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.

Practice

1. Which of the following methods is NOT required when implementing the Iterator interface in PHP?

easy

A. sort()

B. current()

C. next()

D. rewind()

Solution

1. Step 1: Recall Iterator interface methods

   The Iterator interface requires implementing rewind(), current(), key(), next(), and valid() methods.

2. Step 2: Identify the method not in Iterator

   The sort() method is not part of the Iterator interface; it is unrelated to iteration.

3. Final Answer:

   sort() -> Option A

4. Quick Check:

   Iterator methods exclude sort() [OK]

Hint: Remember: Iterator needs rewind, current, key, next, valid only [OK]

Common Mistakes:

• Confusing sort() as part of Iterator
• Forgetting to implement key() or valid()
• Thinking rewind() is optional

2. Which of the following is the correct syntax to declare a class MyIterator that implements the Iterator interface?

easy

A. class MyIterator implements Iterator { }

B. class MyIterator extends Iterator { }

C. class MyIterator uses Iterator { }

D. class MyIterator interface Iterator { }

Solution

1. Step 1: Recall PHP interface implementation syntax

   In PHP, a class implements an interface using the keyword 'implements'.

2. Step 2: Check each option's syntax

   class MyIterator implements Iterator { } uses 'implements' correctly. The other options use invalid keywords or syntax.

3. Final Answer:

   class MyIterator implements Iterator { } -> Option A

4. Quick Check:

   Use 'implements' keyword for interfaces [OK]

Hint: Use 'implements' keyword to implement interfaces in PHP [OK]

Common Mistakes:

• Using 'extends' instead of 'implements' for interfaces
• Using 'uses' or 'interface' keywords incorrectly
• Missing curly braces after class declaration

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

$items = ['a', 'b', 'c'];

class MyIterator implements Iterator {
    private array $items;
    private int $index = 0;

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

    public function current() {
        return $this->items[$this->index];
    }

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

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

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

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

$iterator = new MyIterator($items);
foreach ($iterator as $key => $value) {
    echo "$key:$value ";
}

medium

A. a:b c:d e:f

B. 0:a 1:b 2:c

C. 0:0 1:1 2:2

D. Error: Undefined variable

Solution

1. Step 1: Understand the iterator implementation

   The class stores items and uses an index to track position. Methods current(), key(), next(), rewind(), and valid() are correctly implemented.

2. Step 2: Trace the foreach loop output

   foreach uses key() and current() to print "key:value" pairs. Indexes 0,1,2 correspond to 'a','b','c'. Output is "0:a 1:b 2:c "

3. Final Answer:

   0:a 1:b 2:c -> Option B

4. Quick Check:

   Iterator keys and values match array indexes and elements [OK]

Hint: Trace index and current() output for each loop iteration [OK]

Common Mistakes:

• Confusing keys with values
• Forgetting to implement valid() correctly
• Expecting output without spaces

4. Consider this broken iterator class:

class BrokenIterator implements Iterator {
    private array $data = [1, 2, 3];
    private int $pos = 0;

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

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

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

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

    public function valid() {
        return $this->pos < count($this->data);
    }
}

What is the main issue that could cause unexpected behavior when using this iterator in a foreach loop?

medium

A. The next() method does not have a void return type

B. The valid() method should use isset() instead of count()

C. The valid() method returns a boolean but should return an integer

D. The next() and rewind() methods lack return type declarations

Solution

1. Step 1: Check method signatures against Iterator interface

   Iterator interface requires next() and rewind() to have void return type declarations.

2. Step 2: Identify missing return types

   In the code, next() and rewind() lack ': void' return type, which can cause compatibility issues in strict mode.

3. Final Answer:

   Missing void return types in next() and rewind() -> Option D

4. Quick Check:

   next() and rewind() must declare void return type [OK]

Hint: Check method signatures match interface exactly including return types [OK]

Common Mistakes:

• Ignoring return type declarations
• Thinking valid() must return integer
• Confusing isset() and count() usage in valid()

5. You want to create a custom iterator EvenNumbers that iterates over only even numbers from a given array. Which implementation of the valid() method correctly skips odd numbers during iteration?

hard

A. public function valid(): bool {
return isset($this->items[$this->index]);
}

B. public function valid(): bool {
return $this->items[$this->index] % 2 === 0;
}

C. public function valid(): bool {
while (isset($this->items[$this->index]) && $this->items[$this->index] % 2 !== 0) {
$this->index++;
}
return isset($this->items[$this->index]);
}

D. public function valid(): bool {
return $this->index < count($this->items);
}

Solution

1. Step 1: Understand the goal of skipping odd numbers

   The iterator must advance the index until it points to an even number or end of array.

2. Step 2: Analyze each valid() implementation

   public function valid(): bool { while (isset($this->items[$this->index]) && $this->items[$this->index] % 2 !== 0) { $this->index++; } return isset($this->items[$this->index]); } uses a while loop to skip odd numbers by incrementing index, then checks if current index exists. The other options do not skip odd numbers properly.

3. Final Answer:

   while loop in valid() to skip odd numbers -> Option C

4. Quick Check:

   valid() must skip odd values to find even numbers [OK]

Hint: Use a loop inside valid() to skip unwanted elements [OK]

Common Mistakes:

• Returning true without skipping odd numbers
• Not checking if index exists before accessing array
• Using valid() only to check index range