Michael W. Ryder
3/15/2009 10:14:00 PM
Christopher Dicely wrote:
> On Sat, Mar 14, 2009 at 4:47 PM, Michael W. Ryder
> <_mwryder@worldnet.att.net> wrote:
>> matt neuburg wrote:
>>> Michael W. Ryder <_mwryder@worldnet.att.net> wrote:
>>>
>>>> I am trying to set a group of variables to values stored in a tab
>>>> delimited string. I have no problem splitting the string into an array,
>>>> call it e, or in putting the variable names in a second array, call it
>>>> v. The problem arises when I try to merge the two together. If I enter:
>>>> irb(main):073:0> p v
>>>> ["name", "street", "city", "state", "zip", "*", "telephone"]
>>>> => nil
>>>> irb(main):074:0> p e
>>>> ["John Doe", "123 Main St", "Anywhere", "US", "01234-5678", "ab123",
>>>> "1234567890"]
>>>> => nil
>>>> irb(main):075:0> eval "#{v[0]} = e[0]"
>>>> => "John Doe"
>>>> irb(main):076:0> p name
>>>> "John Doe"
>>>> => nil
>>>>
>>>> it works as I want. But when I try:
>>>> irb(main):077:0> v.each_index {|i| eval "#{v[i]} = e[0]"}
>>>> => ["name", "street", "city", "state", "zip", "*", "telephone"]
>>>>
>>>> and then enter: puts zip
>>>> it returns with a NameError saying that the variable was undefined.
>>> Inside a block, local variables are local to the block. So, you're
>>> creating all those local variables and then the block ends and they are
>>> all thrown away.
>>>
>> I can see that if I create the variable before running the method the
>> variable is set even after the method ends. Which of course brings up the
>> question of why is the change visible outside of the block?
>
> Ruby blocks are closures, and they therefore capture the environment
> where they are created. So if a local variable exists in that
> environment when the block is created, the block has access to it, and
> changes will affect the externally-created local variable. But new
> local variables introduced in the block are local to the block. In
> essence, the environment of the block is subordinate to the
> environment in which it is created.
>
This is what is confusing to me. If I use a variable in a function and
then call another function which uses the same name C does not change
the first instance when I return from the second function unless it was
part of the call. In Basic if I make a change or create a new variable
it is visible everywhere afterwards. Ruby seems to be doing something
like a mix of the two where if the variable is undefined entering the
block it stays undefined outside the block, but if it was defined before
entering the block it is changed to the new value on exit.
>>> Why do you need variables called "name" (etc.) anyway? Why not use a
>>> hash, so that h["name"] contains the name ("John Doe"), h["street"]
>>> contains the street, and so on? So:
>>>
>> I have been programming in Business Basic for over 20 years and am used to
>> reading a file with a statement like:
>> Read(1,Key="1234")Name$,Street$,City$,State$,Zip$,*,Telephone$
>> and then using those variables.
>
> Assuming you have a "keyed_read" method defined properly, you can easily do:
>
> name, street, city, state, zip, _, telephone =
> data_file.keyed_read(:key=>"1234")
>
> But the thing that is harder is dynamically assigning variable names
> based on a array provided. Using hashes or structs are the more
> idiomatic Ruby ways of doing something similar to that. Its possible
> to do something very close to what you were attempting with instance
> variables using Object#instance_variable_set.
>
Part of what brought this up is that I work with a lot of Excel
spreadsheets of information from various clients. I export the data
into tab separated records. I then break up the string and assign it to
the various variables for further processing. Currently in Business
Basic I have to find the position of the first tab, set the first
variable to the data up to that position, chop off the first part of the
string up to the tab and repeat. In Ruby it looks like I can just split
the string and combine it with the variable names in a couple of steps.
> irb(main):036:0> v.zip(e).each { |var, val|
> irb(main):037:1* begin
> irb(main):038:2* self.instance_variable_set(("@"+var).to_sym,val)
> irb(main):039:2> rescue
> irb(main):040:2> end
> irb(main):041:1> }
> => [["name", "John Doe"], ["street", "123 Main St"], ["city",
> "Anywhere"], ["state", "US"], ["zip", "01234-5678"], ["*", "ab123"],
> ["telephone", "1234567890"]]
> irb(main):042:0> @name
> => "John Doe"
>
I will play with this and see how it compares to the other suggestions
from the group.
> So if you want to do it that way, you can. (Note, I'm assuming that
> the '*' is a special symbol for a value you don't want to store, if
> not, you'll have to add some logic to map that to a valid instance
> variable name.)
>