Extracting the last n characters from a ruby string

Question

To get the last n characters from a string, I assumed you could use

ending = string[-n..-1]

but if the string is less than n letters long, you get nil.

What workarounds are available?

Background: The strings are plain ASCII, and I have access to ruby 1.9.1, and I'm using Plain Old Ruby Objects (no web frameworks).

perimosocordiae · Answer 1 · 2010-02-02T08:21:03.227

124

Well, the easiest workaround I can think of is:

ending = str[-n..-1] || str

(EDIT: The or operator has lower precedence than assignment, so be sure to use || instead.)

edited Feb 02 '10 at 08:21

answered Feb 01 '10 at 05:17

perimosocordiae

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+1... I think this way is easier to read than `string.reverse[0..n].reverse`, which gives me a second of "wait, why is he doing that?" (or would if I weren't reading it in the context of this question) – Arkaaito Feb 01 '10 at 05:19
4

Good answer, but it should be `||` instead of `or`, or put parentheses around `str[-n..-1] or str`. – Andrew Grimm Feb 01 '10 at 05:34
Good answer, but I don't like that ruby doesn't treat s[-inf..-1] the same as x[0..inf] – klochner Feb 01 '10 at 07:35
Thanks for noting the operator precedence issue, Andrew. Gets me every time. – perimosocordiae Feb 02 '10 at 08:21
@perimosocordiae you aren't the only one. http://stackoverflow.com/questions/372652/what-are-the-ruby-gotchas-a-newbie-should-be-warned-about – Andrew Grimm Feb 02 '10 at 22:06
I like this solution because [] is so flexible to operate on any position of string. – Donato Jul 21 '15 at 23:56

score 112 · Accepted Answer · edited Nov 02 '13 at 06:40

112

Here you have a one liner, you can put a number greater than the size of the string:

"123".split(//).last(5).to_s

For ruby 1.9+

"123".split(//).last(5).join("").to_s

For ruby 2.0+, join returns a string

"123".split(//).last(5).join

edited Nov 02 '13 at 06:40

nostopbutton

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answered Mar 07 '11 at 04:36

Gareve

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If you're using a recent version of Ruby, you could use `chars` instead of split. – Andrew Grimm Oct 23 '11 at 21:15
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I used "12345678910".split(//).last(7).join.to_s – Hard-Boiled Wonderland Oct 23 '12 at 16:03
@Hard-BoiledWonderland `join` works. I don't think you need the last `to_s` if you use join. – Andrew Grimm Dec 05 '12 at 11:15
@AndrewGrimm You're right, for that case I used the above with Nokogiri `n.xpath('ReferenceID').inner_text.split(//).last(7).join.to_s.to_i` I needed the to_s to perform the to_i to extract the numeric value. – Hard-Boiled Wonderland Dec 05 '12 at 12:31
You are missing __.join__ - now it returns an array of strings. Instead it should be `"123".split(//).last(5).join` (Ruby 2.0.0) – Pavel Nikolov Jul 10 '13 at 13:47
If you're using rails, ActiveSupport defines String#last: http://api.rubyonrails.org/classes/String.html#method-i-last – Gabe Martin-Dempesy Oct 15 '13 at 17:24
would actually be better performance wise to write `"123".split(' ' ).join` in-place of using a regex pattern, but this is a solid breakdown. otherwise, might be nice to explain what `.split(//)` is doing. – thesayhey Jun 01 '18 at 19:48

Zack Xu · Answer 3 · 2017-10-04T09:34:37.137

In straight Ruby (without Rails), you can do

string.chars.last(n).join

For example:

2.4.1 :009 > a = 'abcdefghij'
 => "abcdefghij"
2.4.1 :010 > a.chars.last(5).join
 => "fghij"
2.4.1 :011 > a.chars.last(100).join
 => "abcdefghij"

If you're using Ruby on Rails, you can call methods first and last on a string object. These methods are preferred as they're succinct and intuitive.

For example:

[1] pry(main)> a = 'abcdefg'                                                                                                                
 => "abcdefg"
[2] pry(main)> a.first(3)                                                                                                                   
 => "abc"
[3] pry(main)> a.last(4)                                                                                                                    
 => "defg"

score 14 · Answer 4 · answered Feb 01 '10 at 05:15

14

ending = string.reverse[0...n].reverse

answered Feb 01 '10 at 05:15

Andrew Grimm

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This is the best approach I see on this page that satisfies the requirement of being able to supply an ending character length that exceeds the total string length. – Rob.Kachmar Aug 08 '13 at 18:46
1

For example, if you are intending to take the last 3 character of a group of stings like "abcde", "ab", and "a". This technique will result in "cde", "ab", and "a" using the same code for each. `"abcde".reverse[0,3].reverse` >>> "cde" `"ab".reverse[0,3].reverse` >>> "ab" `"a".reverse[0,3].reverse` >>> "a" – Rob.Kachmar Aug 08 '13 at 18:56

score 10 · Answer 5 · edited Jun 04 '15 at 07:07

10

You can use the following code:

string[string.length-n,string.length]

edited Jun 04 '15 at 07:07

enrico.bacis

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answered Jun 03 '15 at 11:45

rurkss

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code only - answers are not always helpful. explaining why/how this code is the fix would be great – ry8806 Jun 03 '15 at 12:04

score 6 · Answer 6 · answered Sep 01 '11 at 08:32

6

To get the last n characters from a string, you could do this

a[-n, n] if a is the array.

Here's and example if you would want one.

ruby-1.9.2-p180 :006 > a = "911234567890"

=> "911234567890"

ruby-1.9.2-p180 :009 > a[-5,5]

=> "67890"

ruby-1.9.2-p180 :010 > a[-7,7]

=> "4567890"

answered Sep 01 '11 at 08:32

Sunil

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If the number is too large, `nil` is returned, which is what this question was specifically trying to avoid. – Andrew Grimm Sep 07 '11 at 23:52
Good answer. Clean. – Volte Jun 14 '16 at 02:58
Why is this getting upvotes? It has the exact same problem as what the OP was asking how to fix. – jeffdill2 Oct 22 '19 at 13:46

score 5 · Answer 7 · answered Feb 01 '10 at 05:19

5

Have you tried a regex?

string.match(/(.{0,#{n}}$)/)
ending=$1

The regex captures as many characters it can at the end of the string, but no more than n. And stores it in $1.

answered Feb 01 '10 at 05:19

EmFi

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score 1 · Answer 8 · answered Dec 05 '12 at 08:54

1

Improvement on EmFi's answer.

string[/.{,#{n}}\z/m]

answered Dec 05 '12 at 08:54

sawa

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Jeremy Moritz · Answer 9 · 2022-09-30T15:43:08.557

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For those who are using Ruby on Rails, you can simply do this:

str.last(n)

Note: I realize this doesn't answer the OP question because in PLAIN Ruby (w/o Rails) the last method does not exist for String. For that, there are many other good answers here.

edited Sep 30 '22 at 15:43

answered Mar 20 '22 at 21:44

Jeremy Moritz

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The `last` method does not exist for `String`. You should delete this answer. – Casper Jul 09 '22 at 00:59

Extracting the last n characters from a ruby string

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