Confused about private? methods

Hi, just a beginners question,

This is my program:

module Foo
class Bar
def initialize()
arr = [1,2,3,4,5]
arr.collect! { |n| n * -1 }
end
end
end

Now I’ve read that I also can use this (do they call it private ?)
method:

class Array
def multiply_by(x)
collect { |n| n * x }
end
end

and use it later on, like this:

module Foo
class Bar
def initialize()
arr = [1,2,3,4,5]
#arr.collect! { |n| n * -1 }
arr.multiply_by(5) # multiply each member of the array by 5
end
end
end

This does not work, how do i fit the method in my module?

Thnx!

Hi,

I assume you mean “@arr = [1,2,3,4,5]” rather than “arr = [1,2,3,4,5]”.
Because “arr = …” creates a local variable that would simply be
discarded in this case.

Your code doesn’t work because you use “collect” in your “multiply_by”
method instead of “collect!”. When you call “multiply_by”, the array
won’t change. Instead, you get a new array with the multiplied numbers:

ar = [1, 2]
new_ar = ar.multiply_by 4
p ar # still the same: [1, 2]
p new_ar # the new array returned by the method: [4, 8]

To correct your code, you can either change the definition of “multiply
by” to

def multiply_by x
collect! {|n| n * x}
end

or change the initialize method to something like this:

def initialize()

apply the method to the array [1,2,3,4,5] and store the resulting

array [5,10,15,20,25] in the instance variable @arr

@arr = [1,2,3,4,5].multiply_by 5
end

By the way: This has nothing to do with private methods. A private
method is a method that can only be called from within the object
(roughly speaking).

Jacques

Thanx Jacques I got it!

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