What are the differences between `IteratorIterator` and `RecursiveIteratorIterator` in PHP?

Understanding RecursiveIteratorIterator in PHP

Introduction

RecursiveIteratorIterator is a concrete iterator that implements tree traversal. It enables programmers to traverse container objects that implement the RecursiveIterator interface, allowing for looping over nodes in an ordered tree of objects.

Difference between IteratorIterator and RecursiveIteratorIterator

IteratorIterator is also a concrete iterator, but it performs linear traversal on any Traversable. In contrast, RecursiveIteratorIterator requires a RecursiveIterator to traverse a tree. It exposes its sub-iterator (the currently active iterator) through the getInnerIterator() method, while IteratorIterator exposes its main iterator through the same method.

Technical Differences

• RecursiveIteratorIterator takes a RecursiveIterator as input, while IteratorIterator takes any Traversable.
• RecursiveIteratorIterator has knowledge of parent and children nodes, while IteratorIterator does not.
• RecursiveIteratorIterator maintains a stack of iterators and knows the active sub-iterator, while IteratorIterator does not need a stack.
• RecursiveIteratorIterator has more methods than IteratorIterator.

Traversing a Directory Tree Example

To illustrate the use of these iterators, let's consider a directory tree:

tree
├── dirA
│   ├── dirB
│   │   └── fileD
│   ├── fileB
│   └── fileC
└── fileA

• Using IteratorIterator:

$dir = new DirectoryIterator('tree');
foreach ($dir as $file) {
    echo " ├ $file" . PHP_EOL;
}

Output:

 ├ .
 ├ ..
 ├ dirA
 ├ fileA

• Using RecursiveIteratorIterator:

$dir = new RecursiveDirectoryIterator('tree');
$files = new RecursiveIteratorIterator($dir);
foreach ($files as $file) {
    echo " ├ $file" . PHP_EOL;
}

Output:

 ├ tree\.
 ├ tree\..
 ├ tree\dirA\.
 ├ tree\dirA\..
 ├ tree\dirA\dirB\.
 ├ tree\dirA\dirB\..
 ├ tree\dirA\dirB\fileD
 ├ tree\dirA\fileB
 ├ tree\dirA\fileC
 ├ tree\fileA

Customizing Traversal

RecursiveIteratorIterator provides recursion modes that control the order in which nodes are traversed:

• LEAVES_ONLY: List only files
• SELF_FIRST: List directories first, then files
• CHILD_FIRST: List files first, then directories

Conclusion

RecursiveIteratorIterator provides powerful capabilities for traversing tree-like structures in PHP. Its traversal modes offer flexibility, and its meta-information allows for advanced customization of the iteration process.

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