How can I multiply two arrays in Ruby?

Question

I have two arrays, for example

a = [3, 2, 1]
b = [1, 2, 3]

I need to multiply them and create a third array c that will be like this

c = [3 * 1, 2 * 2, 1 * 3]

Which method is the best by speed? I need to do this for huge arrays, and time is important.

I've reopened the question, because the OP is not looking for the sum. — Stefan, Dec 22 '15 at 11:35
Welcome to StackOverflow. For future reference, have a look at [How to select correct answer](http://meta.stackexchange.com/questions/125486/how-to-select-the-correct-answer-when-their-content-is-equal?lq=1) — Wand Maker, Dec 22 '15 at 14:36

score 20 · Accepted Answer · edited Apr 28 '18 at 19:16

20

a.zip(b).map{|x, y| x * y}

This works because zip combines the two arrays into a single array of two element arrays. i.e.:

a = [3, 2, 1]
b = [1, 2, 3]
a.zip(b)
#=> [[3, 1], [2, 2], [1, 3]]

Then you use map to multiply the elements together. This is done by iterating through each two element array and multiplying one element by the other, with map returning the resulting array.

a.zip(b).map{|x, y| x * y}
#=> [3, 4, 3]

edited Apr 28 '18 at 19:16

Neddy

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answered Dec 22 '15 at 11:26

sawa

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If your answer included some informative text you wouldn't have to pad it with a bunch of periods. An answer with only code may be the right answer, but it's never a great answer. – Jordan Running Dec 22 '15 at 16:44

score 4 · Answer 2 · edited Dec 22 '15 at 14:40

4

Try this:

[[3,2,1],[1,2,3]].transpose.map {|a| a.inject(:*)}

edited Dec 22 '15 at 14:40

Wand Maker

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answered Dec 22 '15 at 13:14

Akanksha

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Welcome to StackOverflow. For future reference, have a look at [How do I format my code blocks](http://meta.stackexchange.com/questions/22186/how-do-i-format-my-code-blocks) – Wand Maker Dec 22 '15 at 14:41

score 2 · Answer 3 · edited Dec 22 '15 at 11:40

2

You can try this:

a.map.with_index{ |x, i| a[i]*b[i]}

edited Dec 22 '15 at 11:40

Sergio Tulentsev

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answered Dec 22 '15 at 11:38

Radix

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Is there a way to avoid `x` as it is not used?. For future reference, have a look at [How do I format my code blocks](http://meta.stackexchange.com/questions/22186/how-do-i-format-my-code-blocks) – Wand Maker Dec 22 '15 at 14:43

Wand Maker · Answer 4 · 2015-12-22T14:39:32.650

Since you want to multiply two arrays, we have to assume they are of same size.

Hence, below is a simple way to multiply them - it has O(n) time complexity. Other answers are also equally good, you can pick any one

a = [3, 2, 1]
b = [1, 2, 3]

a.size.times.collect { |i| a[i] * b[i] }
#=> [3, 4, 3]

If time is really of the essence, then, you may want to use multiple threads. A sample program demonstrating the concept is shown below. You can build upon it based on your specific needs.

a = [3, 2, 1]
b = [1, 2, 3]

num_cores = 2 # This decides number of threads, ideally equal to number of cores
ary_size = a.size

# We need to collect result of each thread as separate sub-array.
# We will use result array-of-array so that order of result is preserved
results = Array.new(num_cores, [])

# Divide the array indexes into equal parts    
index_splits = (0...ary_size).each_slice(num_cores)

threads = []

# Process each sub-array in a thread of its own
index_splits.each_with_index do |ary, i| 
    threads << Thread.new do
      results[i] = ary.collect {|j| a[j] * b[j] } 
    end
end
threads.each {|t| t.join} 

p results.flatten
#=> [3,4,3]

Thanks to all, i will try to use multiple threads, maybe it really will work faster — Maxim 666, Dec 22 '15 at 13:50
There may be a bug in my sample code on threads, I am working on it. — Wand Maker, Dec 22 '15 at 13:54

How can I multiply two arrays in Ruby?

4 Answers4