Implanting a new method for a class


#1

Hi there, I have a question regarding how to implant a new method into
the
class Array. I would like to write a ruby file that sums all of the
digits
in an array. I have tried this:

class Array
def Array.sum
until (Array.empty?)
arraySum += Array.pop
end
puts arraySum
end
end

Gave no error when I ran the code. My problem occur when i try to use
this
method:

require “Library/Array_sum.rb”
a = [1…100]
a.sum

I get a NoMethodError on sum. Can anybody tell me what I did wrong here?
Thanks in advance :slight_smile:


#2

On 2/12/06, Jeppe J. removed_email_address@domain.invalid wrote:

end

Thanks in advance :slight_smile:
You are confusing classes and instances of classes. You are defining
methods for the Array class, not for actual array instances.

class A
def A.foo
puts “Class method of #{self.inspect}, instance of class
#{self.class}”
end
def foo
puts “Instance method for #{self.inspect}, of class #{self.class}”
end
end

A.foo # prints “Class method of A, instance of class Class”
a = A.new
a.foo # prints “Instance method of <#A:0xb7ce9000> of class A”

You can implement sum like so:

class Array
def sum
total = 0
# self refers to the current array instance
self.each { |elem| total += elem }
total
end

# but you can also implement sum on the class Array itself.  But 

then
# you need to provide the array, that you want to know the sum of
#
def Array.sum arr
arr.inject(0) { |total, elem| total += elem }
end
end

puts [1,2,3,4].sum
puts Array.sum([1,2,3,4,5])

be careful with “pop”, it changes the Array!

def sum!(arr)
total = 0
total += arr.pop until a.empty?
total
end

arr = [5,4,3]
p arr.sum! #=> 12
p arr #=> [] arr is now empty!

-Levin


#3

Jeppe J. wrote:

Hi there, I have a question regarding how to implant a new method into
the
class Array. I would like to write a ruby file that sums all of the
digits
in an array. I have tried this:

class Array
def Array.sum
until (Array.empty?)
arraySum += Array.pop
end
puts arraySum
end
end

Gave no error when I ran the code. My problem occur when i try to use
this
method:

require “Library/Array_sum.rb”
a = [1…100]
a.sum

I get a NoMethodError on sum. Can anybody tell me what I did wrong here?
Thanks in advance :slight_smile:

Jeppe,
You have a number of problems here. First, if what you are really trying
to do is to sum the array, rather than just learn how to add methods to
a class, then all you have to do is

a = [1,2,3]
a.inject { |v,n| v+n }
=> 6

Second, you created your function as a class method rather than an
instance method by calling it Array.sum. All you need is
def sum

end

This may be why you are getting the MethodNotFound, since the instance
of array “a” does not have this method - the class Array does.

Finally, a = [1…100] will give you an array with the first value being
the range [1…100], not an array with all the values from 1 to 100,
[1,2,3…].

Two suggestions: learn to use irb to see what happens when you call
methods, and (2) pick up Programming Ruby by Dave T. if you have not
already done so. It explains all of this in detail.

Hope this helps,
Keith


#4

Thank you, for sorting things out for me :slight_smile:

2006/2/12, Levin A. removed_email_address@domain.invalid:


#5

Jeppe J. wrote:


I get a NoMethodError on sum. Can anybody tell me what I did wrong here?
Thanks in advance :slight_smile:

You’ve defined a class method (e.g. Array.sum)

You need to do this:

class Array
def sum
until (Array.empty?)


James B.

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