Just for fun, I think every method in Ruby Array can be implemented by the
base map and reduce. The group_by is not easy to re-make, but with the
correct use of fold() and hash() it can be possible.
Here I implemented part of the methods that are available in scala/spark
but not in Ruby Array. 
class Array
def reduceByKey
if block_given?
group_by {|x| x[0]}.map {|x,y| y.reduce {|x,y| [ x[0],
yield(x[1],y[1]) ]}}
else
raise "no block given"
end
end
def mapValues
if block_given?
map {|x| [ x[0], yield(x[1]) ]}
else
raise "no block given"
end
end
def countByKey
group_by {|x| x[0]}.map {|x,y| [x,y.size]}
end
def fold(a)
if block_given?
dup.unshift(a).reduce {|x,y| yield(x,y)}
else
raise "no block given"
end
end
end
Here is the test code:
require '/path/to/module'
li = [["a",1],["b",2],["a",3],["b",5],["a",4],["c",9],["a",11]]
p li.mapValues {|s| s**2 }
p li.reduceByKey {|x,y| x+y }
p li.countByKey
p li.map {|x| x[1]}.fold(10) {|x,y| x+y}
Thanks for all the help.
Adriel
···
On Fri, Jan 21, 2022 at 10:41 AM Adriel Peng <peng.adriel@gmail.com> wrote:
Thank you Frank. I like the dup way.
Now I finished the job, it's a simulation to Scala's fold function:
irb(main):040:0> class Array
irb(main):041:1> def fold(a)
irb(main):042:2> if block_given?
irb(main):043:3> dup.unshift(a).reduce {|x,y| yield(x,y)}
irb(main):044:3> else
irb(main):045:3> raise "no block"
irb(main):046:3> end
irb(main):047:2> end
irb(main):048:1> end
=> :fold
irb(main):049:0> [3,2,1,4].fold(10) {|x,y| x+y}
=> 20
irb(main):050:0> ["a","b","c"].fold("str:") {|x,y| x+y}
=> "str:abc"
Thanks
On Fri, Jan 21, 2022 at 10:31 AM Frank J. Cameron <fjc@fastmail.net> > wrote:
On 1/20/22, Adriel Peng <peng.adriel@gmail.com> wrote:
> class Array; def mytest; unshift(33); end; end
>
> x=[3,2,1,4] # => [3, 2, 1, 4]
> x.mytest # => [33, 3, 2, 1, 4]
> x # => [33, 3, 2, 1, 4]
>
> ... to avoid the original object (Array) to be modified in the method?
Here's two options:
class Array
def mytest
dup.unshift(33)
end
end
class Array
def mytest
[33] + self
end
end
x=[3,2,1,4] # => [3, 2, 1, 4]
x.mytest # => [33, 3, 2, 1, 4]
x # => [3, 2, 1, 4]
Mutation in Ruby | Online Video Tutorial by thoughtbot
Dup vs Clone in Ruby: Understanding The Differences - RubyGuides
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