Array unique and flip in PHP

Introduction

We use array unique and flip to remove duplicate values and swap keys with values in an array. This helps organize data and find unique items easily.

When you want to remove repeated items from a list of names.

When you need to switch keys and values to look up data quickly.

When cleaning up user input to keep only unique entries.

When preparing data for quick search by values instead of keys.

Syntax

PHP

<?php
// Remove duplicates from an array
$uniqueArray = array_unique($array);

// Flip keys and values in an array
$flippedArray = array_flip($array);
?>

array_unique() keeps the first occurrence and removes later duplicates.

array_flip() swaps keys and values; values must be unique and valid keys (strings or integers).

Examples

Removes duplicate 'apple', keeps first one.

PHP

<?php
$array = ['apple', 'banana', 'apple', 'orange'];
$uniqueArray = array_unique($array);
print_r($uniqueArray);
?>

Flips keys and values; last duplicate value 'red' overwrites previous key.

PHP

<?php
$array = ['a' => 'red', 'b' => 'green', 'c' => 'red'];
$flippedArray = array_flip($array);
print_r($flippedArray);
?>

Empty array stays empty after unique.

PHP

<?php
$array = [];
$uniqueArray = array_unique($array);
print_r($uniqueArray);
?>

Single element array flips key 0 and value 'onlyone'.

PHP

<?php
$array = ['onlyone'];
$flippedArray = array_flip($array);
print_r($flippedArray);
?>

Sample Program

This program shows how to remove duplicates from an array and then flip keys and values. It prints the array before and after each step.

PHP

<?php
// Original array with duplicates
$fruits = ['apple', 'banana', 'apple', 'orange', 'banana'];

// Print original array
echo "Original array:\n";
print_r($fruits);

// Remove duplicates
$uniqueFruits = array_unique($fruits);
echo "\nArray after removing duplicates:\n";
print_r($uniqueFruits);

// Flip keys and values
$flippedFruits = array_flip($uniqueFruits);
echo "\nArray after flipping keys and values:\n";
print_r($flippedFruits);
?>

Important Notes

Time complexity: array_unique() and array_flip() both run in O(n) time, where n is the number of elements.

Space complexity: Both functions create new arrays, so space is O(n).

Common mistake: Using array_flip() on arrays with duplicate values causes keys to be overwritten silently.

Use array_unique() when you want to keep only one copy of each value. Use array_flip() when you want to quickly find keys by their values.

Summary

array_unique() removes duplicate values, keeping the first occurrence.

array_flip() swaps keys and values, but values must be unique and valid keys.

These functions help organize and search data efficiently in arrays.

Practice

1. What does the PHP function array_unique() do to an array?

easy

A. Flips keys and values of the array

B. Removes duplicate values, keeping only the first occurrence

C. Sorts the array in ascending order

D. Removes all values except the last occurrence

Solution

1. Step 1: Understand the purpose of array_unique()

   This function scans the array and removes any duplicate values, but keeps the first time each value appears.

2. Step 2: Compare with other options

   Options B, C, and D describe different functions or behaviors. Only Removes duplicate values, keeping only the first occurrence correctly describes array_unique().

3. Final Answer:

   Removes duplicate values, keeping only the first occurrence -> Option B

4. Quick Check:

   array_unique() removes duplicates = A [OK]

Hint: Remember: unique means no duplicates, keep first [OK]

Common Mistakes:

• Thinking it flips keys and values
• Assuming it sorts the array
• Believing it keeps the last occurrence

2. Which of the following is the correct syntax to flip keys and values of an array $arr in PHP?

easy

A. array_flip($arr);

B. flip_array($arr);

C. array_unique($arr);

D. flip($arr);

Solution

1. Step 1: Identify the correct function name

   The PHP function to swap keys and values is array_flip(). Other options are not valid PHP functions.

2. Step 2: Confirm correct usage

   The syntax is array_flip($arr); which returns a new array with keys and values swapped.

3. Final Answer:

   array_flip($arr); -> Option A

4. Quick Check:

   Flip keys and values = array_flip() [OK]

Hint: Flip keys and values with array_flip() [OK]

Common Mistakes:

• Using non-existent functions like flip_array()
• Confusing array_unique() with array_flip()
• Missing parentheses in function call

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

$arr = ['a' => 'apple', 'b' => 'banana', 'c' => 'apple'];
$result = array_flip(array_unique($arr));
print_r($result);

medium

A. Array ( [apple] => a [banana] => b )

B. Array ( [a] => apple [b] => banana [c] => apple )

C. Array ( [apple] => c [banana] => b )

D. Array ( [apple] => 0 [banana] => 1 )

Solution

1. Step 1: Apply array_unique() on $arr

   This removes duplicate values, so 'apple' at key 'c' is removed. Result: ['a' => 'apple', 'b' => 'banana']

2. Step 2: Apply array_flip() on the unique array

   Keys and values swap: 'apple' becomes key with value 'a', 'banana' becomes key with value 'b'.

3. Final Answer:

   Array ( [apple] => a [banana] => b ) -> Option A

4. Quick Check:

   Unique then flip = keys and values swapped without duplicates [OK]

Hint: Unique first, then flip keys and values [OK]

Common Mistakes:

• Flipping before removing duplicates
• Expecting duplicate keys in flipped array
• Confusing keys and values in output

4. Consider this PHP code snippet:

$arr = ['x' => 'dog', 'y' => 'cat', 'z' => 'dog'];
$result = array_flip($arr);
print_r($result);

What is the problem with this code?

medium

A. No problem; output will be correct

B. It causes a syntax error because array_flip requires numeric keys

C. array_flip cannot flip string keys

D. The duplicate values cause keys to be overwritten in the flipped array

Solution

1. Step 1: Understand array_flip() behavior with duplicates

   When flipping, values become keys. Duplicate values cause keys to overwrite previous ones.

2. Step 2: Analyze the example

   'dog' appears twice as a value. After flipping, only the last key ('z') remains for 'dog'.

3. Final Answer:

   The duplicate values cause keys to be overwritten in the flipped array -> Option D

4. Quick Check:

   Duplicate values overwrite keys when flipped [OK]

Hint: Duplicates in values overwrite keys after flip [OK]

Common Mistakes:

• Expecting all keys to be preserved
• Thinking array_flip causes syntax errors
• Assuming string keys cannot be flipped

5. You have an array of user IDs with possible duplicates:

$users = [101 => 'alice', 102 => 'bob', 103 => 'alice', 104 => 'carol'];

You want to create a new array where usernames are keys and user IDs are values, but only keep the first occurrence of each username. Which code correctly achieves this?

hard

A. $uniqueUsers = array_flip($users); $uniqueUsers = array_unique($uniqueUsers);

B. $uniqueUsers = array_unique(array_flip($users));

C. $uniqueUsers = array_flip(array_unique($users));

D. $uniqueUsers = array_unique($users); $uniqueUsers = array_unique($uniqueUsers);

Solution

1. Step 1: Remove duplicate usernames with array_unique()

   This keeps the first occurrence of each username, removing duplicates.

2. Step 2: Flip keys and values with array_flip()

   Swaps usernames to keys and user IDs to values, as required.

3. Final Answer:

   $uniqueUsers = array_flip(array_unique($users)); -> Option C

4. Quick Check:

   Unique usernames first, then flip keys and values [OK]

Hint: Unique first, then flip for correct key-value swap [OK]

Common Mistakes:

• Flipping before removing duplicates
• Applying unique after flipping
• Assuming array_unique works on keys