From: Josh Cheek <josh.cheek@...>
Date: 2010-07-14T07:22:23+09:00
Subject: Re: What does this do?

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On Tue, Jul 13, 2010 at 5:09 PM, Abder-Rahman Ali <
abder.rahman.ali@gmail.com> wrote:

> Josh Cheek wrote:
> > On Tue, Jul 13, 2010 at 4:26 PM, Abder-Rahman Ali <
> > abder.rahman.ali@gmail.com> wrote:
> >
> >>
> >> Thanks.
> >> --
> >> Posted via http://www.ruby-forum.com/.
> >>
> >>
> > I'm not really sure what you're asking, but I think that it is probably
> > answered by the docs:
> >
> >
> >
> > http://ruby-doc.org/core/classes/String.html#M000771
> >
> > str[range] => new str or nil
> >
> > <http://ruby-doc.org/core/classes/String.src/M000771.html>If given a
> > range,
> > a substring containing characters at offsets given by the range is
> > returned.
> > In all three cases, if an offset is negative, it is counted from the end
> > of
> > *str*.
>
> Thanks Josh.
>
> For example, this from the documentation:
> a = "hello there"
> a[-3,2] #=> "er"
>
> -3 ---> Start counting from 0 to 3 from the end (Right -> Left)
> In this case I reached the letter "h" of there
>
> 2 ---> What should I do here?
>
> Thanks.
>
>
>
> --
> Posted via http://www.ruby-forum.com/.
>
>

The first character is zero, the last character is negative one (there is no
negative zero), when you count backwards from the end:
-1: e
-2: r
-3: e
-4: h
-5: t

So, as the docs say "If passed two
Fixnum<http://ruby-doc.org/core/classes/Fixnum.html>objects, returns a
substring starting at the offset given by the first, and
a length <http://ruby-doc.org/core/classes/String.html#M000774> given by the
second."
-3,2 puts you at the e in -3, and returns two characters forward from that
spot. If you are having a difficult time seeing what you could expect, try
giving different numbers in irb, as James suggested.

Open IRB, say "if this thing the docs are telling me is correct, and I
correctly understand it, then I expect xxx to happen when I yyy" Then yyy,
and see if you get xxx. If you do, that means you probably understand it
right. If not, you probably need to experiment more to get a better idea of
what is happening. You can try following patterns to see how the results
change with the pattern:
a[-4,2]
a[-5,2]
a[-6,2]
a[-6,3]
a[-6,4]

Find the patterns in the output, make a hypothesis, see if it gives you what
you expected.

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