Array unique and flip in PHP - Time & Space Complexity

Time Complexity: Array unique and flip

O(n)

Understanding Time Complexity

We want to understand how the time needed changes when we use PHP functions to remove duplicates and flip arrays.

How does the work grow as the array gets bigger?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

$array = [1, 2, 2, 3, 4, 4, 5];
$unique = array_unique($array);
$flipped = array_flip($unique);
print_r($flipped);
    

This code removes duplicate values from an array, then swaps keys and values in the resulting array.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Traversing the array twice -- once for removing duplicates, once for flipping.
• How many times: Each function loops through the array once, so two passes total.

How Execution Grows With Input

As the array size grows, the number of steps grows roughly twice as fast because of two passes.

Input Size (n)	Approx. Operations
10	About 20 steps (2 x 10)
100	About 200 steps (2 x 100)
1000	About 2000 steps (2 x 1000)

Pattern observation: The work grows linearly with the input size, doubling because of two separate loops.

Final Time Complexity

Time Complexity: O(n)

This means the time needed grows directly in proportion to the number of items in the array.

Common Mistake

[X] Wrong: "Using array_unique and array_flip together makes the code run in quadratic time because of nested loops."

[OK] Correct: Each function loops through the array separately, not inside each other, so the total work adds up linearly, not multiplies.

Interview Connect

Understanding how built-in functions work under the hood helps you explain your code clearly and shows you know how performance scales with data size.

Self-Check

"What if we replaced array_unique with a manual loop that checks duplicates? How would the time complexity change?"

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