Equvialent of Python slices?

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.rubyrailways.com

On Oct 4, 2006, at 8:19 AM, Peter S. 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)

I’m not going to call this as pretty, of course, but:

test = “asdfghj”
=> “asdfghj”

test[2…-1].gsub(/(.)./, “\1”)
=> “dgj”

Another way is to use step():

result = “”
=> “”

(2…test.size).step(2) { |i| result << test[i, 1] }
=> 2…7

result
=> “dgj”

We can get that down to one line with enumerator:

require “enumerator”
=> true

(2…test.size).enum_for(:step, 2).inject("") { |r, i| r << test
[i, 1] }
=> “dgj”

I do realize the Python you showed is sexier though.

“asdfghj”[::-1]
‘jhgfdsa’
(= the whole word, 1 steps backwards - i.e. the reverse of the
string)

test.reverse
=> “jhgfdsa”

:wink:

James Edward G. II

James Edward G. II wrote:

On Oct 4, 2006, at 8:19 AM, Peter S. 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

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

On Wed, Oct 04, 2006 at 11:10:10PM +0900, Daniel B. wrote:

‘dgj’
=> “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]. )

Logan C. wrote:

“asdfghj”[2::2]
test[2…-1].gsub(/(.)./, “\1”)
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

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