From: "Michael W. Ryder" <_mwryder@...>
Date: 2007-07-21T04:05:04+09:00
Subject: Re: Is there a replacement for sub?

Martin DeMello wrote:
> On 7/20/07, Michael W. Ryder <_mwryder@worldnet.att.net> wrote:
>>
>> Except where you use b = a.sub!(' ', '').  Then if you loop through this
>> statement 10 times b is nil, not the abcdef I expected.  This is what is
>> so confusing.  At the same time I can not use b = a.sub(' ', '') as it
>> always returns ab c d e f regardless of how many times I execute it,
>> which is what I would expect.  So, how would I do this?
>> 10.times { b = a.sub!(' ', '')} doesn't work, it errors out.
> 
> Remember that a variable is not an object, it is just a label pointing
> to an object.
> 
> sub: returns a *copy* of the string, with the substitution made (i.e.
> a new object)
> sub!: modifies the string in place (i.e. modifies the object itself)
> 
> Therefore
> 
> a = "a b c d e f"
> a.sub(" ", "") # => returns "ab c d e f", but leaves a unchanged.
> also, the return value will be garbage collected, since nothing is
> pointing to it
> 
> b = a.sub(" ", "") # => b is now "ab c d e f", leaves a unchanged
> 
> a = a.sub(" ", "") # => a is now "ab c d e f", the old string is
> garbage collected
> 
> a = "a b c d e f"
> b = a
> a = a.sub(" ", "") # => a is now "ab c d e f", b is still "a b c d e f"
> 
Wrong.  Unless you use b = a.dup b will be the same as a as it is 
pointing to the same object.  The result of the sub operation will 
change b as well as a.


> a = "a b c d e f"
> a.sub!("", '') #=> a is now "ab c d e f"
> 
> a = "a b c d e f"
> b = a.sub!("", '') #=> a is now "ab c d e f", a and b point to the same 
> object
> 
> a = "abcdef"
> b = a.sub!("", '') #=> a is still "abcdef", b is nil
> 
> Note the last bit carefully - sub! modifies the object, and *returns*
> either another pointer to the same object if it was modified, or nil
> if it wasn't. Therefore the return value of sub! should ideally only
> be used to check if the string was modified or not, and then
> discarded.
> 
> As for running in a loop, you want either
> 
> 10.times { a.sub!(" ", '')} #=> modifies a string 10 times
> 
> or
> 
> 10.times {a = a.sub(" ", '')} # creates a series of 10 strings, the
> intermediate ones being GCd
> 
> try this:
> 
> a = "a b c d e f"
> intermediate = [a]
> 10.times {|i| intermediate[i+1] = intermediate[i].sub(" ", '')}
> p intermediate
>

I guess my problem is that the sub operations return a nil if they don't 
make a change rather than the unchanged object.  This seems to be 
inconsistent as you get the changed object back if it works but nothing 
if it doesn't.  Maybe I need to write a version of sub or a method for 
it that returns the string regardless of whether it was changed or not.


> # => ["a b c d e f", "ab c d e f", "abc d e f", "abcd e f", "abcde f",
> "abcdef", "abcdef", "abcdef", "abcdef", "abcdef", "abcdef"]
> 
> 
> martin
>