Iterator interface implementation in PHP - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start the iterator at the first element.

PHP

<?php
class MyIterator implements Iterator {
    private array $items = [1, 2, 3];
    private int $position = 0;

    public function rewind(): void {
        $this->position = [1];
    }

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

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

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

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

Drag options to blanks, or click blank then click option'

A-1

B1

C0

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Setting position to 1 skips the first element.

Using null or -1 causes errors when accessing the array.

The rewind() method resets the iterator position to the first element, which is index 0.

2fill in blank

medium

Complete the code to check if the current position is valid.

PHP

<?php
public function valid(): bool {
    return [1]($this->items[$this->position]);
}

Drag options to blanks, or click blank then click option'

Aisset

Bis_null

Carray_key_exists

Dempty

Attempts:

3 left

💡 Hint

Common Mistakes

Using empty() returns false for zero or empty strings, which may be valid elements.

Using is_null() checks value, not existence.

Using array_key_exists() requires the key, but here we check the value.

The isset() function checks if the current position exists in the array, ensuring the iterator is valid.

3fill in blank

hard

Fix the error in the next() method to correctly advance the iterator.

PHP

<?php
public function next(): void {
    $this->position [1];
}

Drag options to blanks, or click blank then click option'

A--

B-=

C+=

D++

Attempts:

3 left

💡 Hint

Common Mistakes

Using -- moves backward, causing incorrect iteration.

Using += without a value is invalid syntax.

The next() method should increase the position by one using ++ to move forward.

4fill in blank

hard

Fill both blanks to implement the current() method returning the current item.

PHP

<?php
public function current(): mixed {
    return $this->items[1];
}

Drag options to blanks, or click blank then click option'

A->position

B[$this->position]

C[position]

D($this->position)

Attempts:

3 left

💡 Hint

Common Mistakes

Using object property access ->position instead of array index.

Using parentheses instead of square brackets causes errors.

The current() method returns the element at the current position using array access syntax with [$this->position].

5fill in blank

hard

Fill all three blanks to complete the Iterator class with correct methods.

PHP

<?php
class MyIterator implements Iterator {
    private array $items = [10, 20, 30];
    private int $position = 0;

    public function rewind(): void {
        $this->position = [1];
    }

    public function current(): mixed {
        return $this->items[2];
    }

    public function valid(): bool {
        return [3]($this->items[$this->position]);
    }
}
?>

Drag options to blanks, or click blank then click option'

A0

B[$this->position]

Cisset

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Setting position to 1 skips the first element.

Using parentheses instead of square brackets for array access.

Using empty() or is_null() instead of isset().

The rewind() method sets position to 0, current() returns the item at current position using square brackets, and valid() checks if the position exists using isset().

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