# Map more than one array at once

Hi all,

I’ve been using Ruby for a while now, and have enjoyed it immensely.
However, I haven’t yet found how to map more than one array at once.
The common example is calculating the dot product. You’d need to
iterate through each array at the same time to multiply the elements.
However, map only operates on one list at a time. I wanted to avoid
For Loops. Therefore, I end up having to do something like this

def dot_prod(a,b)
return 0 if a.length != b.length
c = []
a.each_with_index { |e, i| c << e * b[i] }
c.inject { |e, t| t += e }
end

Ugly. or use recursion:

def dot_prod(a, b)
return 0 if a.length != b.length
a.empty? ? 0 : a.first * b.first + dot_prod(a[1…-1], b[1…-1])
end

Better, but a map/inject solution would be desired. Does anyone else
know of a way to iterate through two arrays without the use of an
index? Or do people just roll their own multiple array map function?

Wil wrote:

Hi all,

Better, but a map/inject solution would be desired. Does anyone else
know of a way to iterate through two arrays without the use of an
index?

def dot_prod(a, b)
acc = 0
a.zip(b) do |a1, b1|
acc += a1*b1
end
acc
end

best,
Dan

Wil wrote:

Hi all,

I’ve been using Ruby for a while now, and have enjoyed it immensely.
However, I haven’t yet found how to map more than one array at once.
The common example is calculating the dot product.

Here’s an example using inject, no need for a temp variable:

irb(main):001:0> def dot_prod a,b
irb(main):002:1> a.zip(b).inject(0) do |dp,(a,b)|
irb(main):003:2* dp + a*b
irb(main):004:2> end
irb(main):005:1> end
=> nil
irb(main):006:0> dot_prod([1,2,3],[4,5,6])
=> 32

Alle giovedì 19 luglio 2007, Wil ha scritto:

def dot_prod(a,b)
a.empty? ? 0 : a.first * b.first + dot_prod(a[1…-1], b[1…-1])
end

Better, but a map/inject solution would be desired. Does anyone else
know of a way to iterate through two arrays without the use of an
index? Or do people just roll their own multiple array map function?

require ‘generator’

a = [1, 2, 3]
b = [‘a’, ‘b’, ‘c’]
c = [:x, :y, :z]

s = SyncEnumerator.new( a, b, c)
s.each{|i| puts “first: #{i[0]}, second: #{i[1]}, third: #{i[3]}”}
=>
first: 1, second: a, third: x
first: 2, second: b, third: y
first: 3, second: c, third: z

To define the dot product:

def dot_prod(a,b)
raise ArgumentError unless a.size == b.size
SyncEnumerator.new(a,b).inject(0){|res, i| res + i[0] * i[1]}
end

Stefano