PHP Generator Class

Introduction

Traversing a big collection of data using looping construct such as foreach would require large memory and considerable processing time. With generators it is possible to iterate over a set of data without these overheads. A generator function is similar to a normal function. However, instead of return statement in a function, generator uses yield keyword to be executed repeatedly so that it provides values to be iterated.

The yield keyword is the heart of generator mechanism. Even though its use appears to be similar to return, it doesn't stop execution of function. It provides next value for iteration and pauses execution of function.

Syntax

Generator implements Iterator {
   /* Methods */
   public current ( void ) : mixed
   public getReturn ( void ) : mixed
   public key ( void ) : mixed
   public next ( void ) : void
   public rewind ( void ) : void
   public send ( mixed $value ) : mixed
   public throw ( Throwable $exception ) : mixed
   public valid ( void ) : bool
   public __wakeup ( void ) : void
}

Methods

public Generator::current ( void ) − mixed — Get the yielded value

public Generator::getReturn ( void ) : mixed  — Get the return value of a generator

public Generator::key ( void ) − mixed — Gets the key of the yielded value.

public Generator::next ( void ) − void — Resume execution of the generator. same effect as calling Generator::send() with NULL as argument.

public Generator::rewind ( void ) − void — Rewind the iterator. If iteration has already begun, this will throw an exception.

public Generator::send ( mixed $value ) : mixed — Sends given value to generator as result of current yield expression and resumes generator.

public Generator::throw ( Throwable $exception ) − mixed — Throws an exception into the generator and resumes execution of the generator.

public Generator::valid ( void ) − bool — Check if the iterator has been closed

public Generator::__wakeup ( void ) − void — Throws an exception as generators can't be serialized.

Generator class implements Iterator interface. Generator objects cannot be instantiated via new. Any user defined function having yield keyword creates object of generator class.

Generator Example

Since generator implements Iterator interface, for each loop can be used to traverse yielded values.

 Live Demo

<?php
function squaregenerator(){
   for ($i=1; $i<=5; $i++){
      yield $i*$i;
   }
}
$gen=squaregenerator();
foreach ($gen as $val){
   echo $val . " ";
}
?>

Output

Above program shows following output

1 4 9 16 25

following example uses current() and next() methods of generator class to traverse yielded values. Loop condision is checked with valid() method.

Example

 Live Demo

<?php
function squaregenerator(){
   for ($i=1; $i<=5; $i++){
      yield $i*$i;
   }
}
$gen=squaregenerator();
while ( $gen->valid() ){
   echo "key: " . $gen->key(). " value: ". $gen->current() . "
";
   $gen->next();
}
?>

Output

Above program shows following output

key: 0 value: 1
key: 1 value: 4
key: 2 value: 9
key: 3 value: 16
key: 4 value: 25