For enumerators: last, prev?

ruby-talk
1

I see that enumerators have the methods rewind and next.

Are there equivalent methods "end" and "prev"?

I see reverse_each ... but what if I do have a large enumerable object and don't want the intermediate array created?

The documentation ( http://ruby-doc.org/core-1.9.3/Enumerable.html#method-i-reverse_each ) for reverse each says:

- - -

reverse_each(*args) {|item| block } ? enum click to toggle source
reverse_each(*args) ? an_enumerator

Builds a temporary array and traverses that array in reverse order.

If no block is given, an enumerator is returned instead.

- - -

2

Enumerators can only "move" in one direction - forward. #end would be
useless for many (for ex. (1..Float::INFINITY).each - this one never
ends), providing #prev would be equivalent to remembering all of
values yielded so far (unpractical, we don't have infinite memory).

-- Matma Rex

···

2012/3/12 Ralph Shnelvar <ralphs@dos32.com>:

I see that enumerators have the methods rewind and next.

Are there equivalent methods "end" and "prev"?

I see reverse_each ... but what if I do have a large enumerable object and don't want the intermediate array created?

The documentation ( Module: Enumerable (Ruby 1.9.3) ) for reverse each says:

- - -

reverse_each(*args) {|item| block } ? enum click to toggle source
reverse_each(*args) ? an_enumerator

Builds a temporary array and traverses that array in reverse order.

If no block is given, an enumerator is returned instead.

- - -

3

Bartosz,

The convention is to bottom post. I am moving your comment to the right place and responding.

Monday, March 12, 2012, 11:51:54 AM, you wrote:

···

2012/3/12 Ralph Shnelvar <ralphs@dos32.com>:

I see that enumerators have the methods rewind and next.

Are there equivalent methods "end" and "prev"?

I see reverse_each ... but what if I do have a large enumerable object and don't want the intermediate array created?

The documentation ( Module: Enumerable (Ruby 1.9.3) ) for reverse each says:

- - -

reverse_each(*args) {|item| block } ? enum click to toggle source
reverse_each(*args) ? an_enumerator

Builds a temporary array and traverses that array in reverse order.

If no block is given, an enumerator is returned instead.

- - -

Enumerators can only "move" in one direction - forward. #end would be
useless for many (for ex. (1..Float::INFINITY).each - this one never
ends), providing #prev would be equivalent to remembering all of
values yielded so far (unpractical, we don't have infinite memory).

-- Matma Rex

For ranges, all you need define is the inverse of the succ function (which must be defined in order to "go forwards").

By the way,
(99_999_999_990..100_000_000_000).last(3) works just fine but
(1..100_000_000_000) seems to hang in a loop. I suspect Ruby is constructing a temporary array.

Ruby seems not to be clever at all about ranges.