Array unique and flip in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the PHP function array_unique() do?

It removes duplicate values from an array, keeping only the first occurrence of each value.

beginner

What is the purpose of the PHP function array_flip()?

It swaps the keys and values of an array, turning values into keys and keys into values.

beginner

Given $arr = ['apple', 'banana', 'apple', 'orange'];, what is the result of array_unique($arr);?

The result is ['apple', 'banana', 'orange'] with keys preserved as [0 => 'apple', 1 => 'banana', 3 => 'orange'].

intermediate

What happens if you use array_flip() on an array with duplicate values?

Only the last key for each value is preserved because keys must be unique. Earlier keys are overwritten.

intermediate

How can you combine array_unique() and array_flip() to get a flipped array with unique values as keys?

First use array_unique() to remove duplicates, then use array_flip() to swap keys and values, ensuring keys are unique.

What does array_unique() return when given an array with duplicate values?

A An array sorted in ascending order

B An array with duplicates removed, preserving the last occurrence keys

C An array with all values replaced by keys

D An array with duplicates removed, preserving the first occurrence keys

array_unique() removes duplicates and keeps the first occurrence's key.

What will array_flip(['a' => 1, 'b' => 2, 'c' => 1]) return?

A Array with keys and values swapped, last duplicate key kept

B Array with keys and values swapped, first duplicate key kept

C Original array unchanged

D Error because of duplicate values

When flipping, duplicate values become keys; only the last key is kept.

Which function would you use to remove duplicate values from an array in PHP?

A array_flip()

B array_unique()

C array_merge()

D array_slice()

array_unique() removes duplicate values.

If you want to swap keys and values of an array, which function do you use?

A array_unique()

B array_keys()

C array_flip()

D array_reverse()

array_flip() swaps keys and values.

What is a potential problem when using array_flip() on an array with non-unique values?

A It keeps only the last key for duplicate values

B It duplicates keys

C It causes a syntax error

D It sorts the array

Duplicate values become keys, but keys must be unique, so only the last key remains.

Explain how array_unique() and array_flip() work and how they can be used together.

Think about removing duplicates first, then swapping keys and values.

What happens if you flip an array with duplicate values? How does PHP handle the keys?

Consider how keys in PHP arrays must be unique.

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