printing. When $j == 3, break exits inner loop. The output is 12 21 31 32.

Practice

1. What happens when a nested loop runs in PHP?
for ($i = 0; $i < 2; $i++) {
for ($j = 0; $j < 3; $j++) {
// code
}
}

easy

A. The inner loop runs completely for each iteration of the outer loop.

B. The outer loop runs completely for each iteration of the inner loop.

C. Both loops run only once.

D. The loops run simultaneously without nesting.

Solution

1. Step 1: Understand loop nesting order

   The outer loop runs from 0 to 1 (2 times). For each outer loop iteration, the inner loop runs fully.

2. Step 2: Inner loop completes all cycles per outer iteration

   For each $i, $j runs from 0 to 2 (3 times), completing all inner cycles before next outer iteration.

3. Final Answer:

   The inner loop runs completely for each iteration of the outer loop. -> Option A

4. Quick Check:

   Inner loop finishes fully inside outer loop [OK]

Hint: Inner loop runs fully inside each outer loop cycle [OK]

Common Mistakes:

• Thinking outer loop runs inside inner loop
• Assuming loops run only once
• Confusing simultaneous execution

2. Which of the following is the correct syntax for a nested for loop in PHP?

easy

A. for ($i = 0; $i < 2; $i++) { for $j = 0; $j < 3; $j++ { echo $i . $j; } }

B. for ($i = 0; $i < 2; $i++) { for ($j = 0; $j < 3; $j++) { echo $i . $j; } }

C. for ($i = 0; $i < 2; $i++) for ($j = 0; $j < 3; $j++) echo $i . $j;

D. for $i = 0; $i < 2; $i++ { for $j = 0; $j < 3; $j++ { echo $i . $j; } }

Solution

1. Step 1: Check PHP for loop syntax

   PHP requires parentheses around loop conditions and braces for blocks.

2. Step 2: Validate nested loop syntax

   for ($i = 0; $i < 2; $i++) { for ($j = 0; $j < 3; $j++) { echo $i . $j; } } uses correct parentheses and braces for both loops and echo statement inside.

3. Final Answer:

   for ($i = 0; $i < 2; $i++) { for ($j = 0; $j < 3; $j++) { echo $i . $j; } } -> Option B

4. Quick Check:

   Correct parentheses and braces in nested loops [OK]

Hint: Use parentheses and braces properly in PHP loops [OK]

Common Mistakes:

• Omitting parentheses around loop conditions
• Missing braces for loop blocks
• Incorrect for loop syntax without $ sign

3. What is the output of this PHP code?

for ($i = 1; $i <= 2; $i++) {
  for ($j = 1; $j <= 3; $j++) {
    echo $i * $j . ' ';
  }
}

medium

A. 1 2 3 1 2 3

B. 1 4 9 2 5 8

C. 1 2 3 2 4 6

D. 6 12 18 6 12 18

Solution

1. Step 1: Trace outer loop values

   $i runs from 1 to 2.

2. Step 2: Trace inner loop and calculate products

   For $i=1: $j=1,2,3 -> 1*1=1, 1*2=2, 1*3=3
   For $i=2: $j=1,2,3 -> 2*1=2, 2*2=4, 2*3=6

3. Final Answer:

   1 2 3 2 4 6 -> Option C

4. Quick Check:

   Multiply outer and inner loop counters [OK]

Hint: Multiply outer and inner counters for output [OK]

Common Mistakes:

• Mixing up loop variable values
• Printing inner loop values only
• Ignoring multiplication in echo

4. Find the error in this nested loop PHP code:

for ($i = 0; $i < 3; $i++) {
  for ($j = 0; $j < 2; $i++) {
    echo $i . $j . ' ';
  }
}

medium

A. Inner loop increments $i instead of $j causing infinite loop.

B. Outer loop condition is incorrect.

C. Missing semicolon after echo statement.

D. Variables $i and $j are not declared.

Solution

1. Step 1: Check inner loop increment variable

   The inner loop increments $i instead of $j, which is wrong.

2. Step 2: Understand effect of wrong increment

   Incrementing $i inside inner loop changes outer loop variable unexpectedly, causing infinite or wrong behavior.

3. Final Answer:

   Inner loop increments $i instead of $j causing infinite loop. -> Option A

4. Quick Check:

   Increment correct loop variable to avoid errors [OK]

Hint: Increment inner loop variable inside inner loop [OK]

Common Mistakes:

• Incrementing wrong variable in inner loop
• Confusing loop conditions
• Assuming variables need declaration in PHP

5. You want to print a multiplication table from 1 to 3 using nested loops in PHP. Which code correctly prints the table with each row on a new line?

?

hard

A. for ($i = 1; $i <= 3; $i++) { for ($j = 1; $j <= 3; $j++) { echo $i * $j; } echo "\n"; }

B. for ($i = 1; $i <= 3; $i++) { for ($j = 1; $j <= 3; $j++) { echo $i * $j . "\n"; } }

C. for ($i = 1; $i <= 3; $i++) { for ($j = 1; $j <= 3; $j++) { echo $i * $j . ' '; } }

D. for ($i = 1; $i <= 3; $i++) { for ($j = 1; $j <= 3; $j++) { echo $i * $j . ' '; } echo "\n"; }

Solution

1. Step 1: Understand output format

   Each row should print products separated by spaces, then a newline after inner loop ends.

2. Step 2: Check each option for correct newline placement

   for ($i = 1; $i <= 3; $i++) { for ($j = 1; $j <= 3; $j++) { echo $i * $j . ' '; } echo "\n"; } prints products with spaces inside inner loop, then newline after inner loop completes, correctly formatting rows.

3. Final Answer:

   for ($i = 1; $i <= 3; $i++) { for ($j = 1; $j <= 3; $j++) { echo $i * $j . ' '; } echo "\n"; } -> Option D

4. Quick Check:

   Newline after inner loop completes for each row [OK]

Hint: Print newline after inner loop to separate rows [OK]

Common Mistakes:

• Printing newline inside inner loop
• Missing spaces between numbers
• No newline causing all output in one line