Class variable as parameter

Ruby
1

I have the following situation:

class MyClass
def setVar(var,varValue)
var=varValue
end

def initialize
@myVar=0
setVar(@myVar,1)
puts(@myVar.to_s)
end
end

test = MyClass.new

Unfortunately, this piece of code is returning 0 instead of the 1 I
would expect. Is there anything I could do?

Thanks in advance :slight_smile:

2

On Nov 23, 2007, at 4:20 PM, Filipe wrote:

   puts(@myVar.to_s)

end
end

test = MyClass.new

Unfortunately, this piece of code is returning 0 instead of the 1 I
would expect. Is there anything I could do?

Your call to setVar can not change the value of an variable. Only
values of variables are passed in Ruby, not the variables themselves.

def setVar(var,varValue)
   var=varValue # <-- this is the same as: return varValue
end

def initialize
    @myVar=0
    setVar(@myVar,1) # <-- at this point this is the same as:

setVar(0,1)
puts(@myVar.to_s)
end

Ruby has a built-in method, Object#instance_variable_set that you can
use for situations where you need to assign by name, so you don’t
need to define it for any derived class. Also, Ruby can define
accessor methods for you:

class MyClass
attr_accessor :myVar
def initialize
@myVar=0
self.myVar = 1
puts myVar # <-- to_s not needed; puts applies to_s to objects
end
end
test = MyClass.new

This will output 1 and may be what you are looking for.

Regards, Morton

3

On 23 nov, 20:00, Morton G. [email protected] wrote:

would expect. Is there anything I could do?
setVar(@myVar,1) # ← at this point this is the same as:
attr_accessor :myVar
Regards, Morton
The key point is that the setVar method is supposed to do a couple
more things than just set the value of the variable. I removed them
from the example just for the sake of simplicity. Something like:

class MyClass
def setVar(var,varValue)
var=varValue
#do something else…
end

def initialize
@myVar1=0
@myVar2=0
@myVar3=0

   setVar(@myVar,1)
   setVar(@myVar,2)
   setVar(@myVar,3)

end
end

test = MyClass.new

Beeing able to “send” the variable to setVar and have its value
changed would save me many lines of code…

4

Morton G. wrote:

Also, Ruby can define
accessor methods for you:

class MyClass
attr_accessor :myVar
def initialize
@myVar=0
self.myVar = 1
puts myVar # <-- to_s not needed; puts applies to_s to objects
end
end
test = MyClass.new

Here it is explicitly:

class MyClass

def num=(val)
@num = val
end

def num
@num
end

def initialize(val)
@num = val
end

end

t = MyClass.new(1)
puts t.num

which is equivalent to:

class MyClass
attr_accessor :num

def initialize(val)
@num = val
end

end

t = MyClass.new(1)
puts t.num

5

On Nov 23, 2007, at 6:24 PM, Filipe wrote:

end
Unfortunately, this piece of code is returning 0 instead of the 1 I
@myVar=0
class MyClass
This will output 1 and may be what you are looking for.

Regards, Morton

The key point is that the setVar method is supposed to do a couple
more things than just set the value of the variable. I removed them
from the example just for the sake of simplicity.

That was a bad idea, since it made it impossible for anyone to
understand your real problem and give you the help you were looking for.

   @myVar2=0

Beeing able to “send” the variable to setVar and have its value
changed would save me many lines of code…

You can use Object#instance_variable_set to solve your problem by
sending the variable’s name to setVar.

class MyClass def setVar(name, val) instance_variable_set(name, val) #do something else... end 
def initialize
   @myVar1=0
   @myVar2=0
   @myVar3=0
   setVar(:@myVar1, 1)
   setVar(:@myVar2, 2)
   setVar(:@myVar3, 3)

end
end

MyClass.new.inspect # => “#<MyClass:0x24b58 @myVar3=3, @myVar2=2,
@myVar1=1>”

Regards, Morton

6

end
end
test = MyClass.new
Unfortunately, this piece of code is returning 0 instead of the 1 I
would expect. Is there anything I could do?

The most stylish, Ruby way to do what you are trying to do is as
follows:

class MyClass

def initialize
@myVar1 = 0 # direct value manipulation
self.myVar1 = 1 # sent through your wrapper
puts @myVar1
end

def myVar1=(value)
@myVar1 = value
# process extra steps here
end

end

(Ehm, I should admit, there may be a better way to actually get the
accessor method to fire inside of initialize than self.myVar1, but
that’s what I found to work.)

7

Filipe wrote:

I have the following situation:
def initialize
@myVar=0
setVar(@myVar,1)>

Unfortunately, this piece of code is returning 0 instead of the 1 I
would expect. Is there anything I could do?

Yes:

def initialize
@myVar = 0
@myVar = 1
…

8

On Nov 25, 11:50 am, Matt T. [email protected] wrote:

end
@myVar1 = 0 # direct value manipulation

(Ehm, I should admit, there may be a better way to actually get the
accessor method to fire inside of initialize than self.myVar1, but
that’s what I found to work.)

I would still have to writer a myVar=(value) method for every single
variable. That’s what I’m trying to get rid of.

9

You can do something like

–
class Foo

alias_method :method_missing2, :method_missing;

def method_missing(method, *args, &block)
if method.to_s =~ /=$/
set_var($`, args.first);
else
method_missing2(method, *args, &block);
end
end

private
def set_var(name, value)
instance_variable_set("@#{name}", value);
#do stuff here…
end

end

foo = Foo.new;
foo.car = “test”;
p foo #=> #<Foo:0x2fb77e4 @car=“test”>