Generator (computer programming): Difference between revisions

Revision as of 22:18, 19 October 2014

Revision as of 04:29, 26 December 2014

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In the presence of generators, loop constructs of a language – such as for and while – can be reduced into a single loop ... end loop construct; all the usual loop constructs can then be comfortably simulated by using suitable generators in the right way. For example, a ranged loop like <code>for x = 1 to 10</code> can be implemented as iteration through a generator, as in Python's <code>for x in range(10, 1)</code>. Further, <code>break</code> can be implemented as sending ''finish'' to the generator and then using <code>continue</code> in the loop.

== Timeline ==

Generators first appeared in [[CLU programming language|CLU]] (1975),<ref>{{cite web

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use strict;

use warnings;

# enableEnable generator { BLOCK } and yield

use Coro::Generator;

# arrayArray reference to iterate over

my $chars = ['A'...'Z'];

# newNew generator which can be called like a coderef.

# coderef.

my $letters = generator {

my $i = 0;

for my $letter (@$chars) {

# get next letter from $chars

yield $letter;

}

};

# callCall the generator 15 times.

print $letters->(), "\n" for (0..15);

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foreach (fibonacci() as $number) {

echo $number ., "\n";

}

</source>

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<source lang="ruby">

# Generator from an Enumerator object

chars = Enumerator.new(['A', 'B', 'C', 'Z'])

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100.times { puts count.next }

</source>

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<source lang="Java">

// Iterator implemented as anonymous class. This uses generics but doesn't need to.

for (int i: new Iterator<Integer>() {

int counter = 1;

@Override

public boolean hasNext() {

return counter <= 100;

}

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An infinite Fibonacci sequence could also be written as an Iterator:

<source lang="java">

Iterator<Integer> fibo = new Iterator<Integer>() {

int a = 1;

int b = 1;

int total;

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}

// this could then be used as...

for(int f: fibo) {

System.out.println("next Fibonacci number is " + f);

if (someCondition(f)) break;

}

</source>

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<source lang="csharp">

public class CityCollection : IEnumerable<string> {

public IEnumerator<string> GetEnumerator() {

yield return "New York";

yield return "Paris";

yield return "London";

}

}

</source>

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// An implicit declaration of I as an integer is therefore made here

for I in 1..5 loop

IO.WriteLn "I=", I

</source>

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seq { for b in 0 .. 25 do

if b < 15 then

yield b * b }

</source>

forms a sequence of squares of numbers from 0 to 14 by filtering out numbers from the range of numbers from 0 to 25.

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p = []

while True:

# This works in Python 2.5+

if not any( n % f == 0 for f in

itertools.takewhile(lambda f: f*f <= n, p) ):

yield n

p.append( n )

n += 2

</source>

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PEP 380 (implemented in Python 3.3) adds the <tt>yield from</tt> expression, allowing a generator to delegate part of its operations to another generator.<ref name="pep380">[https://www.python.org/dev/peps/pep-0380/ PEP 380 -- Syntax for Delegating to a Subgenerator]</ref>

====Generator Expressionsexpressions====

Python has a syntax modeled on that of [[list comprehension]]s, called a generator expression that aids in the creation of generators.