Array chunk and pad in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of array_chunk with padding

What is the output of the following PHP code?

PHP

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

A Array ( [0] => Array ( [0] => 1 [1] => 2 ) [1] => Array ( [2] => 3 [3] => 4 ) )

B Array ( [0] => Array ( [0] => 1 [1] => 2 ) [1] => Array ( [0] => 3 [1] => 4 ) [2] => Array ( [0] => 5 ) )

C Array ( [0] => Array ( [0] => 1 [1] => 2 ) [1] => Array ( [2] => 3 [3] => 4 ) [2] => Array ( [0] => 5 ) )

D Array ( [0] => Array ( [0] => 1 [1] => 2 ) [1] => Array ( [2] => 3 [3] => 4 ) [2] => Array ( [4] => 5 ) )

💡 Hint

Remember that the third parameter true preserves keys in array_chunk.

When array_chunk is called with preserve_keys=true, the original keys are kept in each chunk. So the keys 0,1 remain in the first chunk, 2,3 in the second, and 4 in the last chunk.

❓ Predict Output

intermediate

Output of array_pad with negative size

What is the output of this PHP code snippet?

PHP

<?php
$array = [10, 20];
$padded = array_pad($array, -5, 0);
print_r($padded);
?>

A Array ( [0] => 0 [1] => 0 [2] => 0 [3] => 10 [4] => 20 )

B Fatal error: array_pad(): Size must be greater than zero

C Array ( [0] => 10 [1] => 20 )

D Array ( [0] => 10 [1] => 20 [2] => 0 [3] => 0 [4] => 0 )

💡 Hint

Negative size pads at the beginning of the array.

array_pad with a negative size pads the array at the beginning until the array length reaches the absolute value of the size. So zeros are added before the original elements.

🔧 Debug

advanced

Identify the error in array_chunk usage

What error does this PHP code produce?

PHP

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

A Array ( )

B Fatal error: array_chunk() expects parameter 2 to be integer, string given

C Warning: array_chunk(): Size parameter expected to be greater than 0

D No output, script runs silently

💡 Hint

Check the minimum allowed value for the size parameter in array_chunk.

array_chunk requires the size parameter to be a positive integer greater than zero. Passing zero triggers a warning.

❓ Predict Output

advanced

Result length after array_pad

How many elements does the array have after this code runs?

PHP

<?php
$array = [5, 6, 7];
$padded = array_pad($array, 6, 9);
echo count($padded);
?>

A 6

B 9

C 3

D Error

💡 Hint

array_pad extends the array to the specified size if needed.

array_pad adds elements to the array until its length matches the absolute value of the size parameter. Here, it pads from 3 to 6 elements.

🧠 Conceptual

expert

Understanding array_chunk with preserve_keys and array_pad combined

Given the code below, what is the output?

PHP

<?php
$array = [0 => 'a', 1 => 'b', 2 => 'c'];
$chunks = array_chunk($array, 2, true);
$chunks[1] = array_pad($chunks[1], 3, 'x');
print_r($chunks);
?>

A Array ( [0] => Array ( [0] => a [1] => b ) [1] => Array ( [2] => c [0] => x [1] => x ) )

B Array ( [0] => Array ( [0] => a [1] => b ) [1] => Array ( [2] => c [3] => x [4] => x ) )

C Array ( [0] => Array ( [0] => a [1] => b ) [1] => Array ( [2] => c [3] => x ) )

D Array ( [0] => Array ( [0] => a [1] => b ) [1] => Array ( [2] => c ) )

💡 Hint

array_chunk with preserve_keys keeps original keys; array_pad adds elements with numeric keys starting from 0.

array_chunk with preserve_keys keeps keys 0 and 1 in first chunk, 2 in second. array_pad pads the second chunk to length 3, adding elements with keys 3 and 4 because keys 0 and 1 are taken by original keys in chunk 1.

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