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.


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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