# Initialize instance variable problem

**URL:** <https://www.ruby-forum.com/t/initialize-instance-variable-problem/194033>\
**Category:** Ruby\
**Created:** [August 28, 2010, 6:17pm UTC](https://www.ruby-forum.com/t/initialize-instance-variable-problem/194033 "2010-08-28T18:17:12Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![cameron1inm](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/779978/32.png) [@cameron1inm](https://www.ruby-forum.com/u/cameron1inm)\
**Post date:** [August 28, 2010, 6:17pm UTC](https://www.ruby-forum.com/t/initialize-instance-variable-problem/194033/1 "2010-08-28T18:17:12Z")

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Why does this return nil for @city?

class Url  
def initialize(city,quary,catagory,minask,maxask)  
@city = city  
@quary = quary  
@cat = catagory  
@min = minask  
@max = maxask  
end

puts @city

end  
a = Url.new(‘spokane’,“tires”,“pts”,“0”,“1000”)  
a

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**Author:** ![cameron1inm](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/779978/32.png) [@cameron1inm](https://www.ruby-forum.com/u/cameron1inm)\
**Post date:** [August 28, 2010, 6:47pm UTC](https://www.ruby-forum.com/t/initialize-instance-variable-problem/194033/2 "2010-08-28T18:47:19Z")

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2010/8/28 Cameron V. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

> Why does this return nil for @city?

Because when you use @city outside a method definition, it is an  
instance variable of you _class_ Url [1] and not of an instance of you  
class.

add

def city  
puts @city  
end

and call a.city to get what you want.

Cheers,

JJ Fleck

[1] I’m not 100% sure of the naming but what is sure is that the @city  
you use is _not_ an instance method of your instance a.

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<div class="post-metadata">

**Author:** ![cameron1inm](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/779978/32.png) [@cameron1inm](https://www.ruby-forum.com/u/cameron1inm)\
**Post date:** [August 29, 2010, 8:58am UTC](https://www.ruby-forum.com/t/initialize-instance-variable-problem/194033/3 "2010-08-29T08:58:06Z")

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I get what your saying,

But the problem I’m having is I want to build a URL string from the  
intit parameters… I need to concantinate them and add some other  
stuff…

So I guess I just pass them to the class and use them as parameters?..  
no need to init them as instance variables?.. alright…

thanks for the reply I will mess with it.

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**Author:** ![cameron1inm](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/779978/32.png) [@cameron1inm](https://www.ruby-forum.com/u/cameron1inm)\
**Post date:** [August 29, 2010, 11:35am UTC](https://www.ruby-forum.com/t/initialize-instance-variable-problem/194033/4 "2010-08-29T11:35:46Z")

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On 28.08.2010 18:17, Cameron V. wrote:

> puts @city
> 
> end  
> a = Url.new(‘spokane’,“tires”,“pts”,“0”,“1000”)  
> a

Others have identified the basic issue (your puts statement accesses a  
member of the class and not of the instance) but I’d like to add a few  
other remarks. First, the name Url might cause confusion because  
someone might think it is a generic URL handling class (like class URI  
for example).

What you basically should do is add attribute accessors for your  
instance variables, e.g.

class Url  
attr :city  
end

Then you can do

a = Url.new …  
puts a.city

But: you can make your life much simpler by resorting to Struct:

CityUrl = Struct.new :city, :quary, :catagory, :minask, :maxask

This gives you a proper initialize, attribute accessors and a few more  
things for free.

irb(main):008:0\> a = CityUrl.new(‘spokane’,“tires”,“pts”,“0”,“1000”)  
=\> #\<struct CityUrl city=“spokane”, quary=“tires”, catagory=“pts”,  
minask=“0”, maxask=“1000”\>  
irb(main):009:0\> a.city  
=\> “spokane”

Btw, from the naming and values of “minask” and “maxask” it seems these  
should be rather used with integer values. Even if you read them as  
strings from some sort of file your application logic will become much  
easier if you convert those strings on reading into ints and then go  
from there. Otherwise you will have to convert them all the time you  
want to use them which is inefficient, error prone (you might forget a  
location) and tedious to change if you need to at one point. Also, you  
depend on the format of the single source which might make it difficult  
to pull the data from other sources later. Basically it’s best to  
always do something like

external representation -\> import (load) converts into internal  
representation -\> work with that internal representation throughout the  
application -\> export (store) converts to external representation

Kind regards

robert

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<div class="post-metadata">

**Author:** ![cameron1inm](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/779978/32.png) [@cameron1inm](https://www.ruby-forum.com/u/cameron1inm)\
**Post date:** [August 29, 2010, 10:12am UTC](https://www.ruby-forum.com/t/initialize-instance-variable-problem/194033/5 "2010-08-29T10:12:40Z")

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On Sat, Aug 28, 2010 at 11:17 AM, Cameron V.  
[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)wrote:

> puts @city
> 
> ## end a = Url.new(‘spokane’,“tires”,“pts”,“0”,“1000”) a
> 
> Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/).

This is basically what Jean said, but I figured it might be easier to  
follow  
if you can actually see it.

class Url

# self is the class Url

puts “outside initialize 1, self is #{self.inspect}”

def initialize(city,quary,catagory,minask,maxask)  
@city = city  
@quary = quary  
@cat = catagory  
@min = minask  
@max = maxask  
# self is an instance of Url  
puts “Inside initialize 3, self is #{self.inspect}”  
end

# self is the class Url

puts “outside initialize 2, self is #{self.inspect}”

def render  
# self is an instance of Url  
puts “Inside initialize 4, self is #{self.inspect}”  
@city + @quary + @cat + @min + @max  
end  
end

a = Url.new(‘spokane’,“tires”,“pts”,“0”,“1000”)  
result = a.render  
puts “result is #{result.inspect}”