Which of the following are iterator functions in php?

The Iterator interface

[PHP 5, PHP 7, PHP 8]

Introduction

Interface for external iterators or objects that can be iterated themselves internally.

Interface synopsis

Predefined iterators

PHP already provides a number of iterators for many day to day tasks. See SPL iterators for a list.

Examples

Example #1 Basic usage

This example demonstrates in which order methods are called when using foreach with an iterator.

The above example will output something similar to:

string[18] "myIterator::rewind"
string[17] "myIterator::valid"
string[19] "myIterator::current"
string[15] "myIterator::key"
int[0]
string[12] "firstelement"

string[16] "myIterator::next"
string[17] "myIterator::valid"
string[19] "myIterator::current"
string[15] "myIterator::key"
int[1]
string[13] "secondelement"

string[16] "myIterator::next"
string[17] "myIterator::valid"
string[19] "myIterator::current"
string[15] "myIterator::key"
int[2]
string[11] "lastelement"

string[16] "myIterator::next"
string[17] "myIterator::valid"

Table of Contents

• Iterator::current — Return the current element
• Iterator::key — Return the key of the current element
• Iterator::next — Move forward to next element
• Iterator::rewind — Rewind the Iterator to the first element
• Iterator::valid — Checks if current position is valid

robert_e_lee at dell dot com ¶

12 years ago

Order of operations when using a foreach loop:

1. Before the first iteration of the loop, Iterator::rewind[] is called.
2. Before each iteration of the loop, Iterator::valid[] is called.
3a. It Iterator::valid[] returns false, the loop is terminated.
3b. If Iterator::valid[] returns true, Iterator::current[] and
Iterator::key[] are called.
4. The loop body is evaluated.
5. After each iteration of the loop, Iterator::next[] is called and we repeat from step 2 above.

This is roughly equivalent to:



The loop isn't terminated until Iterator::valid[] returns false or the body of the loop executes a break statement.

The only two methods that are always executed are Iterator::rewind[] and Iterator::valid[] [unless rewind throws an exception].

The Iterator::next[] method need not return anything. It is defined as returning void. On the other hand, sometimes it is convenient for this method to return something, in which case you can do so if you want.

If your iterator is doing something expensive, like making a database query and iterating over the result set, the best place to make the query is probably in the Iterator::rewind[] implementation.

In this case, the construction of the iterator itself can be cheap, and after construction you can continue to set the properties of the query all the way up to the beginning of the foreach loop since the
Iterator::rewind[] method isn't called until then.

Things to keep in mind when making a database result set iterator:

* Make sure you close your cursor or otherwise clean up any previous query at the top of the rewind method. Otherwise your code will break if the same iterator is used in two consecutive foreach loops when the first loop terminates with a break statement before all the results are iterated over.

* Make sure your rewind[] implementation tries to grab the first result so that the subsequent call to valid[] will know whether or not the result set is empty. I do this by explicitly calling next[] from the end of my rewind[] implementation.

* For things like result set iterators, there really isn't always a "key" that you can return, unless you know you have a scalar primary key column in the query. Unfortunately, there will be cases where either the iterator doesn't know the primary key column because it isn't providing the query, the nature of the query is such that a primary key isn't applicable, the iterator is iterating over a table that doesn't have one, or the iterator is iterating over a table that has a compound primary key. In these cases, key[] can return either:
the row index [based on a simple counter that you provide], or can simply return null.

Iterators can also be used to:

* iterate over the lines of a file or rows of a CSV file
* iterate over the characters of a string
* iterate over the tokens in an input stream
* iterate over the matches returned by an xpath expression
* iterate over the matches returned by a regexp
* iterate over the files in a folder
* etc...

RocketInABog at techno-monks dot net ¶

13 years ago

fetidfrog at gmail dot com ¶

10 years ago

If you have a custom iterator that may throw an exception in it's current[] method, there is no way to catch the exception without breaking a foreach loop.

The following for loop allows you to skip elements for which $iterator->current[] throws an exception, rather than breaking the loop.

mike dot thornton at firstroi dot com ¶

13 years ago

It's important to note that following won't work if you have null values.



Other examples have shown the following which won't work if you have false values:



Instead use:



Or the following if you do not store the position.

Geoffrey Sneddon ¶

13 years ago

So, playing around with iterators in PHP [coming from languages where I'm spoiled with generators to do things like this], I wrote a quick piece of code to give the Fibonacci sequence [to infinity, though only the first terms up to F_{10} are output].

Anonymous ¶

6 years ago

An interesting fact that I didn't read in the doc:

the key[] method is called only if your foreach loop needs it.

For instance, the following loop calls the key[] method:



But the following loop doesn't:

Gilles A ¶

8 years ago

Examples of use


geompse at gmail dot com ¶

11 years ago

Be carefull with Iterator when using nested loops or deleting items inside the collection while looping over it.
It can be tricky to detect.
This unexpected behavior is pertinent if you think about it long enough.

Anthony Sterling ¶

13 years ago

Here's a Fibonacci example using the formula, rather than addition.



Anthony.

InvisibleSmiley ¶

2 months ago

If you implemented Iterator methods next[] and rewind[] by calling array functions next[] and reset[] and returning their results, be advised that this violates the tentative return types [void in both cases] introduced with PHP 8.1.

You can add the #[\ReturnTypeWillChange] attribute to both method implementations but that will only delay the issue until PHP 9.0 comes around.

