Question

Is it possible to “peek ahead” while iterating an array in PHP 5.2? For example, I often use foreach to manipulate data from an array:

foreach($array as $object) {
  // do something
}

But I often need to peek at the next element while going through the array. I know I could use a for loop and reference the next item by it’s index ($array[$i+1]), but it wouldn’t work for associative arrays. Is there any elegant solution for my problem, perhaps involving SPL?

Answer 1

You can use the CachingIterator for this purpose.

Here is an example:

$collection = new CachingIterator(
                  new ArrayIterator(
                      array('Cat', 'Dog', 'Elephant', 'Tiger', 'Shark')));

The CachingIterator is always one step behind the inner iterator:

var_dump( $collection->current() ); // null
var_dump( $collection->getInnerIterator()->current() ); // Cat

Thus, when you do foreach over $collection, the current element of the inner ArrayIterator will be the next element already, allowing you to peek into it:

foreach($collection as $animal) {
     echo "Current: $animal";
     if($collection->hasNext()) {
         echo " - Next:" . $collection->getInnerIterator()->current();
     }
     echo PHP_EOL;
 }

Will output:

Current: Cat - Next:Dog
Current: Dog - Next:Elephant
Current: Elephant - Next:Tiger
Current: Tiger - Next:Shark
Current: Shark

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For some reason I cannot explain, the CachingIterator will always try to convert the current element to string. If you want to iterate over an object collection and need to access properties an methods, pass CachingIterator::TOSTRING_USE_CURRENT as the second param to the constructor.

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On a sidenote, the CachingIterator gets it’s name from the ability to cache all the results it has iterated over so far. For this to work, you have to instantiate it with CachingIterator::FULL_CACHE and then you can fetch the cached results with getCache().