Array map function in PHP - Mini Project: Build & Apply

Array map function

📖 Scenario: You work in a bakery that wants to update the prices of some pastries. You have a list of prices, and you want to increase each price by 10% to cover new costs.

🎯 Goal: Use the array_map function in PHP to create a new list of updated prices with a 10% increase.

📋 What You'll Learn

Create an array called prices with the values 2.50, 3.00, 4.25, 5.00

Create a variable called increase_rate and set it to 0.10

Use array_map with a function that increases each price by increase_rate

Print the new array of updated prices

💡 Why This Matters

🌍 Real World

Updating prices in a list is common in stores and online shops when costs change.

💼 Career

Knowing how to use <code>array_map</code> helps you process lists of data efficiently in PHP, a skill useful for web developers.

1

Create the initial array of prices

Create an array called prices with these exact values: 2.50, 3.00, 4.25, 5.00.

PHP

# Your code here

Hint

Use square brackets [] to create the array and separate values with commas.

2

Set the increase rate

Create a variable called increase_rate and set it to 0.10 to represent a 10% increase.

PHP

<?php
$prices = [2.50, 3.00, 4.25, 5.00];
# Your code here

Hint

Use the = sign to assign the value to the variable.

3

Use array_map to increase prices

Use array_map with a function that takes each price and returns the price increased by increase_rate. Store the result in a variable called updated_prices.

PHP

<?php
$prices = [2.50, 3.00, 4.25, 5.00];
$increase_rate = 0.10;
# Your code here

Hint

Use an anonymous function with use ($increase_rate) to access the variable inside the function.

4

Print the updated prices

Print the updated_prices array using print_r to see the new prices.

PHP

<?php
$prices = [2.50, 3.00, 4.25, 5.00];
$increase_rate = 0.10;
$updated_prices = array_map(function($price) use ($increase_rate) {
    return $price * (1 + $increase_rate);
}, $prices);
# Your code here

Hint

Use print_r($updated_prices); to display the array.

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