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comp.lang.ruby

ruby string slice/[] w/ range, weird end behavior

Gary Yngve

5/8/2009 11:27:00 PM

First the docs:

...If passed two Fixnum objects, returns a substring starting at the
offset given by the first, and a length given by the second. If given
a range, a substring containing characters at offsets given by the
range is returned... Returns nil if the initial offset falls outside
the string, the length is negative, or the beginning of the range is
greater than the end.

Now from irb (1.8):

>> "foo"[2..2]
=> "o"
>> "foo"[3..3]
=> "" # ????????
>> "foo"[4..4]
=> nil

>> "foo"[2,1]
=> "o"
>> "foo"[3,1]
=> "" # ????????
>> "foo"[4,1]
=> nil

>> "foo"[2]
=> 111 # (the 'o' char)
>> "foo"[3]
=> nil # This makes sense to me, but seems inconsistent wrt the above

Seems to me like the null terminator of the string is somehow getting
muddled into all of this.

Is there any meaning/purpose behind this behavior?

Thanks,
Gary
16 Answers

Eleanor McHugh

5/8/2009 11:32:00 PM

0

On 9 May 2009, at 00:26, Gary Yngve wrote:
> First the docs:
>
> ...If passed two Fixnum objects, returns a substring starting at the
> offset given by the first, and a length given by the second. If given
> a range, a substring containing characters at offsets given by the
> range is returned... Returns nil if the initial offset falls outside
> the string, the length is negative, or the beginning of the range is
> greater than the end.
>
> Now from irb (1.8):
>
>>> "foo"[2..2]
> => "o"
>>> "foo"[3..3]
> => "" # ????????
>>> "foo"[4..4]
> => nil
>
>>> "foo"[2,1]
> => "o"
>>> "foo"[3,1]
> => "" # ????????
>>> "foo"[4,1]
> => nil
>
>>> "foo"[2]
> => 111 # (the 'o' char)
>>> "foo"[3]
> => nil # This makes sense to me, but seems inconsistent wrt the above
>
> Seems to me like the null terminator of the string is somehow getting
> muddled into all of this.
>
> Is there any meaning/purpose behind this behavior?

String indices start at zero, so:

"foo"[0..0] => 'f'
"foo"[1..1] => 'o'
"foo"[2..2] => 'o'
"foo"[3..3] => nil


Ellie

Eleanor McHugh
Games With Brains
http://slides.games-with-...
----
raise ArgumentError unless @reality.responds_to? :reason

Sebastian Hungerecker

5/8/2009 11:39:00 PM

0

Am Samstag 09 Mai 2009 01:32:06 schrieb Eleanor McHugh:
> "foo"[3..3] => nil

You sure? I get "", same as the op. If it would return nil, I don't think the
op would have asked his question.

RubyTalk@gmail.com

5/8/2009 11:45:00 PM

0

Element Reference=97If passed a single Fixnum, returns the code of the
character at that position. If passed two Fixnum objects, returns a
substring starting at the offset given by the first, and a length
given by the second. 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.
Returns nil if the initial offset falls outside the string, the length
is negative, or the beginning of the range is greater than the end.

two fixed numbers returns a substring.

On Fri, May 8, 2009 at 4:39 PM, Sebastian Hungerecker
<sepp2k@googlemail.com> wrote:
> Am Samstag 09 Mai 2009 01:32:06 schrieb Eleanor McHugh:
>> "foo"[3..3] =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =3D> nil
>
> You sure? I get "", same as the op. If it would return nil, I don't think=
the
> op would have asked his question.
>
>

Eleanor McHugh

5/8/2009 11:47:00 PM

0

On 9 May 2009, at 00:39, Sebastian Hungerecker wrote:
> Am Samstag 09 Mai 2009 01:32:06 schrieb Eleanor McHugh:
>> "foo"[3..3] => nil
>
> You sure? I get "", same as the op. If it would return nil, I don't
> think the
> op would have asked his question.

Sorry, typo on my part (I'm not having a good week for these it
seems). The point I was trying to make was that

"foo"[3..3] => ""

is a thoroughly valid range, but I guess that would have been clearer
with a fuller explanation. Consider

"foo"[2..3] => "o"

index [3] is actually the end of the string so whilst the range
accesses the string after any characters in it, it's still accessing
the string in a valid range.


Ellie

Eleanor McHugh
Games With Brains
http://slides.games-with-...
----
raise ArgumentError unless @reality.responds_to? :reason

Eleanor McHugh

5/8/2009 11:50:00 PM

0

On 9 May 2009, at 00:44, RubyTalk@gmail.com wrote:
> Element Reference=97If passed a single Fixnum, returns the code of the
> character at that position. If passed two Fixnum objects, returns a
> substring starting at the offset given by the first, and a length
> given by the second. 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.
> Returns nil if the initial offset falls outside the string, the length
> is negative, or the beginning of the range is greater than the end.
>
> two fixed numbers returns a substring.

