Array unique and flip in PHP - Step-by-Step Execution

Concept Flow - Array unique and flip

Start with array

↓

Apply array_unique

↓

Remove duplicate values

↓

Apply array_flip

↓

Swap keys and values

↓

Result: flipped unique array

Start with an array, remove duplicates with array_unique, then swap keys and values using array_flip.

Execution Sample

PHP

<?php
$arr = ["a", "b", "a", "c"];
$unique = array_unique($arr);
$flipped = array_flip($unique);
print_r($flipped);
?>

This code removes duplicates from the array and then flips keys and values.

Execution Table

Step	Action	Array State	Output/Result
1	Start with array	["a", "b", "a", "c"]	Initial array
2	Apply array_unique	[0 => "a", 1 => "b", 3 => "c"]	Duplicates removed, keys preserved
3	Apply array_flip	["a" => 0, "b" => 1, "c" => 3]	Keys and values swapped
4	Print result	Same as step 3	Array printed with flipped keys and values

💡 All duplicates removed and keys/values flipped, process complete.

Variable Tracker

Variable	Start	After array_unique	After array_flip	Final
$arr	["a", "b", "a", "c"]	["a", "b", "a", "c"]	["a", "b", "a", "c"]	["a", "b", "a", "c"]
$unique	N/A	[0 => "a", 1 => "b", 3 => "c"]	[0 => "a", 1 => "b", 3 => "c"]	[0 => "a", 1 => "b", 3 => "c"]
$flipped	N/A	N/A	["a" => 0, "b" => 1, "c" => 3]	["a" => 0, "b" => 1, "c" => 3]

Key Moments - 2 Insights

Why does array_unique keep the original keys instead of reindexing?

array_unique preserves the original keys to keep track of the first occurrence of each unique value, as shown in step 2 of the execution_table where keys 0,1,3 remain.

What happens if array_flip is used on an array with duplicate values?

array_flip will overwrite keys for duplicate values because keys must be unique. Using array_unique first avoids this issue, as shown in step 3 where flipped keys are unique.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the key for value "c" after array_unique?

A 3

B 2

C 1

D 0

💡 Hint

Check the 'Array State' column at step 2 in the execution_table.

At which step does the array keys and values get swapped?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look for the action 'Apply array_flip' in the execution_table.

If the original array had no duplicates, how would the array_unique step change?

A It would reindex the keys

B It would remove some elements

C It would keep the array unchanged

D It would flip keys and values

💡 Hint

Refer to the variable_tracker for $unique after array_unique.

Concept Snapshot

array_unique(array) removes duplicate values but keeps original keys.
array_flip(array) swaps keys and values.
Use array_unique before array_flip to avoid key conflicts.
Resulting array has unique values as keys and original keys as values.

Full Transcript

We start with an array that may have duplicate values. First, we use array_unique to remove duplicates but keep the original keys. Then, we apply array_flip to swap keys and values. This process ensures the flipped array has unique keys. The execution table shows each step's array state and output. Key moments clarify why keys are preserved and why duplicates must be removed before flipping. The visual quiz tests understanding of keys after unique and when flipping happens. The snapshot summarizes the key points for quick recall.

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