Rails - Feb 2011 - Why does variable not change value in an '.each' where an assignment is happening?

I have this line, where the input variables in the array can come in any
form --- string, integer, fixnum, float (ps, I know that sales_price as a
currency should really be BigDecimal or the like on instantiation, this is
just to illustrate my question):

[quantity, sales_price].each { |multiplicand| multiplicand
BigDecimal.new(multiplicand.to_s) if multiplicand.class != BigDecimal}

So if
(rdb:1) quantity = 50
(rdb:1) quantity.class
Fixnum
(rdb:1) sales_price = 50
(rdb:1) sales_price.class
Float

And I call this:
(rdb:1) [quantity, sales_price].each { |multiplicand| multiplicand
BigDecimal.new(multiplicand.to_s) if multiplicand.class != BigDecimal}
[50, 1.99]

Why don''t the variables change?
(rdb:1) quantity.class
Fixnum
(rdb:1) sales_price.class
Float
(rdb:1)

Because if I do this:
[quantity, sales_price].map { |multiplicand| multiplicand
BigDecimal.new(multiplicand.to_s) if multiplicand.class != BigDecimal}

I do get:
[#<BigDecimal:3b4f4f8,''0.5E2'',4(8)>,
#<BigDecimal:3b4f46c,''0.199E1'',8(8)>]

... so I know that the logic is working.

Shouldn''t an operation in an each change the input? --- I thought the
input
should be by ref.

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On Feb 8, 2:44 pm, David Kahn
<d...-rfEMNHKVqOwNic7Bib+Ti1W1rNmOCjRP@public.gmane.org>
wrote:
> I have this line, where the input variables in the array can come in any
> form --- string, integer, fixnum, float (ps, I know that sales_price as a
> currency should really be BigDecimal or the like on instantiation, this is
> just to illustrate my question):
>
> [quantity, sales_price].each { |multiplicand| multiplicand >
BigDecimal.new(multiplicand.to_s) if multiplicand.class != BigDecimal}
>
[snip]

All each does is iterate over a collection, yielding to the block.
when you do multiplicand = blah, you are just setting the local
variable multiplicand to point at some new object - you''re not
mutating the actual object.

Fred

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On Tue, Feb 8, 2011 at 9:06 AM, Frederick Cheung
<frederick.cheung-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org
> wrote:
>
> On Feb 8, 2:44 pm, David Kahn
<d...-rfEMNHKVqOwNic7Bib+Ti1W1rNmOCjRP@public.gmane.org> wrote:
> > I have this line, where the input variables in the array can come in
any
> > form --- string, integer, fixnum, float (ps, I know that sales_price
as a
> > currency should really be BigDecimal or the like on instantiation,
this
> is
> > just to illustrate my question):
> >
> > [quantity, sales_price].each { |multiplicand| multiplicand > >
BigDecimal.new(multiplicand.to_s) if multiplicand.class != BigDecimal}
> >
> [snip]
>
> All each does is iterate over a collection, yielding to the block.
> when you do multiplicand = blah, you are just setting the local
> variable multiplicand to point at some new object - you''re not
> mutating the actual object.
>
I see, I guess I got confused as I use ActiveRecord so much and within a
each block, by setting values and calling save it of course saves the
object. Is there a way to achieve what I am presenting, though, passing a
batch of variables and having them operated on and changed?

This situation:
(rdb:1) v1 = 1
1
(rdb:1) v2 = 2
2
(rdb:1) v3 = 3
3
(rdb:1) arr = [v1,v2,v3]
[1, 2, 3]
(rdb:1) arr = arr.map{|i| i = i + 1}
[2, 3, 4]

However what I would want is to see the variable values changed but they are
not:
(rdb:1) v1
1
(rdb:1) v2
2

Excuse my ingenuity, but I am trying to understand... if I have an array of
variables it seems that when the array is created, the elements which were
variables are substituted by values, and there is no reference:
(rdb:1) arr = [v1,v2,v3]
[1, 2, 3]
(rdb:1) v1
1
(rdb:1) arr[0]=5
5
(rdb:1) v1
1

This must sound like kindergarten but apparently I have been working albeit
successfully under some very false premises.


