I was going through the exercises at http://rubykoans.com/ and got
struck with the following problem:
array = [:peanut, :butter, :and, :jelly]
array[0] => :peanut
array[0,1] => [:peanut]
array[0,2] => [:peanut, :butter]
array[0,0] =>
array[2] => :and
array[2,2] => [:and, :jelly]
array[2,20] => [:and, :jelly]
array[4] => nil # This is OK
array[4,0] => # why is this an empty array and not
nil?
array[4,100] => # why is this an empty array and not
nil?
array[5] => nil # as expected
array[5,0] => nil # as expected
I have gone through the Ruby doc to get an explanation of this behavior,
but there it is marked as '# special cases' but there is no explanation
of this behavior. I being a newb, it would be of immense help if you can
give me some pointers where I can find a correct explanation of this
behaviour.
The python slices seem to behave similarly
arr = [1, 2, 3]
arr[3] # Gives IndexError: list index out of range
arr[3:3] # gives
arr[3:4] # gives
But I could not find an explanation in the python docs either.
Gary Wright's explanation in the thread linked above is very complete and clear.
Strongly recommended if you are at all foggy on this topic.
I will add one terminology/memory tidbit -
The difference in the one-arg/two-arg indexing is ruby's way of dealing with
the classic fence post problem. (i.e. A fence with 3 sections has 4 posts.)
One-arg (e.g. a[1]) -- The argument addresses "fence sections".
Two-arg (e.g. a[1,2]) -- The first argument addresses "fence posts".
(As others have pointed out already, there are reasons for doing both
things from time to time.)
So, to answer the original question using "fence" terminology -
array = [:peanut, :butter, :and, :jelly]
...
array[4,0] => # why is this an empty array and not nil?
Gary Wright's explanation in the thread linked above is very complete and clear.
Strongly recommended if you are at all foggy on this topic.
Thanks for the compliment. I thought I would also point out that while my discussion
in that thread talks about *string* indexing it is equally applicable to *array*
indexing. Good thing too because I would hate for the behavior to be different.
I will add one terminology/memory tidbit -
The difference in the one-arg/two-arg indexing is ruby's way of dealing with
the classic fence post problem. (i.e. A fence with 3 sections has 4 posts.)
Yeah, but what about the grass on the other side? It's still greener. Darn.
Cheers
robert
···
On Thu, Sep 8, 2011 at 10:15 PM, Gary Wright <gwtmp01@mac.com> wrote:
On Sep 8, 2011, at 6:05 AM, Dan Nachbar wrote:
The difference in the one-arg/two-arg indexing is ruby's way of dealing with
the classic fence post problem. (i.e. A fence with 3 sections has 4 posts.)
The difference in the one-arg/two-arg indexing is ruby's way of dealing with
the classic fence post problem. (i.e. A fence with 3 sections has 4 posts.)
I really like this analogy.
Yeah, but what about the grass on the other side? It's still greener. Darn.
I like that!
OffTopic, but vaguely relevant: Have you seen Norman MacLaren's short
film Neighbours?
Fenceposts first appear at just after 3m50s into the 8m5s film.
···
On Fri, Sep 9, 2011 at 1:36 PM, Robert Klemme <shortcutter@googlemail.com> wrote:
On Thu, Sep 8, 2011 at 10:15 PM, Gary Wright <gwtmp01@mac.com> wrote:
No, I haven't. Apparently there's a technical issue - at least I
cannot view the movie.
Kind regards
robert
···
On Sun, Sep 11, 2011 at 3:05 AM, Colin Bartlett <colinb2r@googlemail.com> wrote:
On Fri, Sep 9, 2011 at 1:36 PM, Robert Klemme > <shortcutter@googlemail.com> wrote:
On Thu, Sep 8, 2011 at 10:15 PM, Gary Wright <gwtmp01@mac.com> wrote:
On Sep 8, 2011, at 6:05 AM, Dan Nachbar wrote:
The difference in the one-arg/two-arg indexing is ruby's way of dealing with
the classic fence post problem. (i.e. A fence with 3 sections has 4 posts.)
I really like this analogy.
Yeah, but what about the grass on the other side? It's still greener. Darn.
I like that!
OffTopic, but vaguely relevant: Have you seen Norman MacLaren's short
film Neighbours? Neighbours by Norman McLaren - NFB
Fenceposts first appear at just after 3m50s into the 8m5s film.
OT: I tried last night and seemed to have problems viewing it then,
and also tried just now, and it seems OK. Curious.
···
On Sun, Sep 11, 2011 at 5:10 PM, Robert Klemme <shortcutter@googlemail.com> wrote:
On Sun, Sep 11, 2011 at 3:05 AM, Colin Bartlett <colinb2r@googlemail.com> wrote:
On Fri, Sep 9, 2011 at 1:36 PM, Robert Klemme >> <shortcutter@googlemail.com> wrote:
On Thu, Sep 8, 2011 at 10:15 PM, Gary Wright <gwtmp01@mac.com> wrote:
On Sep 8, 2011, at 6:05 AM, Dan Nachbar wrote:
The difference in the one-arg/two-arg indexing is ruby's way of dealing with
the classic fence post problem. (i.e. A fence with 3 sections has 4 posts.)
I really like this analogy.
Yeah, but what about the grass on the other side? It's still greener. Darn.
I like that!
OffTopic, but vaguely relevant: Have you seen Norman MacLaren's short
film Neighbours? http://www.nfb.ca/film/neighbours_voisins/
Fenceposts first appear at just after 3m50s into the 8m5s film.
No, I haven't. Apparently there's a technical issue - at least I
cannot view the movie.
