Array chunk and pad in PHP

Introduction

We use array chunk and pad to split a big list into smaller parts and make sure each part has the same size by adding extra values if needed.

When you want to show items in groups, like pages of products on a website.

When you need to process data in small pieces instead of all at once.

When you want to fill missing spots in a list to keep the size consistent.

When preparing data for tables where each row must have the same number of columns.

Syntax

PHP

<?php
// Array chunk
$chunks = array_chunk(array $array, int $size, bool $preserve_keys = false);

// Array pad
$padded_array = array_pad(array $array, int $size, mixed $value);
?>

array_chunk splits an array into smaller arrays of a given size.

array_pad adds extra values to an array until it reaches the desired size.

Examples

Splits the array into chunks of 2 elements each.

PHP

<?php
$numbers = [1, 2, 3, 4, 5];
$chunks = array_chunk($numbers, 2);
print_r($chunks);
?>

Chunking an empty array returns an empty array.

PHP

<?php
$empty = [];
$chunks = array_chunk($empty, 3);
print_r($chunks);
?>

Chunking a single-element array with size 2 returns one chunk with that element.

PHP

<?php
$single = [10];
$chunks = array_chunk($single, 2);
print_r($chunks);
?>

Pads the array to size 5 by adding zeros at the end.

PHP

<?php
$values = [1, 2];
$padded = array_pad($values, 5, 0);
print_r($padded);
?>

Sample Program

This program shows how to split an array into chunks of 2 items and then pad the last chunk to have exactly 3 items by adding the word 'empty'.

PHP

<?php
// Original array
$fruits = ['apple', 'banana', 'cherry', 'date', 'elderberry'];

// Show original array
echo "Original array:\n";
print_r($fruits);

// Split into chunks of 2
$fruit_chunks = array_chunk($fruits, 2);
echo "\nArray chunks (size 2):\n";
print_r($fruit_chunks);

// Pad the last chunk to size 3 with 'empty'
$last_chunk_index = count($fruit_chunks) - 1;
$fruit_chunks[$last_chunk_index] = array_pad($fruit_chunks[$last_chunk_index], 3, 'empty');

echo "\nLast chunk after padding to size 3:\n";
print_r($fruit_chunks[$last_chunk_index]);
?>

Important Notes

Time complexity: array_chunk runs in O(n) where n is the number of elements; array_pad also runs in O(m) where m is the padding size.

Space complexity: Both create new arrays, so space grows with the size of the input and output.

A common mistake is forgetting that array_chunk returns a new array of arrays, not modifying the original.

Use array_chunk when you want to split data into parts; use array_pad when you want to fill missing spots to keep size consistent.

Summary

array_chunk splits an array into smaller arrays of a fixed size.

array_pad adds extra values to an array to reach a desired size.

These functions help organize and prepare data for display or processing in equal-sized groups.

Practice

1. What does the PHP function array_chunk do?

easy

A. Splits an array into smaller arrays of a fixed size

B. Adds elements to the end of an array to reach a certain size

C. Removes duplicate values from an array

D. Sorts an array in ascending order

Solution

1. Step 1: Understand the function purpose

   array_chunk is designed to split a large array into smaller arrays each having a fixed number of elements.

2. Step 2: Compare with other options

   Adding elements to reach a size is done by array_pad, not array_chunk. Removing duplicates or sorting are unrelated functions.

3. Final Answer:

   Splits an array into smaller arrays of a fixed size -> Option A

4. Quick Check:

   array_chunk = split array [OK]

Hint: Remember: chunk means break into pieces [OK]

Common Mistakes:

• Confusing array_chunk with array_pad
• Thinking it sorts or removes duplicates
• Assuming it modifies the original array in place

2. Which of the following is the correct syntax to pad an array $arr to size 5 with the value 0 using array_pad?

easy

A. $padded = array_pad($arr, 5);

B. $padded = array_pad(5, $arr, 0);

C. $padded = array_pad($arr, 5, 0);

D. $padded = array_pad($arr, 0, 5);

Solution

1. Step 1: Recall array_pad syntax

   The correct syntax is array_pad(array, size, value), where the first argument is the array, second is the desired size, third is the padding value.