>
> Fred
>
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>
>
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On Feb 8, 3:33 pm, David Kahn
<d...-rfEMNHKVqOwNic7Bib+Ti1W1rNmOCjRP@public.gmane.org>
wrote:
> On Tue, Feb 8, 2011 at 9:06 AM, Frederick Cheung
<frederick.che...@gmail.com
>
> I see, I guess I got confused as I use ActiveRecord so much and within a
> each block, by setting values and calling save it of course saves the
> object. Is there a way to achieve what I am presenting, though, passing a
> batch of variables and having them operated on and changed?
that is very different. doing  some_object.foo = ''bar'' is
calling a
method on an object that changes its state, whereas
foo = bar just points a local variable at something new. (and in
addition Fixnums/BigDecimal are immutable - there is no method you can
call on a number to change it into another nmber)
>
> Excuse my ingenuity, but I am trying to understand... if I have an array of
> variables it seems that when the array is created, the elements which were
> variables are substituted by values, and there is no reference:
> (rdb:1) arr = [v1,v2,v3]
> [1, 2, 3]
Without resorting to dirty tricks, you can''t change the values of
local variables other than by actually doing
v1 = 2. arr does NOT contain [v1,v2,v3]. It contains 3 objects, and
the local variables v1,v2,v3 just happen to also be referencing those
same objects.

Fred
> (rdb:1) v1
> 1
> (rdb:1) arr[0]=5
> 5
> (rdb:1) v1
> 1
>
> This must sound like kindergarten but apparently I have been working albeit
> successfully under some very false premises.
>
>
>
>
>
> > Fred
>
> > --
> > You received this message because you are subscribed to the Google
Groups
> > "Ruby on Rails: Talk" group.
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> > To unsubscribe from this group, send email to
> >
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On Tue, Feb 8, 2011 at 10:34 AM, Frederick Cheung <
frederick.cheung-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> On Feb 8, 3:33 pm, David Kahn
<d...-rfEMNHKVqOwNic7Bib+Ti1W1rNmOCjRP@public.gmane.org> wrote:
> > On Tue, Feb 8, 2011 at 9:06 AM, Frederick Cheung <
> frederick.che...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org
> >
> > I see, I guess I got confused as I use ActiveRecord so much and within
a
> > each block, by setting values and calling save it of course saves the
> > object. Is there a way to achieve what I am presenting, though,
passing a
> > batch of variables and having them operated on and changed?
>
> that is very different. doing  some_object.foo = ''bar'' is
calling a
> method on an object that changes its state, whereas
> foo = bar just points a local variable at something new. (and in
> addition Fixnums/BigDecimal are immutable - there is no method you can
> call on a number to change it into another nmber)
>
I think I am starting to see, maybe. Hopefully not being too annoying here.

It seems rather strange to me that if you have an object, i.e. a string that
it should behave differently than my Account object.... the string#=()
method is still a property, right? -- no different really than
account#name=(), so seems it would be logical that if I can do
account.name= ''x'' within an iterator, that doing
string=''y'' should
also work, but as we
see it does not (lets put the Fixnums and BigDecimal aside as I understand
as a property they are re-created rather than modified).

arr.each {|account| account.name = ''x''}

arr.each {|string| string=(''y'')}

.... they seem to me to be the same thing, unless objects are really not the
same.


But with another experiment I see something interesting --- if I instantiate
a string formally as an object, then I do get the by reference relationship
in an array:

ruby-1.9.2-p136 :098 > str = String.new(''David'')
 => "David"
ruby-1.9.2-p136 :099 > arr = [str]
 => ["David"]
ruby-1.9.2-p136 :100 > arr.each {|s| str = ''new val''}
 => ["David"]
ruby-1.9.2-p136 :101 > str
 => "new val"

But if I just do it as a normal assignment, I do not:

ruby-1.9.2-p136 :106 > string = ''David''
 => "David"
ruby-1.9.2-p136 :107 > arr = [string]
 => ["David"]
ruby-1.9.2-p136 :108 > arr.each {|s| s = ''new val''}
 => ["David"]
ruby-1.9.2-p136 :109 > string
 => "David"


I thought that doing

str = ''David''

was just a shortcut to doing

str = String.new(''David'')

But apparently the behavior at least in the context of an array is
different. Wow!




