Null coalescing operator in PHP - Time & Space Complexity

Time Complexity: Null coalescing operator

O(1)

Understanding Time Complexity

Let's explore how the null coalescing operator affects the time it takes for a PHP program to run.

We want to see how the number of steps changes as the input changes.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

$value = $array['key'] ?? 'default';

This code checks if 'key' exists in the array and is not null; if not, it uses 'default'.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Single key lookup in an array.
• How many times: Exactly once per execution.

How Execution Grows With Input

Checking one key in an array takes about the same time no matter how big the array is.

Input Size (n)	Approx. Operations
10	1 key check
100	1 key check
1000	1 key check

Pattern observation: The time stays about the same even if the array grows larger.

Final Time Complexity

Time Complexity: O(1)

This means the operation takes a fixed amount of time regardless of input size.

Common Mistake

[X] Wrong: "The null coalescing operator checks every element in the array."

[OK] Correct: It only checks one key directly, so it does not loop through the whole array.

Interview Connect

Understanding how simple operations like key lookups work helps you explain your code clearly and confidently.

Self-Check

"What if we used a loop to check multiple keys with the null coalescing operator? How would the time complexity change?"

Practice

1. What does the null coalescing operator ?? do in PHP?

easy

A. Returns the first operand if it is not null, otherwise returns the second operand.

B. Returns true if both operands are null.

C. Checks if a variable is set and throws an error if not.

D. Assigns null to a variable if it is not already set.

Solution

1. Step 1: Understand the operator's purpose

   The null coalescing operator ?? checks if the left value is null.

2. Step 2: Determine the returned value

   If the left value is not null, it returns that; otherwise, it returns the right value.

3. Final Answer:

   Returns the first operand if it is not null, otherwise returns the second operand. -> Option A

4. Quick Check:

   Null coalescing operator returns first non-null value [OK]

Hint: Remember: returns first non-null value [OK]

Common Mistakes:

• Thinking it returns true/false
• Confusing it with the ternary operator
• Assuming it throws errors if variable is unset

2. Which of the following is the correct syntax to use the null coalescing operator in PHP?

easy

A. $value = $a ??= $b;

B. $value = $a ?: $b;

C. $value = $a ? $b;

D. $value = $a ?? $b;

Solution

1. Step 1: Identify the null coalescing operator syntax

   The operator is written as ?? between two expressions.

2. Step 2: Check each option

   $value = $a ?? $b; uses $a ?? $b which is correct syntax. $value = $a ?: $b; uses the ternary shorthand, not null coalescing. $value = $a ? $b; is incomplete syntax. $value = $a ??= $b; uses ??= which is a different operator (null coalescing assignment).

3. Final Answer:

   $value = $a ?? $b; -> Option D

4. Quick Check:

   Correct null coalescing syntax uses ?? between values [OK]

Hint: Use ?? between two values for null coalescing [OK]

Common Mistakes:

• Confusing ?? with ?: operator
• Using incomplete or invalid syntax
• Mixing null coalescing with assignment operator

3. What will be the output of the following PHP code?

$a = null;
$b = 5;
$c = $a ?? $b;
echo $c;

medium

A. null

B. 0

C. 5

D. Error

Solution

1. Step 1: Evaluate the variables

   $a is null, $b is 5.

2. Step 2: Apply null coalescing operator

   $c = $a ?? $b means if $a is not null use $a, else use $b. Since $a is null, $c becomes 5.

3. Final Answer:

   5 -> Option C

4. Quick Check:

   Null coalescing returns 5 when left is null [OK]

Hint: If left is null, output right value [OK]

Common Mistakes:

• Assuming output is null
• Confusing with ternary operator behavior
• Expecting error due to null variable

4. Identify the error in the following PHP code using the null coalescing operator:

$value = ?? 10;

medium

A. Missing left operand before ?? operator.

B. Missing semicolon at the end.

C. Using wrong operator, should be ?: instead of ??.

D. No error, code is correct.

Solution

1. Step 1: Analyze the operator usage

   The null coalescing operator requires a left and right operand, like $a ?? $b.

2. Step 2: Identify the error in the code

   The code has ?? 10 without a left operand before ??, which is invalid syntax.

3. Final Answer:

   Missing left operand before ?? operator. -> Option A

4. Quick Check:

   Null coalescing needs both operands [OK]

Hint: Both sides needed for ?? operator [OK]

Common Mistakes:

• Leaving out left operand
• Confusing ?? with ternary operator
• Ignoring syntax errors

5. Given the array $data = ['name' => null, 'age' => 25];, which code snippet correctly assigns $result to the value of 'name' if it exists and is not null, otherwise to 'Unknown'?

hard

A. $result = $data['name'] ?? 'Unknown';

B. $result = isset($data['name']) ? $data['name'] : 'Unknown';

C. $result = $data['name'] ?: 'Unknown';

D. $result = $data['name'] ??= 'Unknown';

Solution

1. Step 1: Understand the difference between ?? and isset()

   The null coalescing operator returns the right value if the left is null or not set. However, if the key exists with a null value, $data['name'] ?? 'Unknown' returns 'Unknown'.

2. Step 2: Check each option's behavior

   $result = $data['name'] ?? 'Unknown'; returns 'Unknown' because 'name' is null. $result = isset($data['name']) ? $data['name'] : 'Unknown'; uses isset() which returns false if null, so it returns 'Unknown'. $result = $data['name'] ?: 'Unknown'; uses ?: which treats null as false, so returns 'Unknown'. $result = $data['name'] ??= 'Unknown'; uses ??= which assigns 'Unknown' to $data['name'] if null or unset, then returns it.

3. Step 3: Choose the best option for the requirement

   The question asks to assign $result to 'name' if it exists and is not null, else 'Unknown'. Since 'name' exists but is null, only $result = isset($data['name']) ? $data['name'] : 'Unknown'; correctly checks existence and returns 'Unknown'. $result = $data['name'] ?? 'Unknown'; and C treat null as missing, $result = $data['name'] ??= 'Unknown'; modifies the array which is not asked.

4. Final Answer:

   $result = isset($data['name']) ? $data['name'] : 'Unknown'; -> Option B

5. Quick Check:

   Use isset() to check existence and null separately [OK]

Hint: Use isset() to check key exists and not null [OK]

Common Mistakes:

• Assuming ?? checks key existence only
• Using ??= which changes original array
• Confusing ?: with ?? operator