2. Step 2: Check each option

   $padded = array_pad($arr, 5, 0); matches the correct order and arguments. Options B and C have wrong argument order. $padded = array_pad($arr, 5); misses the padding value.

3. Final Answer:

   $padded = array_pad($arr, 5, 0); -> Option C

4. Quick Check:

   array_pad(array, size, value) correct order [OK]

Hint: Remember: array_pad(array, size, value) [OK]

Common Mistakes:

• Swapping argument order
• Omitting the padding value
• Passing size as first argument

3. What is the output of the following PHP code?

$arr = [1, 2, 3, 4, 5];
$chunks = array_chunk($arr, 2);
print_r($chunks);

medium

A. [[1, 2, 3], [4, 5]]

B. [[1, 2], [3, 4], [5]]

C. [[1, 2], [3, 4], [5, null]]

D. [1, 2, 3, 4, 5]

Solution

1. Step 1: Understand array_chunk behavior

   array_chunk splits the array into smaller arrays each with 2 elements, except the last chunk which may have fewer if elements run out.

2. Step 2: Apply to given array

   Splitting [1,2,3,4,5] by 2 gives chunks: [1,2], [3,4], and [5]. No padding is added by array_chunk.

3. Final Answer:

   [[1, 2], [3, 4], [5]] -> Option B

4. Quick Check:

   array_chunk splits into groups of 2 [OK]

Hint: Last chunk may be smaller if not divisible [OK]

Common Mistakes:

• Expecting padding in array_chunk
• Confusing chunk size with number of chunks
• Assuming all chunks have equal length always

4. Identify the error in this PHP code snippet:

$arr = [1, 2];
$padded = array_pad($arr, 5);
print_r($padded);

medium

A. print_r cannot print arrays

B. array_pad cannot increase array size

C. array_pad requires the size to be smaller than original

D. Missing the padding value argument in array_pad

Solution

1. Step 1: Check array_pad function signature

   array_pad requires three arguments: the array, the new size, and the value to pad with.

2. Step 2: Identify missing argument

   The code calls array_pad with only two arguments, missing the padding value, which causes an error.

3. Final Answer:

   Missing the padding value argument in array_pad -> Option D

4. Quick Check:

   array_pad needs 3 arguments [OK]

Hint: array_pad needs array, size, and value [OK]

Common Mistakes:

• Forgetting the padding value
• Assuming array_pad works with two arguments
• Thinking print_r can't print arrays

5. You have an array $data = [10, 20, 30]. You want to split it into chunks of size 2 and ensure each chunk has exactly 2 elements by padding with 0 if needed. Which code correctly achieves this?

hard

A. $chunks = array_chunk($data, 2);
foreach ($chunks as &$chunk) {
$chunk = array_pad($chunk, 2, 0);
}
print_r($chunks);

B. $chunks = array_pad(array_chunk($data, 2), 2, 0);
print_r($chunks);

C. $chunks = array_chunk(array_pad($data, 5, 0), 2);
print_r($chunks);

D. $chunks = array_chunk($data, 2, 0);
print_r($chunks);

Solution

1. Step 1: Understand the goal

   We want chunks of size 2, each chunk exactly length 2, padding with 0 if chunk is smaller.

2. Step 2: Analyze each option

   $chunks = array_chunk($data, 2); foreach ($chunks as &$chunk) { $chunk = array_pad($chunk, 2, 0); } print_r($chunks); splits into chunks, then pads each chunk to size 2 with 0, which is correct.
   $chunks = array_pad(array_chunk($data, 2), 2, 0); print_r($chunks); tries to pad the array of chunks, which is incorrect.
   $chunks = array_chunk(array_pad($data, 5, 0), 2); print_r($chunks); pads the original array to size 5, then chunks, which may add unwanted extra elements.
   $chunks = array_chunk($data, 2, 0); print_r($chunks); passes 0 as third argument to array_chunk, which is invalid.

3. Final Answer:

   $chunks = array_chunk($data, 2); foreach ($chunks as &$chunk) { $chunk = array_pad($chunk, 2, 0); } print_r($chunks); -> Option A

4. Quick Check:

   Pad each chunk after chunking [OK]

Hint: Pad each chunk after chunking, not the whole array [OK]

Common Mistakes:

• Padding the whole array before chunking
• Passing extra arguments to array_chunk
• Trying to pad the array of chunks directly