str1 = str2 is not a copy?!?

[Joe Ruby MUDCRAP-CE]

I have:

File.foreach(data_file) do |line|
line.strip!

puts line

base_name = line

puts line

base_name.sub!('www.', '')
base_name.sub!(/\.\w+$/, '')

puts line
end

Which outputs:

www.domain.com
www.domain.com
domain

WHY is var line getting changed by operations on var base_name? Isn't
'base_name = line' supposed to create a copy? '=' in this case seems to
be acting like an alias or something.

BTW, is there a foreach function that automatically strips off the
newlines from line? Returning the record separators is silly.

Currently hating Ruby,
Joe

 

[Jamey Cribbs]

I have:

File.foreach(data_file) do |line|
line.strip!

puts line

base_name = line

puts line

base_name.sub!('www.', '')
base_name.sub!(/\.\w+$/, '')

puts line
end

Which outputs:

www.domain.com
www.domain.com
domain

WHY is var line getting changed by operations on var base_name? Isn't
'base_name = line' supposed to create a copy? '=' in this case seems to
be acting like an alias or something.

Nope. Now, you have two variables, base_name and line, referencing the
same object. Try:

base_name = line.dup

BTW, is there a foreach function that automatically strips off the
newlines from line? Returning the record separators is silly.

line.chomp!
Currently hating Ruby,

Be a lover not a fighter! ;-)

Jamey

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[Farrel Lifson]

I have:

File.foreach(data_file) do |line|
line.strip!

puts line

base_name = line

puts line

base_name.sub!('www.', '')
base_name.sub!(/\.\w+$/, '')

puts line
end

Which outputs:

www.domain.com
www.domain.com
domain

WHY is var line getting changed by operations on var base_name? Isn't
'base_name = line' supposed to create a copy? '=' in this case seems to
be acting like an alias or something.

BTW, is there a foreach function that automatically strips off the
newlines from line? Returning the record separators is silly.

Currently hating Ruby,
Joe

It doesn't create a copy, both variables reference the same object. If
you don't want to modify the original value use sub rather than gsub!

Farrel

 

[Austin Ziegler]

WHY is var line getting changed by operations on var base_name? Isn't
'base_name = line' supposed to create a copy? '=' in this case seems
to be acting like an alias or something.

No, it isn't. You're not understanding Ruby variables and assignment.

Variables in Ruby aren't locations; they're names. Objects are the only
things that take up (meaningful) space.

Assignment doesn't copy.

BTW, is there a foreach function that automatically strips off the
newlines from line? Returning the record separators is silly.

No, it isn't silly. Sometimes, that's what people want. You should be
using #chomp or #chomp! to strip that (use #strip or #strip! if and only
if you want to remove whitespace from beginning and end of the line).

-austin

 

[snacktime]

I have:

File.foreach(data_file) do |line|
line.strip!

puts line

base_name = line

puts line

base_name.sub!('www.', '')
base_name.sub!(/\.\w+$/, '')

puts line
end

Which outputs:

www.domain.com
www.domain.com
domain

WHY is var line getting changed by operations on var base_name? Isn't
'base_name = line' supposed to create a copy? '=' in this case seems to
be acting like an alias or something.

As far as my understanding goes most assignments just copy object
references. Try something like base_name = line.clone.

Chris

 

[Louis J Scoras]

As has been pointed out, you are creating a new binding to the same
object. Where it can get tricky is with multiple value assignments:

array = ['you', 'loose.']
a, b = array

p [a,b]
p array

b = 'win!'
p array # Doh!!

x = array ; x[1] = 'win!'
p array # array has been changed

 

[dblack]

Hi --

Joe Ruby MUDCRAP-CE wrote:

/ ...


Because in most cases, variable names contain references for the sake of
efficiency. To get the behavior you want, try:

base_name = "" + line

You don't have to resort to a workaround like that; you can do:

base_name = line.dup


David

--
David A. Black | (e-mail address removed)
Author of "Ruby for Rails" [1] | Ruby/Rails training & consultancy [3]
DABlog (DAB's Weblog) [2] | Co-director, Ruby Central, Inc. [4]
[1] http://www.manning.com/black | [3] http://www.rubypowerandlight.com
[2] http://dablog.rubypal.com | [4] http://www.rubycentral.org

 

[Chris Gernon]

WHY is var line getting changed by operations on var base_name? Isn't
'base_name = line' supposed to create a copy? '=' in this case seems to
be acting like an alias or something.

Yep. All variables in Ruby are actually references to objects ... so
using the = operator actually signifies "this variable now points at the
same object as that variable". What you want is probably either:

base_name = line.clone

or

base_name = String.new(line)

but it would probably make more sense to just create base_name at the
first operation where you want the two to be different - for example:

line.strip!
puts line

base_name = line.sub('www.', '')
base_name.sub!(/\.\w+$/, '')

puts line

BTW, is there a foreach function that automatically strips off the
newlines from line? Returning the record separators is silly.

Hmmm, not that I know of. However, I just want to mention that you
probably want String#chomp rather than String#strip, unless you actually
want to strip whitespace from the beginning of the string as well.

Currently hating Ruby,
Joe

Watch it - you may be in danger of losing your MUDCRAP certification ...

 

[David Vallner]

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=20
Because in most cases, variable names contain references for the sake o= f
efficiency.

And I'm guessing the sake of simplicity too. The Ruby basic data item
object is thus one-dimensional, objects hold data, and variables point
to objects. Contrasting with C, where either variables hold data, or
heap memory holds data, and variables point to heap memory, or (ye gods)
C++, where references add yet another dimension to the variable / data
relationship (and that you can override the behaviour to do something
else when a data item changes the way it's being referenced
arbitrarily), this is -much- simpler to understand (when learning as a
first system, not if used to a PHPesque ad-hoc mixture of behaviours),
and what's more important, it's easier to keep a single set of quirks to
watch out for when using other peoples' code, or designing reusable code
(e.g. to avoid clobbering a library's internal state, etc.); although it
-is- a net loss of language expressivity. Indeed, AFAIK, even the
predominant C++ practice is "pick pointers or references and stick with
them throughout your code" for the common case.

David Vallner


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[Gregory Brown]

As has been pointed out, you are creating a new binding to the same
object. Where it can get tricky is with multiple value assignments:

array = ['you', 'loose.']
a, b = array

p [a,b]
p array

b = 'win!'
p array # Doh!!

x = array ; x[1] = 'win!'
p array # array has been changed

I get the point of this example, but it's worth noting that you can
change the array via b if you just use String#replace
=> ["you", "win"]