Array unique and flip in PHP - Mini Project: Build & Apply

Array unique and flip

📖 Scenario: You have a list of fruits with some duplicates. You want to find the unique fruits and then flip the list so that fruits become keys and their positions become values.

🎯 Goal: Create a PHP program that removes duplicate fruits from an array and then flips the array so that fruit names become keys and their indexes become values.

📋 What You'll Learn

Create an array called fruits with the exact values: 'apple', 'banana', 'apple', 'orange', 'banana'

Create a variable called uniqueFruits that holds the unique values from fruits

Create a variable called flippedFruits that flips the keys and values of uniqueFruits

Print the flippedFruits array

💡 Why This Matters

🌍 Real World

Removing duplicates and flipping arrays is useful when you want to quickly find the position of unique items or create lookup tables.

💼 Career

These skills are important for data cleaning, preparing data for search, and optimizing lookups in PHP web applications.

1

Create the initial array

Create an array called fruits with these exact values in order: 'apple', 'banana', 'apple', 'orange', 'banana'

PHP

# Create the array called fruits with the given values
# Your code here

Hint

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

2

Remove duplicates from the array

Create a variable called uniqueFruits that holds the unique values from the fruits array using the array_unique() function

PHP

<?php
$fruits = ['apple', 'banana', 'apple', 'orange', 'banana'];
# Create uniqueFruits using array_unique on fruits
# Your code here

Hint

Use array_unique($fruits) to get unique values.

3

Flip the unique array

Create a variable called flippedFruits that flips the keys and values of the uniqueFruits array using the array_flip() function

PHP

<?php
$fruits = ['apple', 'banana', 'apple', 'orange', 'banana'];
$uniqueFruits = array_unique($fruits);
# Create flippedFruits using array_flip on uniqueFruits
# Your code here

Hint

Use array_flip($uniqueFruits) to swap keys and values.

4

Print the flipped array

Print the flippedFruits array using print_r() to display the result

PHP

<?php
$fruits = ['apple', 'banana', 'apple', 'orange', 'banana'];
$uniqueFruits = array_unique($fruits);
$flippedFruits = array_flip($uniqueFruits);
# Print flippedFruits array
# Your code here

Hint

Use print_r($flippedFruits); to print the array in a readable format.

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