Boo Primer: [Part 16] Generators

Added by Cameron Kenneth Knight

Generator Expressions

Definition: Generator Expression - A phrase that creates a generator based on the syntax:

<expression> for <declarations> [as <type>] in <iterator> [if|unless <condition>]

Generator Expressions have similar syntax to the for loops that we have covered, and serve a similar purpose.

The best way to learn how to use Generator Expressions is by example, so here we load up a booish prompt.

$ booish
>>> List(x for x in range(5)) // simplest Generator Expression
[0, 1, 2, 3, 4]
>>> List(x * 2 for x in range(5)) // get double of values
[0, 2, 4, 6, 8]
>>> List(x**2 for x in range(5)) // get square of values
[0, 1, 4, 9, 16]
>>> List(x for x in range(5) if x % 2 == 0) // check if values are even
[0, 2, 4]
>>> List(x for x in range(10) if x % 2 == 0) // check if values are even
[0, 2, 4, 6, 8]
>>> List(y for y in (x**2 for x in range(10)) if y % 3 != 0) // Generator Expression inside another
[1, 4, 16, 25, 49, 64]
>>> List(cat.Weight for cat in myKitties if cat.Age >= 1.0).Sort()
[6.0, 6.5, 8.0, 8.5, 10.5]
>>> genex = x ** 2 for x in range(5)
generator(System.Int32)
>>> for i in genex:
...     print i
...
0
1
4
9
16

The cat-weight example is probably what Generator Expressions are most useful for. You don't have to create Lists from them either, that's mostly for show. generators are derived from IEnumerable, so you get all the niceties of the for loop as well.

Recommendation: Don't overdo it with Generator Expressions. If they are causing your code to be less readable, then spread them out a little.

Generator Methods

Definition: Generator Expression - A method that creates a generator based on stating the yield keyword within the method.

A Generator Method is like a regular method that can return multiple times. Here's a Generator Method that will return exponents of 2.

def TestGenerator():
    i = 1
    yield i
    for x in range(10):
        i *= 2
        yield i

print List(TestGenerator())

// Output:
// [1, 2, 4, 8, 16, 32, 64, 128, 512, 1024]

Generator Methods are very powerful because they keep all their local variables in memory after a yield. This can allow for certain programming techniques not found in some other languages. Generators are very powerful and useful.

Exercises

1. Create a Generator that will destroy mankind.

Go on to Part 17 - Macros

Back to: Home | Boo Primer