Closures in array functions in PHP - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to filter numbers greater than 5 using array_filter and a closure.

PHP

<?php
$numbers = [2, 5, 8, 1, 10];
$filtered = array_filter($numbers, function($num) {
    return $num [1] 5;
});
print_r($filtered);
?>

Drag options to blanks, or click blank then click option'

A!=

B<

C==

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' will filter numbers less than 5.

Using '==' will only keep numbers equal to 5.

The closure checks if each number is greater than 5 using the '>' operator.

2fill in blank

medium

Complete the code to double each number in the array using array_map and a closure.

PHP

<?php
$numbers = [1, 2, 3, 4];
$doubled = array_map(function($num) {
    return $num [1] 2;
}, $numbers);
print_r($doubled);
?>

Drag options to blanks, or click blank then click option'

A+

B/

C*

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' will add 2 instead of doubling.

Using '-' or '/' will not double the number.

The closure multiplies each number by 2 to double it.

3fill in blank

hard

Fix the error in the closure to correctly filter even numbers using array_filter.

PHP

<?php
$numbers = [1, 2, 3, 4, 5];
$evens = array_filter($numbers, function($num) {
    return $num [1] 2 == 0;
});
print_r($evens);
?>

Drag options to blanks, or click blank then click option'

A%

B/

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '/' will perform division, not remainder.

Using '*' or '+' will not check for even numbers.

The modulus operator '%' returns the remainder. Checking if $num % 2 == 0 finds even numbers.

4fill in blank

hard

Fill both blanks to create an array of squares for numbers greater than 3 using array_map and array_filter with closures.

PHP

<?php
$numbers = [1, 2, 3, 4, 5];
$filtered = array_filter($numbers, function($num) {
    return $num [1] 3;
});
$squares = array_map(function($num) {
    return $num [2] $num;
}, $filtered);
print_r($squares);
?>

Drag options to blanks, or click blank then click option'

A>

B*

C+

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' will filter numbers less than 3.

Using '+' instead of '*' will add numbers instead of squaring.

The first closure filters numbers greater than 3 using '>'. The second closure squares each number by multiplying it by itself using '*'.

5fill in blank

hard

Fill all three blanks to create an associative array with uppercase keys and values filtered by length greater than 4 using closures.

PHP

<?php
$words = ['apple', 'bat', 'carrot', 'dog'];
$result = array_filter(array_combine(array_map(function($word) {
    return [1];
}, $words), $words), function($word) {
    return strlen($word) [2] 4;
});
$result = array_map(function($word) {
    return [3];
}, $result);
print_r($result);
?>

Drag options to blanks, or click blank then click option'

Astrtoupper($word)

B>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' will filter words with length less than or equal to 4.

Not converting keys or values to uppercase will not meet the requirement.

The first blank converts each word to uppercase for keys using strtoupper($word). The second blank filters words with length greater than 4 using '>'. The third blank converts values to uppercase using strtoupper($word).

Practice

1. What is the main purpose of using a closure inside PHP array functions like array_map or array_filter?

easy

A. To define a small, unnamed function that processes array elements inline

B. To declare a global variable accessible inside the array

C. To create a new array without any function

D. To convert arrays into strings automatically

Solution

1. Step 1: Understand what a closure is

   A closure is a small, unnamed function that can be used directly where needed.

2. Step 2: Role in array functions

   Closures let you write the logic for processing each array element inline, making code concise and clear.

3. Final Answer:

   To define a small, unnamed function that processes array elements inline -> Option A

4. Quick Check:

   Closure = small inline function [OK]

Hint: Closures are small functions used directly inside array calls [OK]

Common Mistakes:

• Thinking closures declare global variables
• Confusing closures with array conversion functions
• Assuming closures create arrays without functions

2. Which of the following is the correct syntax to use a closure with array_filter to keep only even numbers from an array $numbers?

easy

A. array_filter($numbers, function $n { return $n % 2 == 0; });

B. array_filter($numbers, function($n) { return $n % 2 == 0; });

C. array_filter($numbers, function($n) => $n % 2 == 0;);

D. array_filter($numbers, function($n) { $n % 2 == 0; });

Solution

1. Step 1: Check closure syntax

   Correct closure syntax uses function($param) { return expression; }.