The same result would occur with

"foo"[3,1] =3D> ""
"foo"[4,1] =3D> nil


Ellie

Eleanor McHugh
Games With Brains
http://slides.games-with-...
----
raise ArgumentError unless @reality.responds_to? :reason

Gary Yngve

5/9/2009 12:12:00 AM

0

Here's an even simpler case:

>> ""[0]
=> nil
>> ""[0..0]
=> ""
>> ""[0..1]
=> ""
>> ""[1..1]
=> nil

Why differentiate between returning "" and nil?
Why isn't this explained in the docs?

Gary Yngve

5/9/2009 12:19:00 AM

0

On May 8, 4:47=A0pm, Eleanor McHugh <elea...@games-with-brains.com>
wrote:
> "foo"[3..3] =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =3D> ""
...
> index [3] is actually the end of the string so whilst the range =A0
> accesses the string after any characters in it, it's still accessing =A0
> the string in a valid range.

Maybe in the native C. But why should that be exposed?
And why shouldn't index[3] return "" if you are correct?

After all:

>> a=3D[]; "asd".each_byte{|x| a << x}; a
=3D> [97, 115, 100]

If I ask for a substring entirely out of bounds, I should consistently
be returned nil or "", not one of the two.

Gary Yngve

5/9/2009 12:23:00 AM

0

Are replies to this group always like this?

someone blindly posts the doc without reading that I already posted
the doc or reading my example (like an implicit RTFM)
other folks automatically assume I'm a newb and respond in five
seconds without fully reading my post or thinking about their own post

do i need to be a kool kid or have a secret code word for this noise
to go away?

i've found the more niche ruby groups to be much more signal.. but
this didn't seem to fit into a niche

Eleanor McHugh

5/9/2009 12:38:00 AM

0


On 9 May 2009, at 01:19, Gary Yngve wrote:

> On May 8, 4:47 pm, Eleanor McHugh <elea...@games-with-brains.com>
> wrote:
>> "foo"[3..3] => ""
> ...
>> index [3] is actually the end of the string so whilst the range
>> accesses the string after any characters in it, it's still accessing
>> the string in a valid range.
>
> Maybe in the native C. But why should that be exposed?

It's not the C implementation, it's the conceptual model of what a
string is: i.e. an array of characters addressable by index and range.

> And why shouldn't index[3] return "" if you are correct?

Because in this case the question you're asking isn't "What substring
occupies the given segment of the string" but "Which character is
stored at the given index in the string". If no character is stored
there (as the case for "foo"[3]) then nil is the only meaningful answer.

"foo"[3] => nil
nil.to_s => ""


> After all:
>
>>> a=[]; "asd".each_byte{|x| a << x}; a
> => [97, 115, 100]
>
> If I ask for a substring entirely out of bounds, I should consistently
> be returned nil or "", not one of the two.

And the substring "foo"[3..3] is in bounds because conceptually you're
dealing with:

f o o
0 1 2 3

so [3..3] equals the slice at the end of the string but not containing
any characters.

And yes, I know this is probably about as clear as mud - my ability to
write English seems to be inversely proportional to the difficulty of
the code I'm working on at any given time, and currently I'm buried in
research so the code is very hairy indeed :(

Whilst it's not directly relevant, being a different language and all,
I recommend "Chapter 3: String Processing" in Programming with Unicon (http://unicon.sourceforge.net/u...
) as it's the same basic model.


Ellie

Eleanor McHugh
Games With Brains
http://slides.games-with-...
----
raise ArgumentError unless @reality.responds_to? :reason

Eleanor McHugh

5/9/2009 1:13:00 AM

0

On 9 May 2009, at 01:23, Gary Yngve wrote:
> Are replies to this group always like this?
>
> someone blindly posts the doc without reading that I already posted
> the doc or reading my example (like an implicit RTFM)
> other folks automatically assume I'm a newb and respond in five
> seconds without fully reading my post or thinking about their own post

Yes. We all have our moron moments. After all, the String.[]
documentation clearly states:

Element Reference---If passed a single +Fixnum+, returns a
substring of one character at that position.

which is precisely what I've just tried to explain in my other message
and confirms that the behaviour you're querying is completely
consistent with the conceptual model of a string of characters.

A Ruby string is not a *char[] and the index points are intersticies
_between_ an array of characters, not the addresses of those characters.

> do i need to be a kool kid or have a secret code word for this noise
> to go away?
>
> i've found the more niche ruby groups to be much more signal.. but
> this didn't seem to fit into a niche



Ellie

Eleanor McHugh
Games With Brains
http://slides.games-with-...
----
raise ArgumentError unless @reality.responds_to? :reason