Nested loop execution in PHP

Introduction

Nested loops let you repeat actions inside other repeated actions. This helps when you work with tables or grids.

When you want to print a multiplication table.

When you need to check every pair of items in two lists.

When you want to create a grid of rows and columns.

When you want to compare each element with every other element.

When you want to generate combinations of items.

Syntax

PHP

<?php
for (initialization; condition; increment) {
    for (initialization; condition; increment) {
        // code to repeat
    }
}
?>

The inner loop runs completely every time the outer loop runs once.

Use different variable names for each loop counter to avoid confusion.

Examples

This prints pairs of numbers where the first number goes from 1 to 3 and the second from 1 to 2.

PHP

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

This prints a 2 by 3 grid of stars.

PHP

<?php
for ($row = 1; $row <= 2; $row++) {
    for ($col = 1; $col <= 3; $col++) {
        echo "*";
    }
    echo "\n";
}
?>

Sample Program

This program prints a 3 by 3 multiplication grid. Each row shows multiples of the row number.

PHP

<?php
// Print a 3x3 grid of numbers
for ($i = 1; $i <= 3; $i++) {
    for ($j = 1; $j <= 3; $j++) {
        echo $i * $j . " ";
    }
    echo "\n";
}
?>

Important Notes

Remember to add a newline after the inner loop to separate rows.

Nested loops can slow down your program if they run many times, so use them carefully.

Summary

Nested loops run one loop inside another to handle repeated tasks in layers.

The inner loop finishes all its cycles for each single cycle of the outer loop.

They are useful for working with tables, grids, and pairs of data.

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