Array merge and combine in PHP

Introduction

We use array merge and combine to join two or more lists into one. This helps us organize and work with data easily.

You have two lists of names and want to put them together into one list.

You want to create a new list where one list's values become keys and the other's values become values.

You want to add items from one list to the end of another list.

You want to pair two lists to create a dictionary-like structure.

Syntax

PHP

<?php
// Merge arrays
$mergedArray = array_merge($array1, $array2);

// Combine arrays (keys and values)
$combinedArray = array_combine($keysArray, $valuesArray);
?>

array_merge joins arrays by adding elements from the second array to the first.

array_combine creates a new array using one array for keys and another for values. Both arrays must have the same number of elements.

Examples

Merges two arrays into one list with all fruits.

PHP

<?php
$array1 = ["apple", "banana"];
$array2 = ["cherry", "date"];
$merged = array_merge($array1, $array2);
print_r($merged);
?>

Combines keys and values into an associative array.

PHP

<?php
$keys = ["a", "b", "c"];
$values = [1, 2, 3];
$combined = array_combine($keys, $values);
print_r($combined);
?>

Merging two empty arrays results in an empty array.

PHP

<?php
// Edge case: empty arrays
$empty1 = [];
$empty2 = [];
$mergedEmpty = array_merge($empty1, $empty2);
print_r($mergedEmpty);
?>

Combining arrays with one element each creates a single key-value pair.

PHP

<?php
// Edge case: one element arrays
$oneKey = ["x"];
$oneValue = [42];
$combinedOne = array_combine($oneKey, $oneValue);
print_r($combinedOne);
?>

Sample Program

This program shows how to merge two fruit lists and how to combine keys and values into an associative array.

PHP

<?php
// Create two arrays
$fruits1 = ["apple", "banana"];
$fruits2 = ["cherry", "date"];

// Show original arrays
echo "Original arrays:\n";
print_r($fruits1);
print_r($fruits2);

// Merge arrays
$allFruits = array_merge($fruits1, $fruits2);
echo "\nAfter merging:\n";
print_r($allFruits);

// Create keys and values arrays
$keys = ["first", "second", "third"];
$values = ["red", "green", "blue"];

// Show keys and values
echo "\nKeys and values arrays:\n";
print_r($keys);
print_r($values);

// Combine keys and values
$colorMap = array_combine($keys, $values);
echo "\nAfter combining keys and values:\n";
print_r($colorMap);
?>

Important Notes

Time complexity: array_merge runs in O(n + m) where n and m are the sizes of the arrays.

Space complexity: Both functions create new arrays, so space grows with input size.

Common mistake: Using array_combine with arrays of different lengths causes an error.

Use array_merge when you want to join lists. Use array_combine when you want to create key-value pairs.

Summary

array_merge joins two or more arrays into one list.

array_combine creates an associative array from two lists: one for keys, one for values.

Both are useful to organize and work with data in PHP.

Practice

1. What does the PHP function array_merge do?

easy

A. Creates an associative array from two arrays

B. Joins two or more arrays into one combined array

C. Sorts an array in ascending order

D. Removes duplicate values from an array

Solution

1. Step 1: Understand the purpose of array_merge

   The function array_merge takes multiple arrays and joins their elements into a single array.

2. Step 2: Compare with other functions

   array_combine creates key-value pairs, sorting and removing duplicates are different functions.

3. Final Answer:

   Joins two or more arrays into one combined array -> Option B

4. Quick Check:

   array_merge joins arrays = B [OK]

Hint: array_merge joins arrays end-to-end [OK]

Common Mistakes:

• Confusing array_merge with array_combine
• Thinking array_merge sorts arrays
• Assuming array_merge removes duplicates

2. Which of the following is the correct syntax to combine two arrays into an associative array using array_combine?

easy

A. array_merge($keys, $values);

B. array_combine($values, $keys);

C. array_combine($keys, $values);

D. array_combine($keys);

Solution

1. Step 1: Recall array_combine parameter order

   The first parameter is the array of keys, the second is the array of values.

