Array chunk and pad in PHP - Step-by-Step Execution

Concept Flow - Array chunk and pad

Start with original array

↓

Split array into chunks of size N

↓

Check last chunk size

↓

Done

↓

Done

We start with an array, split it into smaller arrays (chunks) of a fixed size, then if the last chunk is smaller, we add extra values to fill it.

Execution Sample

PHP

<?php
$array = [1, 2, 3, 4, 5];
$chunks = array_chunk($array, 2);
$chunks[count($chunks) - 1] = array_pad($chunks[count($chunks) - 1], 2, 0);
print_r($chunks);
?>

This code splits an array into chunks of 2 elements, then pads the last chunk with zeros if it has less than 2 elements.

Execution Table

Step	Action	Array State	Chunk Created	Last Chunk Size	Padding Applied
1	Start with array	[1, 2, 3, 4, 5]	-	-	-
2	Chunk array into size 2	[1, 2, 3, 4, 5]	[[1,2],[3,4],[5]]	3rd chunk size = 1	-
3	Check last chunk size	[[1,2],[3,4],[5]]	-	1 < 2 (chunk size)	No
4	Pad last chunk with 0	[[1,2],[3,4],[5]]	-	1 < 2	Last chunk padded to [5,0]
5	Final chunks	[[1,2],[3,4],[5,0]]	-	-	-

💡 Last chunk size reached desired chunk size after padding, process complete.

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
$array	[1,2,3,4,5]	[1,2,3,4,5]	[1,2,3,4,5]	[1,2,3,4,5]
$chunks	N/A	[[1,2],[3,4],[5]]	[[1,2],[3,4],[5,0]]	[[1,2],[3,4],[5,0]]
Last chunk size	N/A	1	2	2

Key Moments - 2 Insights

Why do we need to pad only the last chunk?

Because only the last chunk can be smaller than the chunk size after splitting. Earlier chunks are always full size (see execution_table step 2 and 3).

What happens if the original array size is exactly divisible by chunk size?

No padding is needed because all chunks are full size. The check in step 3 will find last chunk size equals chunk size, so no padding is applied.

Visual Quiz - 3 Questions

Look at the execution table, what is the size of the last chunk after step 2?

A 1

B 2

C 3

D 0

💡 Hint

Check the 'Last Chunk Size' column at step 2 in the execution_table.

At which step is padding applied to the last chunk?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look for 'Padding Applied' column in execution_table where padding is mentioned.

If the original array was [1,2,3,4], how many chunks would be created with chunk size 2?

A 3

B 1

C 2

D 4

💡 Hint

Divide array length 4 by chunk size 2 and check execution_table logic.

Concept Snapshot

array_chunk(array, size) splits array into chunks of given size.
array_pad(array, size, value) adds value to array until it reaches size.
Use array_chunk first, then pad only the last chunk if smaller.
Padding fills missing elements to keep chunk size consistent.
No padding needed if last chunk is already full size.

Full Transcript

This example shows how to split an array into smaller parts called chunks using array_chunk. Each chunk has a fixed size. If the last chunk is smaller, we add extra values using array_pad to fill it. We start with the original array, split it, check the last chunk size, and pad if needed. This ensures all chunks have the same size. Padding is only done on the last chunk because earlier chunks are always full. If the array size divides evenly by chunk size, no padding is needed. This process helps when you want equal-sized groups from an array.

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