Array map function in PHP

Introduction

The array map function helps you change every item in a list quickly. It makes a new list with the changed items.

You want to add 1 to every number in a list of scores.

You need to make all words in a list uppercase.

You want to get the length of each string in a list of names.

You want to convert a list of prices from dollars to euros.

You want to add a prefix to every item in a list of file names.

Syntax

PHP

<?php
function callback_function($item) {
    // change $item and return it
    return $item;
}

$new_array = array_map('callback_function', $original_array);
?>

The callback function is called for each item in the original array.

array_map returns a new array with the changed items, original stays the same.

Examples

Add 1 to each number in the list.

PHP

<?php
$numbers = [1, 2, 3];
$new_numbers = array_map(fn($num) => $num + 1, $numbers);
print_r($new_numbers);

Make all words uppercase using a built-in function.

PHP

<?php
$words = ['apple', 'banana', 'cherry'];
$upper_words = array_map('strtoupper', $words);
print_r($upper_words);

What if the array is empty? Result is also empty.

PHP

<?php
$empty = [];
$result = array_map(fn($x) => $x * 2, $empty);
print_r($result);

Works with one item too.

PHP

<?php
$single = [10];
$doubled = array_map(fn($x) => $x * 2, $single);
print_r($doubled);

Sample Program

This program shows how to use array_map to convert a list of prices from dollars to euros. It prints the original list and the converted list.

PHP

<?php
// Original array of prices in dollars
$prices_in_dollars = [10, 20, 30, 40];

// Function to convert dollars to euros (assume 1 dollar = 0.9 euros)
function convert_to_euros($price_in_dollars) {
    return $price_in_dollars * 0.9;
}

// Print original prices
echo "Original prices in dollars:\n";
print_r($prices_in_dollars);

// Use array_map to convert all prices
$prices_in_euros = array_map('convert_to_euros', $prices_in_dollars);

// Print converted prices
echo "Prices converted to euros:\n";
print_r($prices_in_euros);
?>

Important Notes

Time complexity is O(n) because it changes each item once.

Space complexity is O(n) because it makes a new array of the same size.

Common mistake: forgetting to return the changed item in the callback function.

Use array_map when you want to change every item in a list and keep the same order and size.

Summary

array_map applies a function to every item in an array and returns a new array.

The original array stays the same.

Works with empty arrays and arrays with one item too.

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