Re: Confusing results from string multiplication

On Thu, Dec 8, 2011 at 5:16 PM, Rob Marshall <[e-mail]> wrote:
> Hi,
>
> I actually registered here to ask a different question, but after
> checking the core docs for 1.8.7 I realized that the behavior has
> apparently changed, but as part of the registration process I was asked
> to enter the result of:
>
> puts ((11.to_s * 2).to_i/2)
>
> I don't know Ruby very well, but this one seemed pretty
> straight-forward, in that it was supposed to convert the number (object)
> 11 to a string, double it ('1111') then convert it back to an integer
> and divide by two. Since these are integers the answer would be: 555.
>
> But here's where it gets strange. Since I'm new to Ruby I thought I'd
> play with it a bit, and since I thought that whitespace wasn't supposed
> to matter, I did:
>
>>> 11.to_s * 2
> => "1111"
>>> 11.to_s *2
> => "1011"
>>> 11.to_s*2
> => "1111"
>
> I thought, maybe this is a problem with irb, so I wrote a little
> program:
>
> #!/usr/bin/env ruby
>
> puts (11.to_s * 2)
> puts (11.to_s *2)
> puts (11.to_s*2)
>
> And when I run it, I see:
>
> 1111
> 1011
> 1111
>
> Can anyone explain why the '11.to_s *2' gave me 1011? Is this a 1.8.7
> problem? The Ruby version on my Mac OS X Lion system is: ruby 1.8.7
> (2010-01-10 patchlevel 249) [universal-darwin11.0].

Congratulations, you really hit a strange corner case in the syntax!

irb(main):003:0> 11.to_s *2
=> "1011"
irb(main):004:0> 11.to_s(*2)
=> "1011"
irb(main):005:0> 11.to_s(2)
=> "1011"

The variant with a single space before the star and no space after is
actually the splash operator as in:

irb(main):011:0> def f(*a) p a end
=> nil
irb(main):012:0> args = [1,2,3]
=> [1, 2, 3]
irb(main):013:0> f(args)
[[1, 2, 3]]
=> [[1, 2, 3]]
irb(main):014:0> f(*args)
[1, 2, 3]
=> [1, 2, 3]
irb(main):015:0> f(args, 77, 88)
[[1, 2, 3], 77, 88]
=> [[1, 2, 3], 77, 88]
irb(main):016:0> f(*args, 77, 88)
[1, 2, 3, 77, 88]
=> [1, 2, 3, 77, 88]

In your case:

irb(main):017:0> def f(a) p a end
=> nil
irb(main):018:0> f(2)
2
=> 2
irb(main):019:0> f(*2)
2
=> 2

And 11.to_s(2) ist just the conversion to binary:

irb(main):021:0> 11.to_s 2
=> "1011"
irb(main):022:0> 11.to_s 10
=> "11"
irb(main):023:0> 11.to_s 16
=> "b"

Kind regards

robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/