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More C#, Ruby, and Python Iterators, and JavaScript too

Michael Geary | Thu, 2004-08-05 16:55
category

Making a valiant attempt to post code with my comment system (Sorry, Mike! :-(), Mike Roome points out:

The ruby example isn

Iterators in C#, Python, and Ruby

Michael Geary | Mon, 2004-08-02 17:33
category

Matt Pietrek marvels at C# 2.0 iterators and dissects them right down to the CLR bytecode. I always learn something from Matt, and this whirlwind tour is no exception.

Matt says, “This was the beginning of my descent into the loopy world of C# 2.0 iterators. It took me awhile to wrap my head around them, and when I tried to explain them to other team members I got looks of total confusion.” I wonder if it would have been less confusing if Matt’s team had first been exposed to yield iterators in a language that makes them easier to use.

After using Python and Ruby, the iterators in C# feel right at home to me. They work the same in all three languages, but in Ruby and Python there’s not as much other code to get in the way of understanding them.

Let’s combine all of Matt’s examples into one, and compare the code in each language. First, in C#:

using System;
using System.Collections.Generic;

class SomeContainer
{
    public IEnumerable<string> MyIterator()
    {
        Console.WriteLine( "Before First" );
        yield return "First";
        Console.WriteLine( "After First" );

        for ( int i = 0;  i < 3;  i++ )
        {
            yield return i.ToString();
        }

        Console.WriteLine( "Before Last" );
        yield return "Last";
        Console.WriteLine( "After Last" );
    }
}

class foo
{
    static void  Main()
    {
        SomeContainer container =
            new SomeContainer();

        foreach( string s in
                container.MyIterator() )
            Console.WriteLine( s );
    }
}

When you run that, it should print:

Before First
First
After First
0
1
2
Before Last
Last
After Last

Here’s how you would write the same code in Python:

def MyIterator():
    print "Before First"
    yield "First"
    print "After First"

    for i in xrange(3):
        yield str(i)

    print "Before Last"
    yield "Last"
    print "After Last"

for s in MyIterator():
    print s

And in Ruby, the code looks like this:

def MyIterator()
    p "Before First"
    yield "First"
    p "After First"

    3.times do |i|
        yield i.to_s
    end

    p "Before Last"
    yield "Last"
    p "After Last"
end

MyIterator do |s|
    p s
end

The one unfamiliar thing here may be the |name| notation, which is how a code block such as the body of a loop receives its argument. And the p statements are a kind of print statement.

This Ruby version is even more concise and equally readable once you’re comfortable with the |name| notation:

def MyIterator()
    p "Before First"
    yield "First"
    p "After First"

    3.times { |i| yield i.to_s }

    p "Before Last"
    yield "Last"
    p "After Last"
end

MyIterator { |s| p s }

Either way, the Python and Ruby versions make it easier to see what the iterator function does and how yield interacts with the rest of the code.

You may note that the Python and Ruby versions don’t create and instantiate a SomeContainer class as the C# version does. That’s true, and it would make the code in those languages a bit longer (but still simpler than the C# code). But, if you don’t need to—and you especially don’t need to when you’re experimenting and trying to understand a radical new technique like yield iterators—why bother?