From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2014-06-27T09:16:44+02:00
Subject: [ruby-core:63354] Unexpected behaviour of cycle and zip

Hi,

(I already posted this in Ruby Talk, and it was suggested to check in Core)

I was surprised today with a behaviour, maybe my expectation was wrong
and someone can explain why it works this way:

I have an array of elements, and I want to cycle through it assigning
each element to another list of elements. But the cycle can be already
in the middle. For example:

a = [1,2,3]

b = [6,7,8,9,10]

the desired result should be (if the next element in the cycle was 2):

[[6,2], [7,3], [8,1],[9,2],[10,3]]

So I, intuitively tried this:

2.0.0-p195 :469 > b = [6,7,8,9,10]
 => [6, 7, 8, 9, 10]
2.0.0-p195 :471 > a = [1,2,3].cycle
 => #<Enumerator: [1, 2, 3]:cycle>
2.0.0-p195 :472 > a.next
 => 1
2.0.0-p195 :473 > a.peek
 => 2
2.0.0-p195 :474 > b.zip(a)
 => [[6, 1], [7, 2], [8, 3], [9, 1], [10, 2]]

So, it seems that the enumerator a is starting from the beginning,
instead of from the "next". If I read correctly the source code (MRI)
it seems that zip uses take, and testing this shows that it always
starts from the beginning:

2.0.0-p195 :475 > a.take 2
 => [1, 2]
2.0.0-p195 :476 > a.take 2
 => [1, 2]
2.0.0-p195 :477 > a.take 2
 => [1, 2]

Is it wrong to expect the enumerator to behave as a cycle starting
from its current position for operations like zip and take?

Thanks,

Jesus.