2. Step 2: Validate each option

   array_filter($numbers, function($n) { return $n % 2 == 0; }); uses correct syntax with return statement. array_filter($numbers, function $n { return $n % 2 == 0; }); misses parentheses. array_filter($numbers, function($n) => $n % 2 == 0;); uses arrow syntax incorrectly. array_filter($numbers, function($n) { $n % 2 == 0; }); lacks return keyword.

3. Final Answer:

   array_filter($numbers, function($n) { return $n % 2 == 0; }); -> Option B

4. Quick Check:

   Closure syntax needs parentheses and return [OK]

Hint: Closures need parentheses and return keyword inside curly braces [OK]

Common Mistakes:

• Omitting parentheses around parameters
• Using arrow syntax incorrectly in PHP 7+
• Forgetting to return a value inside closure

3. What will be the output of the following PHP code?

$nums = [1, 2, 3, 4];
$result = array_map(function($x) { return $x * 3; }, $nums);
print_r($result);

medium

A. [0, 3, 6, 9]

B. [1, 2, 3, 4]

C. [3, 6, 9, 12]

D. Syntax error

Solution

1. Step 1: Understand array_map with closure

   array_map applies the closure to each element, multiplying by 3.

2. Step 2: Calculate each element

   1*3=3, 2*3=6, 3*3=9, 4*3=12, so result is [3, 6, 9, 12].

3. Final Answer:

   [3, 6, 9, 12] -> Option C

4. Quick Check:

   Multiply each by 3 = [3,6,9,12] [OK]

Hint: array_map applies closure to each element and returns new array [OK]

Common Mistakes:

• Expecting original array unchanged
• Confusing array_map with array_filter
• Thinking output is a string, not array

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

$values = [10, 15, 20];
$filtered = array_filter($values, function($v) {
  if ($v > 10) {
    return true;
  }
});
print_r($filtered);

medium

A. The closure does not return false for values <= 10, causing unexpected filtering

B. The array_filter function requires a named function, not a closure

C. The array_filter call is missing the array parameter

D. The closure syntax is invalid without a semicolon after return

Solution

1. Step 1: Analyze closure return behavior

   The closure returns true only if value > 10, but returns nothing (null) otherwise.

2. Step 2: Understand array_filter behavior

   array_filter expects explicit boolean returns. Returning implicit null (falsy) for values <=10 removes them but can cause unexpected filtering behavior.

3. Final Answer:

   The closure does not return false for values <= 10, causing unexpected filtering -> Option A

4. Quick Check:

   Closure must return true or false explicitly [OK]

Hint: Always return true or false explicitly in closures for array_filter [OK]

Common Mistakes:

• Not returning false explicitly in closure
• Thinking closures must be named functions
• Missing array parameter in array_filter

5. Given an array $data = [1, 2, 3, 4, 5, 6], how can you use a closure with array_filter and array_map together to get an array of squares of only the odd numbers?

hard

A. array_filter(fn($x) => $x % 2 !== 0, array_map($data, fn($x) => $x * $x));

B. array_filter(array_map($data, fn($x) => $x * $x), fn($x) => $x % 2 !== 0);

C. array_map(array_filter($data, fn($x) => $x % 2 !== 0), fn($x) => $x * $x);

D. array_map(fn($x) => $x * $x, array_filter($data, fn($x) => $x % 2 !== 0));

Solution

1. Step 1: Filter odd numbers first

   Use array_filter with closure to keep only odd numbers: fn($x) => $x % 2 !== 0.

2. Step 2: Map squares of filtered numbers

   Apply array_map with closure to square each filtered number: fn($x) => $x * $x.

3. Step 3: Combine calls correctly

   Pass filtered array as second argument to array_map, so filtering happens before mapping.

4. Final Answer:

   array_map(fn($x) => $x * $x, array_filter($data, fn($x) => $x % 2 !== 0)); -> Option D

5. Quick Check:

   Filter odds then map squares [OK]

Hint: Filter first, then map to transform filtered results [OK]

Common Mistakes:

• Reversing order of array_map and array_filter
• Passing arguments in wrong order
• Using array_map or array_filter with wrong parameter order