Yield keyword behavior in PHP - Time & Space Complexity

Time Complexity: Yield keyword behavior

O(n)

Understanding Time Complexity

Let's explore how using the yield keyword affects the time it takes for a PHP function to run.

We want to know how the number of steps changes as the input grows when using yield.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

function countToN(int $n) {
    for ($i = 1; $i <= $n; $i++) {
        yield $i;
    }
}

foreach (countToN(5) as $number) {
    echo $number . " ";
}

This code yields numbers from 1 to n one by one, instead of returning all at once.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The for loop runs from 1 to n, yielding one value each time.
• How many times: Exactly n times, once per number.

How Execution Grows With Input

Each number from 1 to n is processed one at a time, so the steps grow directly with n.

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

Pattern observation: The number of steps grows in a straight line as n increases.

Final Time Complexity

Time Complexity: O(n)

This means the time to run grows directly in proportion to the size of the input n.

Common Mistake

[X] Wrong: "Using yield makes the function run instantly regardless of input size."

[OK] Correct: yield delays output but the loop still runs n times, so the total work grows with n.

Interview Connect

Understanding how yield affects time helps you explain efficient data processing in real projects.

Self-Check

What if we changed the loop to yield only every other number? How would the time complexity change?

Practice

1. What does the yield keyword do in a PHP function?

easy

A. It returns a value and pauses the function, resuming later to produce more values.

B. It immediately stops the function and returns a single value.

C. It creates a new variable inside the function.

D. It converts the function into a class method automatically.

Solution

1. Step 1: Understand yield behavior

   The yield keyword pauses the function and returns a value without ending it.

2. Step 2: Compare with return

   Unlike return, yield allows the function to resume and produce more values later.

3. Final Answer:

   It returns a value and pauses the function, resuming later to produce more values. -> Option A

4. Quick Check:

   Yield pauses and resumes function = A [OK]

Hint: Yield pauses function to produce values one by one [OK]

Common Mistakes:

• Confusing yield with return which ends function
• Thinking yield creates variables
• Assuming yield converts function to class method

2. Which of the following is the correct syntax to yield values from a function in PHP?

easy

A. yield($value);

B. return yield $value;

C. yield => $value;

D. yield $value;

Solution

1. Step 1: Recall yield syntax

   The correct syntax to yield a value is simply yield $value; inside the function.

2. Step 2: Check other options

   Options B, C, and D use invalid or incorrect syntax for yield in PHP.

3. Final Answer:

   yield $value; -> Option D

4. Quick Check:

   Simple yield syntax is 'yield $value;' [OK]

Hint: Yield uses simple 'yield $value;' syntax [OK]

Common Mistakes:

• Adding return before yield
• Using arrow (=>) with yield
• Calling yield like a function

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

function gen() {
  yield 1;
  yield 2;
  yield 3;
}
foreach (gen() as $val) {
  echo $val . ' ';
}

medium

A. 1 2 3

B. Array

C. 123

D. Error: yield not allowed here

Solution

1. Step 1: Understand generator yields

   The function gen() yields values 1, 2, and 3 one by one.

2. Step 2: foreach prints each yielded value

   The foreach loop prints each value followed by a space, so output is '1 2 3 '.

3. Final Answer:

   1 2 3 -> Option A

4. Quick Check:

   Yielded values printed with spaces = B [OK]

Hint: Yield outputs values one by one in foreach loop [OK]

Common Mistakes:

• Expecting concatenated string without spaces
• Thinking yield returns an array
• Assuming syntax error with yield

4. Identify the error in this PHP code using yield:

function numbers() {
  yield 1
  yield 2;
}
foreach(numbers() as $n) {
  echo $n;
}

medium

A. foreach loop syntax is wrong

B. Missing semicolon after first yield statement

C. Cannot use yield inside a function

D. Yield must be followed by parentheses

Solution

1. Step 1: Check yield statements

   The first yield statement is missing a semicolon at the end, causing a syntax error.

2. Step 2: Validate other parts

   Using yield inside a function and foreach syntax are correct; yield does not require parentheses.

3. Final Answer:

   Missing semicolon after first yield statement -> Option B

4. Quick Check:

   Each yield needs semicolon [OK]

Hint: Check semicolons after yield statements [OK]

Common Mistakes:

• Forgetting semicolon after yield
• Thinking yield needs parentheses
• Assuming foreach syntax error

5. Given this PHP function using yield:

function filterPositive(array $numbers) {
  foreach ($numbers as $num) {
    if ($num > 0) {
      yield $num;
    }
  }
}

$values = [-2, 0, 3, 5, -1];
foreach (filterPositive($values) as $val) {
  echo $val . ',';
}

What is the output?

hard

A. 0,3,5,

B. -2,0,3,5,-1,

C. 3,5,

D. Error: yield cannot be used with if

Solution

1. Step 1: Analyze filtering condition

   The function yields only numbers greater than 0, so it skips -2, 0, and -1.

2. Step 2: Identify yielded values

   Only 3 and 5 satisfy the condition and are yielded, printed with commas.

3. Final Answer:

   3,5, -> Option C

4. Quick Check:

   Yield filters positive numbers only = D [OK]

Hint: Yield can filter values with if condition [OK]

Common Mistakes:

• Including zero or negative numbers
• Thinking yield can't be inside if
• Expecting all values printed