Compact and extract functions in PHP - Time & Space Complexity

Time Complexity: Compact and extract functions

O(n)

Understanding Time Complexity

We want to understand how the time it takes to run PHP's compact and extract functions changes as the number of variables grows.

How does the work inside these functions grow when we add more variables?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

$var1 = 10;
$var2 = 20;
$var3 = 30;

// Create an array from variables
$array = compact('var1', 'var2', 'var3');

// Extract variables back from array
extract($array);
    

This code creates an array from named variables using compact, then restores those variables using extract.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Both compact and extract loop through the list of variable names or array keys.
• How many times: Once for each variable name given to compact or each key in the array for extract.

How Execution Grows With Input

As the number of variables increases, the functions do more work, roughly one step per variable.

Input Size (n)	Approx. Operations
10	About 10 steps
100	About 100 steps
1000	About 1000 steps

Pattern observation: The work grows directly in proportion to the number of variables.

Final Time Complexity

Time Complexity: O(n)

This means the time to run compact or extract grows linearly with the number of variables involved.

Common Mistake

[X] Wrong: "compact and extract run instantly no matter how many variables there are."

[OK] Correct: Both functions must look at each variable name or array key, so more variables mean more work and more time.

Interview Connect

Understanding how built-in functions like compact and extract scale helps you reason about performance in real projects and shows you can think about code efficiency clearly.

Self-Check

"What if we passed an array with nested arrays to extract? How would the time complexity change?"

Practice

1. What does the PHP function compact do?

easy

A. Creates an array from given variable names and their values

B. Extracts variables from an array into the current symbol table

C. Deletes variables from the current scope

D. Converts an array into a JSON string

Solution

1. Step 1: Understand the purpose of compact

   The compact function takes variable names as strings and creates an associative array where keys are those names and values are the variables' values.

2. Step 2: Differentiate from extract

   extract does the opposite by creating variables from array keys, so it is not the correct description for compact.

3. Final Answer:

   Creates an array from given variable names and their values -> Option A

4. Quick Check:

   compact = array from variables [OK]

Hint: Remember: compact packs variables into an array [OK]

Common Mistakes:

• Confusing compact with extract
• Thinking compact extracts variables
• Assuming compact converts arrays to strings

2. Which of the following is the correct syntax to use extract on an array $data?

easy

A. extract = $data;

B. extract($data);

C. extract->$data();

D. extract($data[]);

Solution

1. Step 1: Recall the function call syntax

   In PHP, functions are called by their name followed by parentheses containing arguments. So extract($data); is the correct syntax.

2. Step 2: Identify incorrect syntax

   Options B, C, and D misuse assignment, object operator, or array syntax, which are invalid for function calls.

3. Final Answer:

   extract($data); -> Option B

4. Quick Check:

   Function call syntax = extract($data); [OK]

Hint: Use parentheses to call extract with array argument [OK]

Common Mistakes:

• Using assignment instead of function call
• Using object operator (->) incorrectly
• Adding brackets after variable name

3. What is the output of the following PHP code?

$name = "Alice";
$age = 30;
$arr = compact('name', 'age');
print_r($arr);

medium

A. Array ( [Alice] => name [30] => age )

B. Array ( [0] => name [1] => age )

C. Error: Undefined variables

D. Array ( [name] => Alice [age] => 30 )

Solution

1. Step 1: Understand compact behavior

   The compact function creates an array where keys are the variable names as strings and values are the current values of those variables.

2. Step 2: Apply to given variables

   Variables $name and $age have values "Alice" and 30, so the array will be ['name' => 'Alice', 'age' => 30].

3. Final Answer:

   Array ( [name] => Alice [age] => 30 ) -> Option D

4. Quick Check:

   compact('name','age') = array with keys and values [OK]

Hint: compact creates array with variable names as keys [OK]

Common Mistakes:

• Thinking compact returns variable values as keys
• Expecting numeric keys instead of variable names
• Assuming compact prints variables directly

4. Identify the error in this PHP code snippet:

$data = ['color' => 'red', 'size' => 'large'];
extract($data);
echo $colour;

medium

A. Variable name mismatch: $colour vs 'color' key

B. extract() cannot be used on associative arrays

C. Missing semicolon after extract()

D. echo cannot print variables created by extract()

Solution

1. Step 1: Check variable names created by extract()

   The extract function creates variables named exactly as the array keys, so it creates $color and $size.

2. Step 2: Compare echoed variable name

   The code tries to echo $colour which is different from $color, causing an undefined variable notice.

3. Final Answer:

   Variable name mismatch: $colour vs 'color' key -> Option A

4. Quick Check:

   extract keys = variable names; spelling must match [OK]

Hint: Check spelling of variable names after extract [OK]

Common Mistakes:

• Assuming extract changes variable names
• Thinking extract only works on indexed arrays
• Ignoring PHP notices for undefined variables

5. Given the array $info = ['name' => 'Bob', 'age' => 25, 'city' => 'Paris', 'age' => 30];, what will be the result of extract($info); echo $age;?

hard

A. 25

B. Error: Duplicate keys in array

C. 30

D. Undefined variable error

Solution

1. Step 1: Understand PHP array key behavior

   When an array has duplicate keys, the last value overwrites previous ones. Here, 'age' appears twice with values 25 and 30, so 30 is kept.

2. Step 2: Apply extract() and echo

   Extract creates variables from keys, so $age will be 30, the last value. Echoing $age prints 30.

3. Final Answer:

   30 -> Option C

4. Quick Check:

   Duplicate keys keep last value; extract uses that [OK]

Hint: Last duplicate key value is used in arrays [OK]

Common Mistakes:

• Expecting first duplicate key value to be used
• Thinking extract throws error on duplicates
• Assuming undefined variable error due to duplicates