Array map function in PHP - Step-by-Step Execution

Concept Flow - Array map function

Start with array

↓

Pick first element

↓

Apply callback function

↓

Store result in new array

↓

More elements?

Yes→Pick next element

No↓

Return new array

The map function takes each element of an array, applies a function to it, and collects the results into a new array.

Execution Sample

PHP

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

This code squares each number in the array and prints the new array.

Execution Table

Step	Current Element	Callback Applied	Result Stored	New Array State
1	1	1 * 1 = 1	1	[1]
2	2	2 * 2 = 4	4	[1, 4]
3	3	3 * 3 = 9	9	[1, 4, 9]
4	No more elements	-	-	[1, 4, 9]

💡 All elements processed, map returns new array [1, 4, 9]

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
numbers	[1, 2, 3]	[1, 2, 3]	[1, 2, 3]	[1, 2, 3]	[1, 2, 3]
current element	N/A	1	2	3	N/A
result	N/A	1	4	9	N/A
squares	[]	[1]	[1, 4]	[1, 4, 9]	[1, 4, 9]

Key Moments - 2 Insights

Why does the original array 'numbers' not change after using array_map?

array_map creates a new array with the results and does not modify the original array, as shown in the variable_tracker where 'numbers' stays the same.

What happens if the callback function returns a different type than the input?

The new array will contain the returned values exactly as produced by the callback, regardless of type, because array_map stores the callback results directly.

Visual Quiz - 3 Questions

Look at the execution_table, what is the 'Result Stored' at step 2?

A 2

B 4

C 1

D 9

💡 Hint

Check the 'Result Stored' column in row 2 of the execution_table.

At which step does the map function finish processing all elements?

A Step 4

B Step 3

C Step 2

D Step 1

💡 Hint

Look for the step where 'No more elements' appears in the 'Current Element' column.

If the callback function was changed to return $n + 1, what would be the 'New Array State' after step 3?

A [1, 2, 3]

B [1, 4, 9]

C [2, 3, 4]

D [0, 1, 2]

💡 Hint

Consider how adding 1 to each element changes the array values in the variable_tracker.

Concept Snapshot

array_map(callback, array) applies callback to each element
Returns a new array with results
Original array stays unchanged
Callback can be any function or closure
Useful for transforming arrays easily

Full Transcript

The PHP array_map function takes an array and a callback function. It applies the callback to each element in the array one by one. Each result is stored in a new array. The original array remains unchanged. After all elements are processed, the new array is returned. For example, squaring each number in [1, 2, 3] produces [1, 4, 9]. This process is stepwise: pick element, apply function, store result, repeat until done.

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