Iterator interface implementation in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple Iterator implementation

What is the output of this PHP code that implements Iterator to loop over an array of colors?

PHP

<?php
class ColorIterator implements Iterator {
    private array $colors;
    private int $position = 0;

    public function __construct(array $colors) {
        $this->colors = $colors;
    }

    public function current(): string {
        return $this->colors[$this->position];
    }

    public function key(): int {
        return $this->position;
    }

    public function next(): void {
        $this->position++;
    }

    public function rewind(): void {
        $this->position = 0;
    }

    public function valid(): bool {
        return isset($this->colors[$this->position]);
    }
}

$colors = ['red', 'green', 'blue'];
$iterator = new ColorIterator($colors);
foreach ($iterator as $key => $color) {
    echo "$key:$color ";
}
?>

A 0:red 1:green 2:blue

B red green blue

C 0:red 1:green

D 1:red 2:green 3:blue

💡 Hint

Remember the Iterator interface methods control the keys and values during foreach.

The Iterator interface methods current(), key(), next(), rewind(), and valid() control the iteration. The code outputs keys 0,1,2 with their colors separated by spaces.

🧠 Conceptual

intermediate

Understanding valid() method in Iterator

What is the role of the valid() method in a PHP Iterator implementation?

A It returns the current element's value.

B It resets the iterator to the first element.

C It checks if the current position is valid and iteration should continue.

D It moves the iterator to the next element.

💡 Hint

Think about how foreach knows when to stop looping.

valid() returns true if the current position is valid, so foreach continues looping. If false, iteration stops.

🔧 Debug

advanced

Identify the error in this Iterator implementation

What error will this PHP code produce when trying to iterate over the object?

PHP

<?php
class NumberIterator implements Iterator {
    private array $numbers = [1, 2, 3];
    private int $pos = 0;

    public function current() {
        return $this->numbers[$this->pos];
    }

    public function key() {
        return $this->pos;
    }

    public function next() {
        $this->pos++;
    }

    public function rewind() {
        $this->pos = 0;
    }

    // Missing valid() method
}

$iter = new NumberIterator();
foreach ($iter as $num) {
    echo $num . ' ';
}
?>

A Outputs: 1 2 3

B Fatal error: Class must implement method valid() from Iterator interface

C Warning: Undefined offset 3

D No output, infinite loop

💡 Hint

Check if all required Iterator methods are implemented.

The Iterator interface requires valid() method. Missing it causes a fatal error.

📝 Syntax

advanced

Which option correctly implements the rewind() method?

Choose the correct rewind() method implementation for a PHP Iterator that resets position to zero.

A public function rewind() { $this->position = 0; }

B public function rewind() { return 0; }

C public function rewind() { $this->position++; }

D public function rewind() { unset($this->position); }

💡 Hint

rewind() should reset the iterator to the start.

rewind() must reset the internal pointer to the first element, usually by setting position to 0.

🚀 Application

expert

Custom Iterator to iterate only even numbers

Given this PHP Iterator class, what is the output when iterating over it?

PHP

<?php
class EvenNumberIterator implements Iterator {
    private array $numbers;
    private int $position = 0;

    public function __construct(array $numbers) {
        $this->numbers = $numbers;
        $this->rewind();
    }

    public function current(): int {
        return $this->numbers[$this->position];
    }

    public function key(): int {
        return $this->position;
    }

    public function next(): void {
        do {
            $this->position++;
        } while (isset($this->numbers[$this->position]) && $this->numbers[$this->position] % 2 !== 0);
    }

    public function rewind(): void {
        $this->position = 0;
        if (isset($this->numbers[$this->position]) && $this->numbers[$this->position] % 2 !== 0) {
            $this->next();
        }
    }

    public function valid(): bool {
        return isset($this->numbers[$this->position]);
    }
}

$nums = [1, 2, 3, 4, 5, 6];
$iter = new EvenNumberIterator($nums);
foreach ($iter as $key => $val) {
    echo "$key:$val ";
}
?>

A 2:3 4:5 6:7

B 0:1 1:2 2:3 3:4 4:5 5:6

C 0:2 1:4 2:6

D 1:2 3:4 5:6

💡 Hint

The iterator skips odd numbers by advancing position until an even number is found.

The iterator starts at position 0 (value 1, odd), calls next() to skip to position 1 (value 2, even). Then next() skips odd numbers similarly. Output keys are original positions of even numbers.