Yield keyword behavior in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PHP generator?

Consider the following PHP code using yield. What will it output when run?

PHP

<?php
function gen() {
    yield 1;
    yield 2;
    yield 3;
}
foreach (gen() as $value) {
    echo $value . ' ';
}
?>

A Error: yield used outside function

B 123

C 1 2 3

D Array

💡 Hint

Remember that yield produces values one by one, and the foreach loop prints each.

The generator function gen() yields values 1, 2, and 3 sequentially. The foreach loop prints each value followed by a space, resulting in 1 2 3 .

❓ Predict Output

intermediate

What does this PHP generator return?

What will be the output of this PHP code snippet?

PHP

<?php
function gen() {
    yield 'a' => 1;
    yield 'b' => 2;
}
foreach (gen() as $key => $value) {
    echo "$key:$value ";
}
?>

A a:1 b:2

B 1:a 2:b

C a b

D Error: Invalid key in yield

💡 Hint

Keys can be assigned in yield and are preserved in foreach.

The generator yields keys 'a' and 'b' with values 1 and 2 respectively. The foreach loop prints each key and value pair separated by a colon and space.

❓ Predict Output

advanced

What is the output of this nested generator?

Analyze the following PHP code with nested generators. What will it output?

PHP

<?php
function gen1() {
    yield 1;
    yield 2;
}
function gen2() {
    yield from gen1();
    yield 3;
}
foreach (gen2() as $v) {
    echo $v . ' ';
}
?>

A Error: yield from not supported

B 3 1 2

C 1 3 2

D 1 2 3

💡 Hint

The yield from keyword delegates yielding to another generator.

The gen2() function uses yield from gen1() to yield all values from gen1() first, then yields 3. The output is 1 2 3 .

❓ Predict Output

advanced

What error does this PHP code raise?

What error will this PHP code produce when executed?

PHP

<?php
function gen() {
    yield 1;
}
$gen = gen();
$gen->send(10);
?>

A No error, outputs 10

B Fatal error: Cannot send() to a generator that has not started

C 1

D 10

💡 Hint

You must start the generator before sending a value.

The send() method requires the generator to be started (at least one yield executed). Calling send() before starting causes a fatal error.

🧠 Conceptual

expert

How many values does this generator produce?

Given the following PHP generator code, how many values will be yielded when iterated?

PHP

<?php
function gen() {
    for ($i = 0; $i < 3; $i++) {
        if ($i === 1) {
            yield $i;
            continue;
        }
        yield $i * 2;
    }
}
$values = iterator_to_array(gen());
echo count($values);
?>

A 3

B 2

C 4

D 1

💡 Hint

Count how many times yield is called in the loop.

The loop runs 3 times (0,1,2). For i=0 and i=2, it yields i*2. For i=1, it yields i. So total yields: 3.

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