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.