>
> >
> > Excuse my ingenuity, but I am trying to understand... if I have an
array
> of
> > variables it seems that when the array is created, the elements which
> were
> > variables are substituted by values, and there is no reference:
>
>
> > (rdb:1) arr = [v1,v2,v3]
> > [1, 2, 3]
>
> Without resorting to dirty tricks, you can''t change the values of
> local variables other than by actually doing
> v1 = 2. arr does NOT contain [v1,v2,v3]. It contains 3 objects, and
> the local variables v1,v2,v3 just happen to also be referencing those
> same objects.
>
> Fred
>
> > (rdb:1) v1
> > 1
> > (rdb:1) arr[0]=5
> > 5
> > (rdb:1) v1
> > 1
> >
> > This must sound like kindergarten but apparently I have been working
> albeit
> > successfully under some very false premises.
> >
> >
> >
> >
> >
> > > Fred
> >
> > > --
> > > You received this message because you are subscribed to the
Google
> Groups
> > > "Ruby on Rails: Talk" group.
> > > To post to this group, send email to
rubyonrails-talk-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org
> .
> > > To unsubscribe from this group, send email to
> > >
rubyonrails-talk+unsubscribe-/JYPxA39Uh5TLH3MbocFF+G/Ez6ZCGd0@public.gmane.org
> > > For more options, visit this group at
> > >http://groups.google.com/group/rubyonrails-talk?hl=en.
>
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Phil


On Tue, Feb 8, 2011 at 11:26 AM, David Kahn
<dk-rfEMNHKVqOwNic7Bib+Ti1W1rNmOCjRP@public.gmane.org>wrote:
>
>
> On Tue, Feb 8, 2011 at 10:34 AM, Frederick Cheung <
> frederick.cheung-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>
>> On Feb 8, 3:33 pm, David Kahn
<d...-rfEMNHKVqOwNic7Bib+Ti1W1rNmOCjRP@public.gmane.org> wrote:
>> > On Tue, Feb 8, 2011 at 9:06 AM, Frederick Cheung <
>> frederick.che...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org
>> >
>> > I see, I guess I got confused as I use ActiveRecord so much and
within a
>> > each block, by setting values and calling save it of course saves
the
>> > object. Is there a way to achieve what I am presenting, though,
passing
>> a
>> > batch of variables and having them operated on and changed?
>>
>> that is very different. doing  some_object.foo =
''bar'' is calling a
>> method on an object that changes its state, whereas
>> foo = bar just points a local variable at something new. (and in
>> addition Fixnums/BigDecimal are immutable - there is no method you can
>> call on a number to change it into another nmber)
>>
>
> I think I am starting to see, maybe. Hopefully not being too annoying here.
>
>
> It seems rather strange to me that if you have an object, i.e. a string
> that it should behave differently than my Account object.... the string#=()
> method is still a property, right? -- no different really than
> account#name=(), so seems it would be logical that if I can do
> account.name = ''x'' within an iterator, that doing
string=''y'' should also
> work, but as we see it does not (lets put the Fixnums and BigDecimal aside
> as I understand as a property they are re-created rather than modified).
>
> arr.each {|account| account.name = ''x''}
>
> arr.each {|string| string=(''y'')}
>
> .... they seem to me to be the same thing, unless objects are really not
> the same.
>
>
> But with another experiment I see something interesting --- if I
> instantiate a string formally as an object, then I do get the by reference
> relationship in an array:
>
> ruby-1.9.2-p136 :098 > str = String.new(''David'')
>  => "David"
> ruby-1.9.2-p136 :099 > arr = [str]
>  => ["David"]
> ruby-1.9.2-p136 :100 > arr.each {|s| str = ''new val''}
>  => ["David"]
> ruby-1.9.2-p136 :101 > str
>  => "new val"
>
> But if I just do it as a normal assignment, I do not:
>
> ruby-1.9.2-p136 :106 > string = ''David''
>  => "David"
> ruby-1.9.2-p136 :107 > arr = [string]
>  => ["David"]
> ruby-1.9.2-p136 :108 > arr.each {|s| s = ''new val''}
>  => ["David"]
> ruby-1.9.2-p136 :109 > string
>  => "David"
>
>
In the first case, you explicitly changed the value of the local variable
str... it didn''t change the array contents. In the second case you only
changed the value inside the scope of the block that was enumerating
through. Whether you created the string with String.new or not won''t
make a
difference.

Ruby does pass object by reference, but when you do s = "whatever" in
the
block, you''re creating a new local variable that only lives as long as
the
scope of the block.

If you want to modify an array of strings, for example, you could do:

arr = ["John", "Doe"]
=> ["John","Doe"]
arr.each{|s| s.replace("blah") }
=> ["blah","blah"]

s.replace actually modifies the object, instead of creating a new one.


