[SOLVED] [Ruby] Strange results when returning Enumerator from function

Snark1994 · 12-30-2012, 01:13 PM

Hiya,

I hit a bug in one of my ruby scripts, and narrowed it down to this unexpected behaviour:

Code:

#!/usr/bin/env/ruby

def get_enum
    numbers = [1,2,3,4,5,6]
    return numbers.each
end

puts get_enum().inspect

get_enum do |num|
    p num 
end

puts "\n"+get_enum().each.inspect
get_enum.each do |num|
    p num 
end

prints

Code:

#<Enumerator: [1, 2, 3, 4, 5, 6]:each>

#<Enumerator: [1, 2, 3, 4, 5, 6]:each>
1
2
3
4
5
6

while I would expect it to print

Code:

#<Enumerator: [1, 2, 3, 4, 5, 6]:each>
1
2
3
4
5
6

#<Enumerator: [1, 2, 3, 4, 5, 6]:each>
1
2
3
4
5
6

(as calling .each() on an Enumerator just returns itself).

Does anyone have any idea why this is happening?

Thanks,

grail · 12-31-2012, 04:34 AM

Looks correct to me Snark as you do not call each on your enumerator that you created, ie:

Code:

#!/usr/bin/env/ruby

def get_enum
    numbers = [1,2,3,4,5,6]
    return numbers.each
end

puts get_enum().inspect

get_enum.each do |num|
    p num 
end

The loop is never entered without this.

Snark1994 · 01-01-2013, 04:17 AM

Hm. But why doesn't it work with the Enumerator returned from the function? I would expect the behaviour I got if the function I had was

Code:

def get_enum
    numbers = [1,2,3,4,5,6]
    return numbers #not numbers.each
end

but I called '.each' on the array before returning it...

Or is that just not how it works?

Thanks,

grail · 01-01-2013, 05:07 AM

What you now have is an array being returned. My simple solution to following the issue was to put the following line into the code:

Code:

puts get_enum.class

This then tells you that with 'each' on the return item you are now returning an enumerator, but without it you are returning an array.

Let me know if that makes sense?

Maybe also remember that as everything is an object, when you call get_enum.<whatever>, the <whatever> is called on the item being returned (sorry if that is obvious)

Snark1994 · 01-01-2013, 05:39 PM

I think I understand what you're saying, but if I do then we're talking at cross purposes!

Code:

$numbers = [1,2,3,4,5,6]

def get_enum
    return $numbers.each
end

puts get_enum.class

prints "Enumerator" (what I would expect) and

Code:

$numbers = [1,2,3,4,5,6]

def get_array
    return $numbers
end

puts get_array.class

prints "Array" (also what I would expect). However, I can run

Code:

$numbers = [1,2,3,4,5,6]

$numbers.each do |x|
    p x
end

and get the numbers printed out (still what I would expect) but when I try

Code:

$numbers = [1,2,3,4,5,6]

def get_enum
    return $numbers.each
end

get_enum do |x|
    p x
end

I don't get the numbers. If I'm returning "$numbers.each" from "get_enum", why does it behave differently if I change "$numbers.each" to "get_enum" in the line which has "do |x|" in it?

Sorry if I've misunderstood what you said!

Thanks,

sundialsvcs · 01-01-2013, 06:07 PM

If you use a variable the "wrong" way, Ruby will try to do what you told it to do, even to the point of transmogrifying the variable into the sort of thing that's appropriate for what you tried to do. Don't put a "right" and "wrong" example back-to-back in either order; set up the situation twice cleanly. Welcome to dynamic typecasting.

Snark1994 · 01-02-2013, 04:47 AM

And returning an enumerator is the "wrong" thing?

I see, this is a "feature" then

okay, I guess that's my question solved, more or less. Thanks grail and sundialsvcs for your time

grail · 01-02-2013, 08:26 AM

Quote:

If I'm returning "$numbers.each" from "get_enum", why does it behave differently if I change "$numbers.each" to "get_enum" in the line which has "do |x|" in it?

Maybe think of assigning your return to a new variable:

Code:

$test = get_enum

This now makes "$test" into an enumerator and to cycle over the items in an enumerator you need to call something like:

Code:

$test.each {|x| ... }

You would not simply call "$test" and expect it to perform a block, you still need a method, such as each, to perform the action.

Not sure if that helps anymore?

I would not say that you are using the variables the wrong way but you are expecting an action from a variable instead of using a method.

ntubski · 01-02-2013, 09:28 AM

Quote:

Originally Posted by Snark1994

I don't get the numbers. If I'm returning "$numbers.each" from "get_enum", why does it behave differently if I change "$numbers.each" to "get_enum" in the line which has "do |x|" in it?

Calling each does something different depending on whether you pass a block or not. You're expecting the no-argument version to return a thing-which-expects-a-block (more formally, you think you've done a partial application), but actually it just returns the enumerator (i.e. it's a nop), so to actually enumerate you have to call "each" (again) with a block argument.

Snark1994 · 01-02-2013, 04:50 PM

Ah, yes, I guess I had the haskell-ish idea that once I've called ".each" then I should just be able to stick it into a block and it would work.

Thanks, ntubski!