# maps - Mappatura dei valori da due array in Ruby

original title: "maps - Mapping values from two array in Ruby"

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I'm wondering if there's a way to do what I can do below with Python, in Ruby:

``````sum = reduce(lambda x, y: x + y, map(lambda x, y: x * y, weights, data))
``````

I have two arrays of equal sizes with the weights and data but I can't seem to find a function similar to map in Ruby, reduce I have working.

Mi chiedo se c'è un modo per fare quello che posso fare di seguito con Python, in Ruby: sum = reduce (lambda x, y: x + y, map (lambda x, y: x * y, pesi, dati)) Ho due matrici di dimensioni uguali con il ...

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@Michiel de Mare

Your Ruby 1.9 example can be shortened a bit further:

``````weights.zip(data).map(:*).reduce(:+)
``````

Also note that in Ruby 1.8, if you require ActiveSupport (from Rails) you can use:

``````weights.zip(data).map(&:*).reduce(&:+)
``````

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In Ruby 1.9:

``````weights.zip(data).map{|a,b| a*b}.reduce(:+)
``````

In Ruby 1.8:

``````weights.zip(data).inject(0) {|sum,(w,d)| sum + w*d }
``````

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The Array.zip function does an elementwise combination of arrays. It's not quite as clean as the Python syntax, but here's one approach you could use:

``````weights = [1, 2, 3]
data = [4, 5, 6]
result = Array.new
a.zip(b) { |x, y| result << x * y } # For just the one operation

sum = 0
a.zip(b) { |x, y| sum += x * y } # For both operations
``````

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Ruby has a `map` method (a.k.a. the `collect` method), which can be applied to any `Enumerable` object. If `numbers` is an array of numbers, the following line in Ruby:

``````numbers.map{|x| x + 5}
``````

is the equivalent of the following line in Python:

``````map(lambda x: x + 5, numbers)
``````

For more details, see here or here.

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An alternative for the map that works for more than 2 arrays as well:

``````def dot(*arrays)
arrays.transpose.map {|vals| yield vals}
end

dot(weights,data) {|a,b| a*b}

# OR, if you have a third array

dot(weights,data,offsets) {|a,b,c| (a*b)+c}
``````

This could also be added to Array:

``````class Array
def dot
self.transpose.map{|vals| yield vals}
end
end

[weights,data].dot {|a,b| a*b}

#OR

[weights,data,offsets].dot {|a,b,c| (a*b)+c}
``````

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``````weights = [1,2,3]
data    = [10,50,30]

require 'matrix'
Vector[*weights].inner_product Vector[*data] # => 200
``````