Array map function in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of array_map with a simple callback

What is the output of the following PHP code?

PHP

<?php
$numbers = [1, 2, 3, 4];
$result = array_map(fn($n) => $n * 2, $numbers);
print_r($result);
?>

A Array ( [0] => 2 [1] => 4 [2] => 6 [3] => 8 )

B Array ( [1] => 2 [2] => 4 [3] => 6 [4] => 8 )

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

D Array ( [0] => 1 [1] => 4 [2] => 9 [3] => 16 )

💡 Hint

array_map applies the function to each element and returns a new array with the results.

The callback multiplies each number by 2, so the output array contains doubled values with the same keys starting from 0.

❓ Predict Output

intermediate

Using array_map with multiple arrays

What is the output of this PHP code?

PHP

<?php
$a = [1, 2, 3];
$b = [4, 5, 6];
$result = array_map(fn($x, $y) => $x + $y, $a, $b);
print_r($result);
?>

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

B Array ( [0] => 5 [1] => 7 [2] => 9 )

C Array ( [0] => 4 [1] => 5 [2] => 6 )

D Array ( [0] => 1 [1] => 7 [2] => 9 )

💡 Hint

array_map can take multiple arrays and pass corresponding elements to the callback.

The callback adds elements from both arrays at the same positions: 1+4=5, 2+5=7, 3+6=9.

🔧 Debug

advanced

Identify the error in array_map usage

What error does this PHP code produce?

PHP

<?php
$values = [1, 2, 3];
$result = array_map('strtoupper', $values);
print_r($result);
?>

A Array ( [0] => 1 [1] => 4 [2] => 9 )

B Fatal error: Call to undefined function strtoupper()

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

D Warning: strtoupper() expects parameter 1 to be string, int given

💡 Hint

strtoupper expects strings, but the array contains integers.

strtoupper cannot convert integers to uppercase, so PHP warns about wrong argument type.

🧠 Conceptual

advanced

How does array_map handle arrays of different lengths?

Given these arrays, what will be the output of array_map?

PHP

<?php
$a = [1, 2, 3, 4];
$b = [10, 20];
$result = array_map(fn($x, $y) => $x + $y, $a, $b);
print_r($result);
?>

A Array ( [0] => 11 [1] => 22 [2] => 3 [3] => 4 )

B Array ( [0] => 11 [1] => 22 )

C Warning: array_map(): Argument #2 is shorter than Argument #1

D Array ( [0] => 11 [1] => 22 [2] => 13 [3] => 24 )

💡 Hint

array_map uses NULL for missing elements in shorter arrays.

For missing elements in $b, NULL is passed, so adding NULL to int returns the int itself.

📝 Syntax

expert

Which option produces the output: Array ( [0] => 1 [1] => 4 [2] => 9 )?

Select the PHP code snippet that produces the output shown when run.

A

<?php
$nums = [1, 2, 3];
$result = array_map(fn($n) => $n * 3, $nums);
print_r($result);
?>

B

<?php
$nums = [1, 2, 3];
$result = array_map(fn($n) => $n + 2, $nums);
print_r($result);
?>

C

<?php
$nums = [1, 2, 3];
$result = array_map(fn($n) => $n ** 2, $nums);
print_r($result);
?>

D

<?php
$nums = [1, 2, 3];
$result = array_map(fn($n) => $n - 1, $nums);
print_r($result);
?>

💡 Hint

The output shows squares of the numbers.

Option C squares each number: 1^2=1, 2^2=4, 3^2=9.

Practice

1. What does the PHP function array_map do?

easy

A. Deletes elements from an array based on a condition

B. Merges two or more arrays into one

C. Sorts the elements of an array in ascending order

D. Applies a function to each element of an array and returns a new array

Solution

1. Step 1: Understand the purpose of array_map

   The function applies a given callback function to each element of the input array.

2. Step 2: Identify the output of array_map

   It returns a new array with the results of the callback function applied to each element.

3. Final Answer:

   Applies a function to each element of an array and returns a new array -> Option D

4. Quick Check:

   array_map = apply function to array elements [OK]

Hint: Remember: array_map transforms each item, returns new array [OK]

Common Mistakes:

• Thinking array_map modifies the original array
• Confusing array_map with array_filter
• Assuming array_map merges arrays

2. Which of the following is the correct syntax to use array_map in PHP?

easy

A. array_map(function, array);

B. array_map(array, function);

C. array_map->function(array);

D. array_map(function => array);

Solution

1. Step 1: Recall the function signature

   The correct syntax is array_map(callback_function, array_to_process).

2. Step 2: Match the options with the syntax

   array_map(function, array); matches the correct order: function first, then array.

3. Final Answer:

   array_map(function, array); -> Option A

4. Quick Check:

   array_map(callback, array) syntax [OK]

Hint: Function comes first, array second in array_map [OK]

Common Mistakes:

• Swapping the order of function and array
• Using arrow or assignment syntax incorrectly
• Using object method syntax instead of function call

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

$numbers = [1, 2, 3];
$result = array_map(fn($n) => $n * 2, $numbers);
print_r($result);

medium

A. [2, 4, 6]

B. [1, 2, 3]

C. [3, 4, 5]

D. Error: Invalid callback

Solution

1. Step 1: Understand the callback function

   The callback doubles each number: $n * 2.

2. Step 2: Apply the callback to each element

   1*2=2, 2*2=4, 3*2=6, so the new array is [2, 4, 6].

3. Final Answer:

   [2, 4, 6] -> Option A

4. Quick Check:

   Each element doubled = [2, 4, 6] [OK]

Hint: Multiply each element by 2 in the callback [OK]

Common Mistakes:

• Expecting original array unchanged
• Confusing the callback operation
• Thinking print_r outputs a string, not array

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

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

medium

A. array_map requires an anonymous function, not a named function

B. No error, code runs correctly

C. Function double is called before it is defined

D. Missing semicolon after array_map call

Solution

1. Step 1: Check function definition and usage

   The function double is defined after the array_map call, but PHP allows this because functions are hoisted.

2. Step 2: Verify syntax and function type

   array_map accepts named functions as callbacks, and semicolons are correctly placed.

3. Final Answer:

   No error, code runs correctly -> Option B

4. Quick Check:

   Named function callback works fine [OK]

Hint: PHP functions can be used before definition [OK]

Common Mistakes:

• Assuming function must be defined before use
• Thinking only anonymous functions work in array_map
• Missing semicolon errors where none exist

5. Given an array of strings ['apple', 'banana', 'cherry'], how can you use array_map to create a new array with the length of each string?

hard

A. array_map(fn($s) => size($s), ['apple', 'banana', 'cherry']);

B. array_map(fn($s) => count($s), ['apple', 'banana', 'cherry']);

C. array_map(fn($s) => strlen($s), ['apple', 'banana', 'cherry']);

D. array_map(fn($s) => $s.length, ['apple', 'banana', 'cherry']);

Solution

1. Step 1: Identify the correct function to get string length

   In PHP, strlen() returns the length of a string.

2. Step 2: Apply strlen in array_map callback

   Using fn($s) => strlen($s) applies length calculation to each string.

3. Final Answer:

   array_map(fn($s) => strlen($s), ['apple', 'banana', 'cherry']); -> Option C

4. Quick Check:

   Use strlen() for string length in array_map [OK]

Hint: Use strlen() inside array_map callback for string lengths [OK]

Common Mistakes:

• Using count() which counts array elements, not string length
• Trying to access string length with dot notation
• Using non-existent size() function