Equvialent of Python slices?

[Peter Szinek]

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

 

[James Edward Gray II]

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:
[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"



James Edward Gray II

 

[Daniel Berger]

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]

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]

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]. )

 

[Daniel Berger]

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