Array unique and flip in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of array_unique and array_flip combination

What is the output of the following PHP code?

PHP

<?php
$arr = ['apple', 'banana', 'apple', 'orange', 'banana'];
$result = array_flip(array_unique($arr));
print_r($result);
?>

A Array ( [apple] => 0 [banana] => 1 [orange] => 3 )

B Array ( [0] => apple [1] => banana [3] => orange )

C Array ( [apple] => 2 [banana] => 4 [orange] => 3 )

D Array ( [apple] => 0 [banana] => 2 [orange] => 3 )

💡 Hint

Remember that array_unique keeps the first occurrence of each value and array_flip swaps keys and values.

array_unique removes duplicate values but keeps the first key for each unique value. Then array_flip swaps keys and values, so the unique values become keys and their original keys become values.

❓ Predict Output

intermediate

Count of items after array_unique and array_flip

How many items are in the resulting array after running this code?

PHP

<?php
$input = ['cat', 'dog', 'cat', 'bird', 'dog', 'fish'];
$output = array_flip(array_unique($input));
echo count($output);
?>

A 6

B 4

C 3

D 5

💡 Hint

Count unique values before flipping.

array_unique removes duplicates, leaving ['cat', 'dog', 'bird', 'fish'] which is 4 items. array_flip does not change the count.

🔧 Debug

advanced

Identify the error in array_flip usage

What error will this PHP code produce?

PHP

<?php
$arr = ['a' => 'x', 'b' => 'x', 'c' => 'y'];
$result = array_flip($arr);
print_r($result);
?>

A Array ( [x] => c [y] => c )

B Array ( [x] => b [y] => c )

C Warning: array_flip(): Can only flip STRING and INTEGER values!

D Array ( [x] => b [y] => c ) but keys overwritten silently

💡 Hint

Check what happens when array_flip has duplicate values in the original array.

array_flip swaps keys and values. If values are duplicated, the last key overwrites previous ones silently. So 'x' maps to 'b' (last key with value 'x').

📝 Syntax

advanced

Syntax error in array_unique usage

Which option contains a syntax error when trying to get unique values from an array?

A $unique = array_unique arr;

B $unique = array_unique($arr);

C $unique = array_unique($arr, SORT_STRING);

D $unique = array_unique($arr, SORT_REGULAR);

💡 Hint

Check the function call syntax carefully.

Option A misses parentheses around the argument, causing a syntax error.

🚀 Application

expert

Result of combined array_unique and array_flip with numeric keys

Given the code below, what is the output?

PHP

<?php
$input = [2 => 'red', 5 => 'blue', 3 => 'red', 7 => 'green'];
$output = array_flip(array_unique($input));
print_r($output);
?>

A Array ( [red] => 0 [blue] => 1 [green] => 2 )

B Array ( [red] => 3 [blue] => 5 [green] => 7 )

C Array ( [red] => 2 [blue] => 5 [green] => 7 )

D Array ( [2] => red [5] => blue [7] => green )

💡 Hint

array_unique keeps the first key for each unique value. array_flip swaps keys and values.

array_unique keeps the first occurrence of 'red' at key 2, so unique array is [2=>'red',5=>'blue',7=>'green']. array_flip swaps keys and values, so 'red' => 2, 'blue' => 5, 'green' => 7.

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