> I thought that doing
>
> str = ''David''
>
> was just a shortcut to doing
>
> str = String.new(''David'')
>
> But apparently the behavior at least in the context of an array is
> different. Wow!
>
>
>
>>
>> >
>> > Excuse my ingenuity, but I am trying to understand... if I have an
array
>> of
>> > variables it seems that when the array is created, the elements
which
>> were
>> > variables are substituted by values, and there is no reference:
>>
>>
>> > (rdb:1) arr = [v1,v2,v3]
>> > [1, 2, 3]
>>
>> Without resorting to dirty tricks, you can''t change the values
of
>> local variables other than by actually doing
>> v1 = 2. arr does NOT contain [v1,v2,v3]. It contains 3 objects, and
>> the local variables v1,v2,v3 just happen to also be referencing those
>> same objects.
>>
>> Fred
>>
>> > (rdb:1) v1
>> > 1
>> > (rdb:1) arr[0]=5
>> > 5
>> > (rdb:1) v1
>> > 1
>> >
>> > This must sound like kindergarten but apparently I have been
working
>> albeit
>> > successfully under some very false premises.
>> >
>> >
>> >
>> >
>> >
>> > > Fred
>> >
>> > > --
>> > > You received this message because you are subscribed to the
Google
>> Groups
>> > > "Ruby on Rails: Talk" group.
>> > > To post to this group, send email to
>> rubyonrails-talk-/JYPxA39Uh5TLH3MbocFF+G/Ez6ZCGd0@public.gmane.org
>> > > To unsubscribe from this group, send email to
>> > >
rubyonrails-talk+unsubscribe-/JYPxA39Uh5TLH3MbocFF+G/Ez6ZCGd0@public.gmane.org
>> > > For more options, visit this group at
>> > >http://groups.google.com/group/rubyonrails-talk?hl=en.
>>
>> --
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Groups
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>>
>>
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On Tue, Feb 8, 2011 at 12:06 PM, Phil Crissman
<phil.crissman-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>wrote:
>
> Phil
>
>
> On Tue, Feb 8, 2011 at 11:26 AM, David Kahn
<dk-rfEMNHKVqOwNic7Bib+Ti1W1rNmOCjRP@public.gmane.org>wrote:
>
>>
>>
>> On Tue, Feb 8, 2011 at 10:34 AM, Frederick Cheung <
>> frederick.cheung-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>>
>>> On Feb 8, 3:33 pm, David Kahn
<d...-rfEMNHKVqOwNic7Bib+Ti1W1rNmOCjRP@public.gmane.org> wrote:
>>> > On Tue, Feb 8, 2011 at 9:06 AM, Frederick Cheung <
>>> frederick.che...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org
>>> >
>>> > I see, I guess I got confused as I use ActiveRecord so much
and within
>>> a
>>> > each block, by setting values and calling save it of course
saves the
>>> > object. Is there a way to achieve what I am presenting,
though, passing
>>> a
>>> > batch of variables and having them operated on and changed?
>>>
>>> that is very different. doing  some_object.foo =
''bar'' is calling a
>>> method on an object that changes its state, whereas
>>> foo = bar just points a local variable at something new. (and in
>>> addition Fixnums/BigDecimal are immutable - there is no method you
can
>>> call on a number to change it into another nmber)
>>>
>>
>> I think I am starting to see, maybe. Hopefully not being too annoying
>> here.
>>
>> It seems rather strange to me that if you have an object, i.e. a string
>> that it should behave differently than my Account object.... the
string#=()
>> method is still a property, right? -- no different really than
>> account#name=(), so seems it would be logical that if I can do
>> account.name = ''x'' within an iterator, that doing
string=''y'' should also
>> work, but as we see it does not (lets put the Fixnums and BigDecimal
aside
>> as I understand as a property they are re-created rather than
modified).
>>
>> arr.each {|account| account.name = ''x''}
>>
>> arr.each {|string| string=(''y'')}
>>
>> .... they seem to me to be the same thing, unless objects are really
not
>> the same.
>>
>>
>> But with another experiment I see something interesting --- if I
>> instantiate a string formally as an object, then I do get the by
reference
>> relationship in an array:
>>
>> ruby-1.9.2-p136 :098 > str = String.new(''David'')
>>  => "David"
>> ruby-1.9.2-p136 :099 > arr = [str]
>>  => ["David"]
>> ruby-1.9.2-p136 :100 > arr.each {|s| str = ''new
val''}
>>  => ["David"]
>> ruby-1.9.2-p136 :101 > str
>>  => "new val"
>>
>> But if I just do it as a normal assignment, I do not:
>>
>> ruby-1.9.2-p136 :106 > string = ''David''
>>  => "David"
>> ruby-1.9.2-p136 :107 > arr = [string]
>>  => ["David"]
>> ruby-1.9.2-p136 :108 > arr.each {|s| s = ''new
val''}
>>  => ["David"]
>> ruby-1.9.2-p136 :109 > string
>>  => "David"
>>
>>
> In the first case, you explicitly changed the value of the local variable
> str... it didn''t change the array contents. In the second case you
only
> changed the value inside the scope of the block that was enumerating
> through. Whether you created the string with String.new or not
won''t make a
> difference.
>
> Oh, right, I see my errror
> Ruby does pass object by reference, but when you do s =
"whatever" in the
> block, you''re creating a new local variable that only lives as
long as the
> scope of the block.
>
> If you want to modify an array of strings, for example, you could do:
>
> arr = ["John", "Doe"]
> => ["John","Doe"]
> arr.each{|s| s.replace("blah") }
> => ["blah","blah"]
>
> s.replace actually modifies the object, instead of creating a new one.
>
Perfect, now I think I''ve got it, thanks!

