Array filter function in PHP - Time & Space Complexity

Time Complexity: Array filter function

O(n)

Understanding Time Complexity

When we use the array filter function, we want to know how the time it takes changes as the array gets bigger.

We ask: How does filtering more items affect the work done?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

$array = [1, 2, 3, 4, 5, 6];
$filtered = array_filter($array, fn($x) => $x % 2 === 0);
print_r($filtered);
    

This code filters an array to keep only even numbers.

Identify Repeating Operations

• Primary operation: Checking each element with the callback function.
• How many times: Once for every element in the array.

How Execution Grows With Input

As the array gets bigger, the number of checks grows in the same way.

Input Size (n)	Approx. Operations
10	10 checks
100	100 checks
1000	1000 checks

Pattern observation: The work grows directly with the number of items.

Final Time Complexity

Time Complexity: O(n)

This means the time to filter grows in a straight line with the array size.

Common Mistake

[X] Wrong: "Filtering an array is faster than checking every item."

[OK] Correct: The filter must look at each item to decide if it stays, so it takes time proportional to the array size.

Interview Connect

Understanding how filtering scales helps you explain efficiency clearly and shows you know how common functions behave with bigger data.

Self-Check

"What if the callback function itself loops over another array for each element? How would the time complexity change?"

Practice

1. What does the PHP function array_filter do to an array?

easy

A. It removes all keys from the array.

B. It keeps only the elements where the callback returns true.

C. It changes all values to strings.

D. It sorts the array in ascending order.

Solution

1. Step 1: Understand the purpose of array_filter

   The function array_filter tests each element with a callback function.

2. Step 2: Determine what happens based on callback result

   Elements for which the callback returns true are kept; others are removed.

3. Final Answer:

   It keeps only the elements where the callback returns true. -> Option B

4. Quick Check:

   array_filter keeps elements with true callback [OK]

Hint: Remember: true keeps, false removes elements [OK]

Common Mistakes:

• Thinking array_filter sorts the array
• Assuming it changes values to strings
• Believing it removes keys

2. Which of the following is the correct syntax to use array_filter with a callback function named isPositive on array $numbers?

easy

A. array_filter($numbers, isPositive);

B. array_filter($numbers, &isPositive);

C. array_filter(isPositive, $numbers);

D. array_filter($numbers, 'isPositive');

Solution

1. Step 1: Recall array_filter syntax

   The second argument must be a callable, passed as a string or anonymous function.

2. Step 2: Identify correct callable format

   Passing the function name as a string, like 'isPositive', is correct.

3. Final Answer:

   array_filter($numbers, 'isPositive'); -> Option D

4. Quick Check:

   Callback function name as string [OK]

Hint: Callback function names go in quotes [OK]

Common Mistakes:

• Passing callback without quotes
• Swapping argument order
• Using & before function name

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

$arr = [1, 2, 3, 4, 5];
$result = array_filter($arr, fn($n) => $n % 2 === 0);
print_r($result);

medium

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

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

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

D. Array ( [0] => 2 [2] => 4 )

Solution

1. Step 1: Understand the callback function

   The callback keeps numbers where $n % 2 === 0, meaning even numbers.

2. Step 2: Identify even numbers and keys

   Even numbers in the array are 2 and 4 at keys 1 and 3 respectively.

3. Final Answer:

   Array ( [1] => 2 [3] => 4 ) -> Option C

4. Quick Check:

   Only even numbers kept with original keys [OK]

Hint: Check which elements return true, keys stay same [OK]

Common Mistakes:

• Assuming keys are reindexed
• Keeping odd numbers by mistake
• Confusing array keys with values

4. Identify the error in this PHP code using array_filter:

$data = [10, 15, 20];
$result = array_filter($data, function($x) {
  if ($x > 15) return true;
});
print_r($result);

medium

A. The callback function does not return false for some values.

B. array_filter requires a third argument for keys.

C. The array must be associative for array_filter to work.

D. The print_r function cannot print arrays.

Solution

1. Step 1: Check callback return values

   The callback returns true only if $x > 15, but returns nothing (null) otherwise.

2. Step 2: Understand array_filter behavior

   Callback must return true or false explicitly; null is treated as false but is unclear and risky.

3. Final Answer:

   The callback function does not return false for some values. -> Option A

4. Quick Check:

   Callback must return true or false explicitly [OK]

Hint: Always return true or false in callback [OK]

Common Mistakes:

• Missing explicit false return
• Thinking array_filter needs 3 arguments
• Believing print_r can't print arrays

5. Given an array $items = ['apple' => 3, 'banana' => 0, 'cherry' => 5, 'date' => null];, which array_filter call will keep only items with values that are positive numbers (greater than zero)?

hard

A. array_filter($items, fn($v) => $v > 0);

B. array_filter($items, fn($v) => (bool)$v);

C. array_filter($items);

D. array_filter($items, fn($v) => $v !== 0);

Solution

1. Step 1: Understand the goal

   We want to keep only values greater than zero (positive numbers).

2. Step 2: Analyze each option

   array_filter($items, fn($v) => $v > 0); keeps values where $v > 0, exactly what we want.
   array_filter($items, fn($v) => (bool)$v); keeps all truthy values, but 0 is false and null is false, so it removes them but also keeps negative numbers if any.
   array_filter($items); uses default filter which removes falsy values (0, null, false), but does not check for > 0.
   array_filter($items, fn($v) => $v !== 0); removes only zeros but keeps null and negative numbers.

3. Final Answer:

   array_filter($items, fn($v) => $v > 0); -> Option A

4. Quick Check:

   Filter with $v > 0 keeps only positive numbers [OK]

Hint: Use explicit condition for positive numbers [OK]

Common Mistakes:

• Using default filter which keeps all truthy values
• Filtering only non-zero but allowing null
• Confusing truthy with positive