Generator (computer science)

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In computer science, a generator is a special routine that can be used to control the iteration behaviour of a loop. A generator is very similar to a function that returns an array, in that a generator has parameters, can be called, and generates a sequence of values. However, instead of building an array containing all the values and returning them all at once, a generator yields the values one at a time, which requires less memory and allows the caller to get started processing the first few values immediately. Therefore, lazy evaluation can be used in the implementation of generators in certain cases. However, generators are not referentially transparent. In short, a generator looks like a function but behaves like an iterator.

Contents 1 History 2 Uses 2.1 Python 2.2 C++ 2.3 PHP 3 See also 4 References

[edit] History

Generators first appeared in CLU (1975)^[1] and are now available in Python^[2], C#, Ruby^[3], and JavaScript^[4]. (In CLU and C#, generators are called iterators.)

[edit] Uses

Generators are usually invoked inside loops. The first time that a generator invocation is reached in a loop, an iterator object is created that encapsulates the state of the generator routine at its beginning, with arguments bound to the corresponding parameters. The generator's body is then executed in the context of that iterator until a special yield action is encountered; at that time, the value provided with the yield action is used as the value of the invocation expression. The next time the same generator invocation is reached in a subsequent iteration, the execution of the generator's body is resumed after the yield action, until yet another yield action is encountered. In addition to the yield action, execution of the generator body can also be terminated by a finish action, at which time the innermost loop enclosing the generator invocation is terminated.

Because generators compute their yielded values only on demand, they are useful for representing sequences that are expensive to compute, or even infinite.

A pseudorandom number generator is an example of a generator.

In the presence of generators, loop constructs of a language 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.

[edit] Python

An example Python generator:

def countfrom(n):
    while True:
        yield n
        n += 1

# Example use: printing out the integers from 10 to 20.
# Note that this iteration terminates normally, despite countfrom() being
# written as an infinite loop.

for i in countfrom(10):
    if i <= 20:
        print i
    else:
        break

# Another generator, which produces prime numbers indefinitely as needed.

def primes():
    n = 2
    p = []
    while 1:
        if not any( n % f == 0 for f in p ):
            yield n
            p.append( n )
        n += 1

>>> f = primes()
>>> f.next()
2
>>> f.next()
3
>>> f.next()
5
>>> f.next()
7

In Python, a generator can be thought of as an iterator that contains a frozen stack frame. Whenever the iterator's next() method is called, Python resumes the frozen frame, which executes normally until the next yield statement is reached. The generator's frame is then frozen again, and the yielded value is returned to the caller.

Generators can be implemented in terms of more expressive control flow constructs, such as coroutines or first-class continuations.^[5]

[edit] C++

Defines a generator (implemented as a function object) that counts from 10 up, and we invoke the generator 11 times and print the results.

#include <iostream>
#include <iterator>
#include <algorithm>

class countfrom {
private:
  int count;
public:
  countfrom(int n) : count(n) {}
  int operator()() { return count++; }
};

int main() {
  std::generate_n(std::ostream_iterator<int>(std::cout, "\n"), 11, countfrom(10));
  return 0;
}

[edit] PHP

In PHP, especially when working with MySQL databases, it is often useful to use the build-in function mysql_fetch_array, which is a generator.

   $sql = 'SELECT * FROM `users` ORDER BY `user_id` DESC LIMIT 10';
   $res = mysql_query( $sql );
   // this generator will keep returning data, satisfying the while condition
   // as long as there is something new to return, or false when there is no
   // more data to return.
   // Keep in mind that the equals sign is not a comparison operator, but
   // an assignment operator
   while ( $row = mysql_fetch_array( $res ) ) { 
       // process $row
   }

[edit] See also

• List comprehension for another construct that generates a sequence of values
• Iterator for the concept of producing a list one element at a time
• Lazy evaluation for producing values when needed
• Corecursion for potentially infinite data by recursion instead of yield
• Coroutine for even more generalization from subroutine
• Continuation for generalization of control flow

[edit] References

1. ^ Liskov, Barbara (April 1992). A History of CLU (pdf).
2. ^ Python Enhancement Proposals: PEP 255: Simple Generators, PEP 289: Generator Expressions, PEP 342: Coroutines via Enhanced Generators
3. ^ Python Feature Equivalents in Ruby.
4. ^ New In JavaScript 1.7. Retrieved on October 10, 2006.
5. ^ Kiselyov, Oleg (January 2004). General ways to traverse collections in Scheme.

• Stephan Murer, Stephen Omohundro, David Stoutamire and Clemens Szyperski: Iteration abstraction in Sather. ACM Transactions on Programming Languages and Systems, 18(1):1-15 (1996) [1]