Better adapt your implementations now [stop returning anything from these methods] and, if need be, only add the return type declarations later.

jeroen at asystance dot nl ¶

2 years ago

RocketInABog's seemingly trivial tIterator_array class has one huge problem [which just cost me a couple of hours].

Consider this example, using their class:

The problem is that foreach resets arrays, but doesn't call Iterator::rewind on objects!

I also think it's a design mistake that foreach works with Iterator, but current[], key[] and end[] don't - these iterate over the objects fields.

I just refactored some code to use an Iterator instead of an array, and it broke in several very unexpected ways because of these differences.

Andrew Roffey ¶

1 year ago

/**
* Iterate  a directory tree by walking the tree. For each directory in
* the tree rooted at directory $parent_dir [including $parent_dir
* itself], it returns $dirpath => array[$dirnames, $filenames].
*
* $dirpath is a string, the path to the directory. $dirnames is a list
* of the names of the subdirectories in dirpath [excluding '.' and
* '..']. $filenames is a list of the names of the non-directory files in
* $dirpath.
*
* Inspired by Python os.walk; see
* //docs.python.org/3/library/os.html#os.walk
* Implemented as an iterator rather than a generator.
*
* @return array
*/

class walker implements Iterator {
  private $parent_dir = '';
  private $d = 0; // iterator
  private $dirs = array[]; // indexed by iterator
  private $dirstack;
  private $discovered = array[]; // indexed by directory
  private $v; // current directory
  private $cur_dirnames = array[]; // current subdirs
  private $cur_filenames = array[]; // current files in directory

  public function __construct[$parent_dir] {
    $this->parent_dir = $parent_dir;
    $this->d = 0;
    $this->dirs[$this->d] = $parent_dir;
    $this->dirstack = new SplStack[];
    $this->dirstack->push[$parent_dir];
    $this->discovered = array[$parent_dir => true];
    $this->v = $parent_dir;
    $this->cur_dirnames = array[];
    $this->cur_filenames = array[];
    $this->next[];
  }

  public function rewind[] {
    $this->__construct[$this->parent_dir];
  }

  public function current[] {
    return array[$this->cur_dirnames, $this->cur_filenames];
  }

  public function key[] {
    return $this->v;
  }

  public function next[] {
    ++$this->d;
    $this->v = $this->dirstack->pop[];
    $this->dirs[$this->d] = $this->v;
    $this->cur_dirnames = array[];
    $this->cur_filenames = array[];
    if [!$dh = opendir[$this->v]] {
      // opendir emits E_WARNING if unable to open directory, likely due
      // to a permissions issue or directory removed before we could get
      // there
      return;
    }
    // discover the directories, return directories and files
    while [false !== [$fn = readdir[$dh]]] {
      if [$fn != '.' && $fn !== '..'] {
        $fullfn = $this->v . '/' . $fn;
        if [is_dir[$fullfn]] {
      $this->cur_dirnames[] = $fn;
          if [!array_key_exists[$fullfn, $this->discovered]] {
            $this->discovered[$fullfn] = true;
        $this->dirstack->push[$fullfn];
      }
        } else {
          $this->cur_filenames[] = $fn;
        }
      }
    }
    closedir[$dh];
  }

  public function valid[] {
    return $this->dirstack->count[];
  }
}

$tree = new walker["/tmp"];
foreach [$tree as $parent_dir => $nodes] {
  $subdirs = $nodes[0];
  $files = $nodes[1];
  printf["%s\n", $parent_dir];
  if [$subdirs] printf["  %s\n", implode["/\n  ", $subdirs]];
  if [$files] printf["  %s\n", implode["\n  ", $files]];
  print["\n"];
}

sofe2038 at gmail dot com ¶

2 years ago

The "scalar" restriction on key[] is no longer true. A simple example is that Generators can yield non-scalar keys.

dwlnetnl at users dot sourceforge dot net ¶

10 years ago

Be aware that when you call a method like current[$this] within the Iterator class, the properties of the class are returned and the Iterator's current[] isn't called. This is because current[] applies to arrays and the Iterator class is then interpretated as an array.

ct ¶

11 years ago


* ...
*
* 
*      one
* 
* 
*      two
* 
*  ...
*
*
*/class parseXML implements Iterator {

        private

kent at marketacumen dot com ¶

11 years ago

For Iterators implement database queries, what I've found is that if you want to chain multiple iterators together using a "MultipleIterator" then you *do not* want ::rewind[] to actually execute your query, especially if it's expensive.

Instead, what I've done is implement that portion in "valid."

e.g.



That way if you chain multiple queries in a "MultipleIterator" together, the "rewind" call [which rewinds all iterators at once] does not execute every query at once.

In addition, I found that the MultipleIterator may now work best for other reasons, but still, the above is a good idea to postpone queries until the last possible moment they are needed.

Daniel Lo Nigro ¶

9 years ago

If you're using PHP 5.5 or above and are creating a simple iterator, consider using a generator function instead. There is significantly less boilerplate code and the code is easier to read. //au1.php.net/generators

shaun at slickdesign dot com dot au ¶

3 years ago

With a large number of `current`, `next`, `key`, and `reset` array function implementations, care needs to be taken to ensure that deletions and nested loops are accounted for appropriately for each situation.

The following class has been copied from a previous comment and modified to allow it to be used within nested loops.