2. Step 2: Check each option

   array_combine($keys, $values); correctly uses array_combine($keys, $values);. array_combine($values, $keys); reverses parameters, C uses wrong function, D misses second parameter.

3. Final Answer:

   array_combine($keys, $values); -> Option C

4. Quick Check:

   Keys first, values second in array_combine = A [OK]

Hint: Keys go first, values second in array_combine [OK]

Common Mistakes:

• Swapping keys and values parameters
• Using array_merge instead of array_combine
• Missing one of the required parameters

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

$a = [1, 2];
$b = [3, 4];
$result = array_merge($a, $b);
print_r($result);

medium

A. [1, 2, 3, 4]

B. [3, 4, 1, 2]

C. [1 => 2, 3 => 4]

D. Array keys and values combined

Solution

1. Step 1: Understand array_merge behavior

   array_merge joins arrays by appending elements of second array to first, preserving order.

2. Step 2: Apply to given arrays

   $a = [1, 2], $b = [3, 4], merged result is [1, 2, 3, 4].

3. Final Answer:

   [1, 2, 3, 4] -> Option A

4. Quick Check:

   array_merge appends arrays = [1, 2, 3, 4] [OK]

Hint: array_merge appends arrays in order given [OK]

Common Mistakes:

• Thinking array_merge sorts or combines keys
• Confusing output with associative array
• Expecting keys to be preserved as in original arrays

4. Identify the error in this PHP code snippet:

$keys = ['a', 'b'];
$values = [1];
$result = array_combine($keys, $values);
print_r($result);

medium

A. array_combine merges arrays instead of combining keys and values

B. array_combine cannot use string keys

C. array_combine needs only one parameter

D. array_combine requires both arrays to have the same number of elements

Solution

1. Step 1: Check array_combine requirements

   array_combine requires both arrays to have equal length to pair keys and values correctly.

2. Step 2: Analyze given arrays

   $keys has 2 elements, $values has 1 element, causing an error.

3. Final Answer:

   array_combine requires both arrays to have the same number of elements -> Option D

4. Quick Check:

   Equal length arrays needed for array_combine = D [OK]

Hint: Keys and values arrays must be same length [OK]

Common Mistakes:

• Ignoring array length mismatch
• Thinking array_combine works with one array
• Confusing array_combine with array_merge

5. Given two arrays:
$keys = ['name', 'age', 'city'];
$values = ['Alice', 30, 'Paris'];
Which PHP code correctly creates an associative array and then merges it with ['country' => 'France']?

hard

A. $assoc = array_combine($keys, $values); $final = array_merge($assoc, ['country' => 'France']);

B. $assoc = array_merge($keys, $values); $final = array_combine($assoc, ['country' => 'France']);

C. $final = array_combine($keys, $values, ['country' => 'France']);

D. $final = array_merge(array_combine($values, $keys), ['country' => 'France']);

Solution

1. Step 1: Create associative array from keys and values

   Use array_combine($keys, $values) to pair keys with values correctly.

2. Step 2: Merge associative array with another array

   Use array_merge to add the 'country' key-value pair to the associative array.

3. Step 3: Check other options for errors

   $assoc = array_merge($keys, $values); $final = array_combine($assoc, ['country' => 'France']); swaps functions incorrectly, C uses wrong parameter count, D swaps keys and values.

4. Final Answer:

   $assoc = array_combine($keys, $values); $final = array_merge($assoc, ['country' => 'France']); -> Option A

5. Quick Check:

   Combine keys-values, then merge extra array = A [OK]

Hint: Combine keys-values first, then merge extra pairs [OK]

Common Mistakes:

• Swapping keys and values in array_combine
• Trying to combine more than two arrays at once
• Using array_merge before array_combine