Yield from delegation in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of yield from delegation with nested generators

What is the output of the following PHP code?

PHP

<?php
function gen1() {
    yield 1;
    yield 2;
}

function gen2() {
    yield from gen1();
    yield 3;
}

foreach (gen2() as $value) {
    echo $value . ' ';
}
?>

A 3 1 2

B 1 3 2

C 2 1 3

D 1 2 3

💡 Hint

Remember that yield from passes all values from the delegated generator in order.

The function gen2 uses 'yield from gen1()', which yields 1 and 2 in order, then yields 3. So the output is '1 2 3 '.

❓ Predict Output

intermediate

Value returned by yield from delegation

What is the output of this PHP code?

PHP

<?php
function inner() {
    yield 1;
    yield 2;
    return 42;
}

function outer() {
    $result = yield from inner();
    yield $result;
}

foreach (outer() as $value) {
    echo $value . ' ';
}
?>

A 1 2

B 42 1 2

C 1 2 42

D 1 2 0

💡 Hint

The return value of the inner generator is captured by yield from and can be yielded afterwards.

The inner generator yields 1 and 2, then returns 42. The outer generator yields from inner(), capturing 42 in $result, then yields $result. So output is '1 2 42 '.

🔧 Debug

advanced

Identify the error in yield from usage

What error does this PHP code produce?

PHP

<?php
function gen() {
    yield from 123;
}

foreach (gen() as $v) {
    echo $v;
}
?>

A Syntax error: unexpected 'from'

B Fatal error: Cannot iterate over an integer

C Warning: Undefined variable

D No output, code runs silently

💡 Hint

yield from expects an iterable or generator, not a scalar value.

The code tries to yield from an integer 123, which is not iterable. This causes a fatal error about iterating over a non-iterable.

📝 Syntax

advanced

Correct yield from syntax in PHP

Which option shows the correct syntax to delegate yielding to another generator in PHP?

A yield from anotherGenerator();

B yield anotherGenerator();

C yieldfrom anotherGenerator();

D yield_from anotherGenerator();

💡 Hint

The keyword is two words separated by a space.

The correct syntax uses 'yield from' with a space. Other options are invalid syntax or yield the generator object itself.

🚀 Application

expert

Count total yielded values using yield from delegation

Given the following PHP code, what is the value of $count after execution?

PHP

<?php
function genA() {
    yield 1;
    yield 2;
}

function genB() {
    yield 3;
    yield 4;
}

function genC() {
    yield from genA();
    yield from genB();
    yield 5;
}

$count = 0;
foreach (genC() as $v) {
    $count++;
}
echo $count;
?>

A 5

B 4

C 6

D 3

💡 Hint

Count all values yielded by genA, genB, and the last yield in genC.

genA yields 2 values, genB yields 2 values, and genC yields one more value (5). Total is 2 + 2 + 1 = 5.

Practice

1.

What does the yield from statement do in PHP generators?

easy

A. It delegates yielding values from another generator or iterable.

B. It stops the generator immediately.

C. It converts a generator into an array.

D. It throws an exception inside the generator.

Solution

1. Step 1: Understand the purpose of yield from

   yield from is used to delegate yielding values from another generator or iterable, so you don't have to write a loop to yield each value manually.

2. Step 2: Compare with other options

   Stopping a generator, converting it to an array, or throwing exceptions are unrelated to yield from.

3. Final Answer:

   It delegates yielding values from another generator or iterable. -> Option A

4. Quick Check:

   yield from delegates values [OK]

Hint: Remember: yield from passes values from another generator [OK]

Common Mistakes:

• Thinking yield from stops the generator
• Confusing yield from with array conversion
• Assuming it throws exceptions

2.

Which of the following is the correct syntax to delegate yielding from another generator $gen inside a generator function?

function gen() {
    // your code here
}

easy

A. yield $gen;

B. yield to $gen;

C. yield($gen);

D. yield from $gen;

Solution

1. Step 1: Recall correct yield from syntax

   The correct syntax to delegate yielding from another generator is yield from $gen;.

2. Step 2: Check other options for syntax errors

   yield $gen; yields the generator object itself, not its values. yield($gen); is invalid syntax. yield to $gen; is not valid PHP syntax.

3. Final Answer:

   yield from $gen; -> Option D

4. Quick Check:

   Correct syntax uses yield from [OK]

Hint: Use exact phrase yield from to delegate generators [OK]

Common Mistakes:

• Using just yield $gen; instead of yield from
• Adding parentheses incorrectly
• Writing invalid keywords like 'yield to'

3.

What will be the output of the following PHP code?

function gen1() {
    yield 1;
    yield 2;
}

function gen2() {
    yield from gen1();
    yield 3;
}

foreach (gen2() as $value) {
    echo $value . ' ';
}

medium

A. 1 3 2

B. 3 1 2

C. 1 2 3

D. 1 2

Solution

1. Step 1: Understand yield from gen1() in gen2()

   This delegates yielding values 1 and 2 from gen1() inside gen2().

2. Step 2: Trace the output of the foreach loop

   The loop yields 1, then 2 from gen1(), then yields 3 from gen2(). So output is "1 2 3 "

3. Final Answer:

   1 2 3 -> Option C

4. Quick Check:

   Delegation yields 1,2 then 3 [OK]

Hint: Remember yield from outputs all delegated values first [OK]

Common Mistakes:

• Assuming order is changed
• Forgetting the last yield after delegation
• Thinking yield from yields the generator object

4.

Identify the error in this PHP generator code:

function genA() {
    yield 1;
    yield 2;
}

function genB() {
    yield genA();
    yield 3;
}

foreach (genB() as $val) {
    echo $val . ' ';
}

medium

A. It yields the generator object instead of its values.

B. Missing semicolon after yield statements.

C. Cannot yield inside a function.

D. The foreach loop is invalid syntax.

Solution

1. Step 1: Analyze yield genA(); in genB()

   This yields the generator object genA() itself, not the values it produces.

2. Step 2: Understand correct delegation with yield from

   To yield values from genA(), use yield from genA(); instead of yield genA();.

3. Final Answer:

   It yields the generator object instead of its values. -> Option A

4. Quick Check:

   Use yield from to delegate values [OK]

Hint: Use yield from to yield values, not the generator object [OK]

Common Mistakes:

• Using yield instead of yield from for delegation
• Assuming yielding a generator yields its values
• Ignoring the need to delegate properly

5.

Given two generators genX() yielding numbers 1 to 3 and genY() yielding numbers 4 to 6, how can you create a generator genZ() that yields all numbers from 1 to 6 using yield from?

function genX() {
    yield 1;
    yield 2;
    yield 3;
}

function genY() {
    yield 4;
    yield 5;
    yield 6;
}

function genZ() {
    // Fill in code here
}

hard

A. yield genX(); yield genY();

B. yield from genX(); yield from genY();

C. yield genX() + genY();

D. yield from genY(); yield from genX();

Solution

1. Step 1: Use yield from to delegate both generators in order

   To yield all numbers from 1 to 6 in sequence, delegate genX() first, then genY() using yield from.

2. Step 2: Check order and syntax

   yield from genX(); yield from genY(); yields 1,2,3 then 4,5,6. yield from genY(); yield from genX(); reverses order. Options B and C yield generator objects or invalid expressions.

3. Final Answer:

   yield from genX(); yield from genY(); -> Option B

4. Quick Check:

   Delegate generators in order with yield from [OK]

Hint: Chain generators with multiple yield from statements [OK]

Common Mistakes:

• Yielding generators instead of delegating
• Reversing the order of delegation
• Trying to add generators directly