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.