Generator function execution model in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple generator with yield

What is the output of this PHP code when executed?

PHP

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

A 1 2 3

B 123

C Array

D Error: Cannot use foreach on generator

💡 Hint

Remember that yield produces values one by one and foreach iterates over them.

The generator 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

Generator state after partial iteration

What will be the output of this PHP code?

PHP

<?php
function gen() {
    yield 'a';
    yield 'b';
    yield 'c';
}
$g = gen();
echo $g->current();
$g->next();
echo $g->current();
?>

A Error: Cannot call current() before next()

B bc

C ab

D a

💡 Hint

The generator starts at the first yield value before calling next().

When the generator is created, current() returns the first yielded value 'a'. After calling next(), current() returns the second yielded value 'b'. So the output is 'ab'.

🔧 Debug

advanced

Identify the error in generator usage

What error does this PHP code produce when run?

PHP

<?php
function gen() {
    yield 1;
    yield 2;
}
$g = gen();
$g->next();
$g->next();
$g->next();
echo $g->current();
?>

A Fatal error: Call to undefined method

B NULL

C Notice: Undefined offset

D Error: Cannot advance generator beyond end

💡 Hint

What happens when you call next() past the last yield?

After the generator yields 1 and 2, the third next() call moves the generator past the end. current() then returns NULL because the generator is finished.

🧠 Conceptual

advanced

Behavior of yield in generator with return

What is the output of this PHP code?

PHP

<?php
function gen() {
    yield 10;
    return 20;
}
$g = gen();
foreach ($g as $value) {
    echo $value . ' ';
}
echo $g->getReturn();
?>

A Error: getReturn() not supported

B 10

C 20

D 10 20

💡 Hint

The return value of a generator can be retrieved after iteration.

The generator yields 10, which is printed with a space. After iteration, getReturn() returns the value 20 from the return statement, which is then printed.

❓ Predict Output

expert

Generator with keys and values

What is the output of this PHP code?

PHP

<?php
function gen() {
    yield 'x' => 100;
    yield 'y' => 200;
}
foreach (gen() as $key => $value) {
    echo "$key:$value ";
}
?>

A x:100 y:200

B 100:x 200:y

C 0:100 1:200

D Error: Invalid key syntax

💡 Hint

Yield can specify keys for the generated values.

The generator yields keys 'x' and 'y' with values 100 and 200 respectively. The foreach loop prints each key and value separated by a colon and space.

Practice

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

easy

A. It pauses the function and returns a value to the caller.

B. It immediately ends the function and returns a value.

C. It creates a new variable inside the function.

D. It calls another function asynchronously.

Solution

1. Step 1: Understand the role of yield in generators

   The yield keyword pauses the function execution and sends a value back to the caller without ending the function.

2. Step 2: Compare with other options

   Unlike return, yield does not end the function but allows it to resume later.

3. Final Answer:

   It pauses the function and returns a value to the caller. -> Option A

4. Quick Check:

   yield pauses and returns value [OK]

Hint: Remember: yield pauses, return ends function [OK]

Common Mistakes:

• Confusing yield with return
• Thinking yield creates variables
• Assuming yield calls other functions

2. Which of the following is the correct syntax to define a generator function in PHP?

easy

A. function gen() { return yield 1; }

B. function gen() { yield 1; }

C. function gen() => yield 1;

D. generator gen() { yield 1; }

Solution

1. Step 1: Identify correct generator syntax

   In PHP, a generator function is a normal function that uses yield inside its body.

2. Step 2: Check each option

   function gen() { yield 1; } uses a normal function with yield correctly. function gen() { return yield 1; } misuses return yield. function gen() => yield 1; uses invalid arrow syntax. generator gen() { yield 1; } uses an invalid keyword generator.

3. Final Answer:

   function gen() { yield 1; } -> Option B

4. Quick Check:

   Normal function + yield = generator [OK]

Hint: Generator = function with yield inside braces [OK]

Common Mistakes:

• Using return with yield incorrectly
• Trying arrow function syntax for generators
• Using non-existent keywords like 'generator'

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

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

medium

A. 123

B. Error: Cannot use foreach on generator

C. 1,2,3

D. 1 2 3

Solution

1. Step 1: Understand generator yields

   The generator yields values 1, then 2, then 3 in sequence.

2. Step 2: Analyze foreach output

   The foreach loops over each yielded value and echoes it followed by a space, so output is "1 2 3 "

3. Final Answer:

   1 2 3 -> Option D

4. Quick Check:

   foreach prints each yield with space [OK]

Hint: foreach prints each yield value in order [OK]

Common Mistakes:

• Ignoring spaces in echo output
• Thinking foreach can't iterate generators
• Confusing output format with commas

4. Identify the error in this PHP generator function:

function gen() {
  yield 1;
  return 2;
  yield 3;
}
foreach (gen() as $v) {
  echo $v;
}

medium

A. Syntax error: return not allowed in generator

B. No error, outputs 123

C. The return ends generator early, so yield 3 is never reached

D. foreach cannot iterate generator with return

Solution

1. Step 1: Understand return in generators

   Return ends the generator function immediately, so code after return is not executed.

2. Step 2: Analyze code flow

   After yielding 1, the return 2 ends the generator, so yield 3 is never reached or yielded.

3. Final Answer:

   The return ends generator early, so yield 3 is never reached -> Option C

4. Quick Check:

   Return stops generator, later yields skipped [OK]

Hint: Return ends generator; yields after return ignored [OK]

Common Mistakes:

• Thinking return causes syntax error
• Expecting all yields to run despite return
• Assuming foreach can't handle return in generator

5. Given this generator function, what will be the output?

function gen() {
  for ($i = 0; $i < 3; $i++) {
    yield $i => $i * 2;
  }
}
foreach (gen() as $key => $value) {
  echo "$key:$value ";
}

hard

A. 0:0 1:2 2:4

B. 0:0 1:1 2:2

C. 0 2 4

D. Error: Cannot yield key => value in PHP

Solution

1. Step 1: Understand yield with key => value

   The generator yields keys 0,1,2 with values 0,2,4 respectively (value = key * 2).

2. Step 2: Analyze foreach output

   foreach prints each key and value separated by colon and space, so output is "0:0 1:2 2:4 "

3. Final Answer:

   0:0 1:2 2:4 -> Option A

4. Quick Check:

   Yield key=>value prints key:value pairs [OK]

Hint: Yield key=>value pairs print as key:value [OK]

Common Mistakes:

• Confusing value with key
• Thinking yield key=>value is invalid
• Ignoring spaces in output