From: Robert Klemme <shortcutter@...>
Date: 2007-07-20T18:27:28+09:00
Subject: Re: Is there a replacement for sub?

2007/7/20, dblack@wobblini.net <dblack@wobblini.net>:
> Hi --
>
> On Fri, 20 Jul 2007, Robert Klemme wrote:
>
> > 2007/7/20, Michael W. Ryder <_mwryder@worldnet.att.net>:
> >> Chris Shea wrote:
> >> > On Jul 19, 8:59 pm, "Michael W. Ryder" <_mwry...@worldnet.att.net>
> >> > wrote:
> >> >> I was trying to come up with a way to remove x instances of a character
> >> >> from a string and came up with a problem.  If I enter:
> >> >>
> >> >> a = "a b c d e f"
> >> >> for i in 1..3
> >> >>    a = a.sub!(' ', '')
> >> >> end
> >> >> puts a  ==> returns 'abcd e f' which is correct.
> >> >>
> >> >> But if I enter:
> >> >>
> >> >> a = "a b c d e f"
> >> >> for i in 1..10
> >> >>    a = a.sub!(' ', '')
> >> >> end
> >> >> puts a  ==> returns error.rb:3: private method `sub!' called for
> >> >> nil:NilClass (NoMethodError, and a is now nil.
> >> >>
> >> >> What I am looking for is a way to remove the first n instances of a
> >> >> blank from the string without wiping out the string if it does not
> >> >> contain at least n blanks.  I assume there is a way to do this with
> >> >> regular expressions, but have not found it yet.  This is something an
> >> >> editor I liked, UCEDIT, on the CDC Cyber had in the 70's.
> >> >
> >> > sub! modifies the string in place, so you don't need to say a = a.sub!
> >> > (' ','').  a is already changing.  And since sub! is modifying in
> >> > place, it returns nil if no changes are being made, and you end up
> >> > setting a to nil when that happens.
> >> >
> >> > a = 'a b c d e f'
> >> > 10.times { a.sub!(' ','')}
> >> > puts a     # 'abcdef'
> >> >
> >> > HTH,
> >> > Chris
> >> >
> >>
> >> I think this is where I am having problems understanding Ruby.  I have
> >> to use a.sub(' ', '') in a for loop but use a.sub!(' ', '') when using a
> >> times loop.  Why the difference?
> >
> > There is none.  a.sub! is an alternative to a = a.sub - wherever you use it.
>
> a.sub! will modify the same string, though, whereas a = a.sub
> changes the binding of 'a' to a new string.  (Which you probably
> didn't mention because it doesn't matter in a given case, but could be
> misunderstood out of context :-)

Yes. I was just referring to the assumed different usage in for and
while loops (see question). Thanks for clarifying!

Kind regards

robert