From: Robert Klemme <bob.news@...>
Date: 2004-10-25T17:24:07+09:00
Subject: Re: [rcr] String#first / String#last


"Simon Strandgaard" <neoneye@adslhome.dk> schrieb im Newsbeitrag
news:200410241739.14922.neoneye@adslhome.dk...
> On Sunday 24 October 2004 15:37, ts wrote:
> > >>>>> "S" == Simon Strandgaard <neoneye@adslhome.dk> writes:
> >
> > S>     raise TypeError, "cannot convert #{n.class} to Integer" unless
> > n.kind_of? S> (Integer)
> >
> > svg% ruby -e 'class A; def to_int() 2 end; end; p
[1,2,3].first(A.new)'
> > [1, 2]
> > svg%
> >
> >
> > Guy Decoux
>
>
> Ok.. I think I have fixed this issue now.
>
>
> irb(main):001:0> 'abc'.last(3)
> => "abc"
> irb(main):002:0> class A; def to_int; 2 end; end
> => nil
> irb(main):003:0> 'abc'.last(A.new)
> => "bc"
> irb(main):004:0> class B; def to_int; 'a' end; end
> => nil
> irb(main):005:0> 'abc'.last(B.new)
> TypeError: B#to_int should return Integer
>         from ./a.rb:22:in `last'
>         from (irb):5
> irb(main):006:0>
>
>
>
> New implementation is here:
>
> class String
>   def first(length=nil)
>     length ||= 1
>     unless length.respond_to?(:to_int)
>       raise TypeError, "cannot convert #{length.class} to Integer"
>     end
>     n = length.to_int
>     unless n.kind_of?(Integer)
>       raise TypeError, "#{length.class}#to_int should return Integer"
>     end
>     raise ArgumentError, "negative string size" if n < 0
>     n = [n, self.size].min
>     self[0, n]
>   end
>   def last(length=nil)
>     length ||= 1
>     unless length.respond_to?(:to_int)
>       raise TypeError, "cannot convert #{length.class} to Integer"
>     end
>     n = length.to_int
>     unless n.kind_of?(Integer)
>       raise TypeError, "#{length.class}#to_int should return Integer"
>     end
>     raise ArgumentError, "negative string size" if n < 0
>     n = [n, self.size].min
>     self[-n, n]
>   end
> end
>
>
> btw: Is this better?


I'd prefer

class String
  def first(n)
    n = n.to_int
    raise ArgumentError, "negative string size" if n < 0
    self[0,n]
  end

  def last(n)
    n = n.to_int
    raise ArgumentError, "negative string size" if n < 0
    self[-n,n] || self[-length,length]
  end
end

Notes:

 - You don't need the type check for n because #to_int will throw if n
does not have it.

 - You don't need the min in first, because String#[] takes care of that
already.

 - A default value does not make sense IMHO, because with #first() (i.e.
without arg) I'd
   expect to get the first character ("foo"[0]) which is something
different).

Kind regards

    robert