>
>
>
>> I thought that doing
>>
>> str = ''David''
>>
>> was just a shortcut to doing
>>
>> str = String.new(''David'')
>>
>> But apparently the behavior at least in the context of an array is
>> different. Wow!
>>
>
>>
>>
>>>
>>> >
>>> > Excuse my ingenuity, but I am trying to understand... if I
have an
>>> array of
>>> > variables it seems that when the array is created, the
elements which
>>> were
>>> > variables are substituted by values, and there is no
reference:
>>>
>>>
>>> > (rdb:1) arr = [v1,v2,v3]
>>> > [1, 2, 3]
>>>
>>> Without resorting to dirty tricks, you can''t change the
values of
>>> local variables other than by actually doing
>>> v1 = 2. arr does NOT contain [v1,v2,v3]. It contains 3 objects, and
>>> the local variables v1,v2,v3 just happen to also be referencing
those
>>> same objects.
>>>
>>> Fred
>>>
>>> > (rdb:1) v1
>>> > 1
>>> > (rdb:1) arr[0]=5
>>> > 5
>>> > (rdb:1) v1
>>> > 1
>>> >
>>> > This must sound like kindergarten but apparently I have been
working
>>> albeit
>>> > successfully under some very false premises.
>>> >
>>> >
>>> >
>>> >
>>> >
>>> > > Fred
>>> >
>>> > > --
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the Google
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On 8 February 2011 18:06, Phil Crissman
<phil.crissman-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> If you want to modify an array of strings, for example, you could do:
> arr = ["John", "Doe"]
> => ["John","Doe"]
> arr.each{|s| s.replace("blah") }
> => ["blah","blah"]
> s.replace actually modifies the object, instead of creating a new one.
>
Also, depending on your use-case, consider the enumerable methods
".inject", ".collect", et al:

  >> x = [1,2,3]
  => [1, 2, 3]
  >> x.collect { |y| y*y }
  => [1, 4, 9]
  >>

http://www.ruby-doc.org/core/classes/Enumerable.html

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On Tue, Feb 8, 2011 at 2:17 PM, Michael Pavling
<pavling-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> On 8 February 2011 18:06, Phil Crissman
<phil.crissman-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> > If you want to modify an array of strings, for example, you could do:
> > arr = ["John", "Doe"]
> > => ["John","Doe"]
> > arr.each{|s| s.replace("blah") }
> > => ["blah","blah"]
> > s.replace actually modifies the object, instead of creating a new one.
> >
>
> Also, depending on your use-case, consider the enumerable methods
> ".inject", ".collect", et al:
>
Thanks... #inject is new to me and looks very cool. From what I understand
collect and map are the same function, is that your understanding too?

>
>  >> x = [1,2,3]
>  => [1, 2, 3]
>  >> x.collect { |y| y*y }
>  => [1, 4, 9]
>  >>
>
> http://www.ruby-doc.org/core/classes/Enumerable.html
>
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On 8 February 2011 20:35, David Kahn
<dk-rfEMNHKVqOwNic7Bib+Ti1W1rNmOCjRP@public.gmane.org>
wrote:
> On Tue, Feb 8, 2011 at 2:17 PM, Michael Pavling
<pavling-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>> Also, depending on your use-case, consider the enumerable methods
>> ".inject", ".collect", et al:
>
> Thanks... #inject is new to me and looks very cool. From what I understand
> collect and map are the same function, is that your understanding too?
>
That''s what the documentation says...

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