Dummy question about method in class

Ruby
1

Dear all

I have a dummy question.

[1,2,3,4,5].inject { |sum,n| sum+n }
The inject method in this case is to calculate the sum of array.

I searched the RDoc. Why the inject method is belongs to Enumerable
Class, but not Array Class?

Many thanks
Valentino

2

On Feb 18, 2009, at 4:36 PM, Valentino L. wrote:

[1,2,3,4,5].inject { |sum,n| sum+n }
The inject method in this case is to calculate the sum of array.

I searched the RDoc. Why the inject method is belongs to Enumerable
Class, but not Array Class?

Enumerable is a Module, not a Class. It can be mixed into other
classes, so the Class gains the Methods of the module. Arrays uses
Enumerable instead of implementing #inject on its own, so they respond
to Enumerables #inject.

As a short presentation, i redefine Enumerable#inspect to do something
different (never try this at home!):

===
module Enumerable
def inject
puts “hey”
end
end

[].inject #=> puts “hey” now

Regards,
Florian G.

–
Florian G.

smtp: [email protected]
jabber: [email protected]
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3

On 18.02.2009 16:36, Valentino L. wrote:

[1,2,3,4,5].inject { |sum,n| sum+n }
The inject method in this case is to calculate the sum of array.

Additional to Florian’s good explanation: be aware that the version you
posted has a special property: it will return nil for empty Enumerables.

Consider this version which in most cases is the one you want (an empty
list summed up results in 0):

[1,2,3,4,5].inject(0) { |sum,n| sum+n }

Kind regards

robert

4

Valentino L. wrote:

I have a dummy question.

[1,2,3,4,5].inject { |sum,n| sum+n }
The inject method in this case is to calculate the sum of array.

I searched the RDoc. Why the inject method is belongs to Enumerable
Class, but not Array Class?

The Enumerable module can be shared by many classes. Any class which
implements its own ‘each’ method can mixin Enumerable and gain a whole
load of functionality.

For example:

class Fib
include Enumerable
def initialize(a,b,count)
@a, @b, @count = a, b, count
end
def each
@count.times do
yield @a
@a, @b = @b, @a+@b
end
end
end

p Fib.new(1,1,10).inspect
p Fib.new(1,1,10).max
p Fib.new(1,1,10).inject(0) { |sum,n| sum+n }