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comp.lang.ruby

Equvialent of Python slices?

Peter Szinek

10/4/2006 1:19:00 PM

Hello,

Sorry for the lame question again, but even after a lot of googling I
still couldn't find the $subj., ie. something like:

>>> "asdfghj"[2::2]
'dgj'
(= every second letter, beginning from the 2nd index in the string)
>>> "asdfghj"[::-1]
'jhgfdsa'
(= the whole word, 1 steps backwards - i.e. the reverse of the string)

ranges in Ruby are similar, without the 3rd 'step' param, which I am
missing frequently... Also, with slices, you can omit one or both
delimiters (i.e. if you omit the first (last) one, it defaults to the
0th (last) index of the list - is there some simiar possibility in Ruby,
too?



Thx
Peter
http://www.rubyra...
4 Answers

Daniel Berger

10/4/2006 2:07:00 PM

0


James Edward Gray II wrote:
> On Oct 4, 2006, at 8:19 AM, Peter Szinek wrote:
>
> > Hello,
> >
> > Sorry for the lame question again, but even after a lot of googling
> > I still couldn't find the $subj., ie. something like:
> >
> > >>> "asdfghj"[2::2]
> > 'dgj'
> > (= every second letter, beginning from the 2nd index in the string)

Interesting, though I'm surprised Python has explicit syntax for what
seems like a highly specific type of operation. But, hey, cool.

> I'm not going to call this as pretty, of course, but:
>
> >> test = "asdfghj"
> => "asdfghj"
> >> test[2..-1].gsub(/(.)./, "\\1")
> => "dgj"

Slightly OT here....

I'm trying to remember why test[2, -1] returns nil. Positive numbers
for the second integer work as expected:

test[2,1] => 'd'
test[2,2] => 'df'
test[2,100] => dfghj
test[2,-1] => nil

/me considers a patch to string.c

Regards,

Dan

MonkeeSage

10/4/2006 2:45:00 PM

0

Hi Peter,

In ruby you can extend built-in objects, so you can get the same
(general) functionality by adding a method to Enumerable (which is
mixed-in to String and Array [and Hash, too, but step is not meaningful
for that class]):

module Enumerable
def step(*args)
start, stop = 0, -1
case args.size
when 1
step = args[0]
when 2
start = args[0]
step = args[1]
when 3
start = args[0]
stop = args[1]
step = args[2]
else
raise(ArgumentError,
"`step': wrong number of " +
"arguments (#{args.size} for 3)",
caller)
end
if self.is_a?(Hash)
raise(NoMethodError,
"`step': not implemented for Hash",
caller)
end
seq = self[start..stop]
pos = 0
if self.is_a?(Array)
out = seq[0,1]
elsif self.is_a?(String)
out = seq[0,1].to_s
end
while pos < seq.size
val = seq[(pos += step)]
out << val unless val.nil?
end
out
end
end

# start at index 2 and step by 2...
p "abcdefg".step(2, 2) # => "ceg"
p %w{a b c d e f g}.step(2, 2) # => ["c", "e", "g"]

# this breaks
{'a'=>'b'}.step(2) # => -:60: `step': not implemented for Hash
(NoMethodError)

Regards,
Jordan

Logan Capaldo

10/4/2006 3:00:00 PM

0

On Wed, Oct 04, 2006 at 11:10:10PM +0900, Daniel Berger wrote:
>
> James Edward Gray II wrote:
> > On Oct 4, 2006, at 8:19 AM, Peter Szinek wrote:
> >
> > > Hello,
> > >
> > > Sorry for the lame question again, but even after a lot of googling
> > > I still couldn't find the $subj., ie. something like:
> > >
> > > >>> "asdfghj"[2::2]
> > > 'dgj'
> > > (= every second letter, beginning from the 2nd index in the string)
>
> Interesting, though I'm surprised Python has explicit syntax for what
> seems like a highly specific type of operation. But, hey, cool.
>
> > I'm not going to call this as pretty, of course, but:
> >
> > >> test = "asdfghj"
> > => "asdfghj"
> > >> test[2..-1].gsub(/(.)./, "\\1")
> > => "dgj"
>
> Slightly OT here....
>
> I'm trying to remember why test[2, -1] returns nil. Positive numbers
> for the second integer work as expected:
>
> test[2,1] => 'd'
> test[2,2] => 'df'
> test[2,100] => dfghj
> test[2,-1] => nil
I can see it logically, test[start position, length]. A negative one
length doesn't make much sense. test[2..-1] works though, since it's
start postion..(.)end position. Unfortunately you can't coun't backwards
with a negative length ( like if I want the last two characters I can't
say test[-1, -2]. )
>
> /me considers a patch to string.c
>
> Regards,
>
> Dan
>

Daniel Berger

10/4/2006 3:07:00 PM

0

Logan Capaldo wrote:
> On Wed, Oct 04, 2006 at 11:10:10PM +0900, Daniel Berger wrote:
> >
> > James Edward Gray II wrote:
> > > On Oct 4, 2006, at 8:19 AM, Peter Szinek wrote:
> > >
> > > > Hello,
> > > >
> > > > Sorry for the lame question again, but even after a lot of googling
> > > > I still couldn't find the $subj., ie. something like:
> > > >
> > > > >>> "asdfghj"[2::2]
> > > > 'dgj'
> > > > (= every second letter, beginning from the 2nd index in the string)
> >
> > Interesting, though I'm surprised Python has explicit syntax for what
> > seems like a highly specific type of operation. But, hey, cool.
> >
> > > I'm not going to call this as pretty, of course, but:
> > >
> > > >> test = "asdfghj"
> > > => "asdfghj"
> > > >> test[2..-1].gsub(/(.)./, "\\1")
> > > => "dgj"
> >
> > Slightly OT here....
> >
> > I'm trying to remember why test[2, -1] returns nil. Positive numbers
> > for the second integer work as expected:
> >
> > test[2,1] => 'd'
> > test[2,2] => 'df'
> > test[2,100] => dfghj
> > test[2,-1] => nil
> I can see it logically, test[start position, length]. A negative one
> length doesn't make much sense. test[2..-1] works though, since it's
> start postion..(.)end position. Unfortunately you can't coun't backwards
> with a negative length ( like if I want the last two characters I can't
> say test[-1, -2]. )

Ok, I just need to remember the second value is a length, not an index,
although that still confuses me.

I wonder if making that change would cause havoc with the regex engine.
Best to leave it alone I suppose.

Regards,

Dan