CODEWITHSUNDEEP
rubyarrays
zsljulius
zsljulius

Reputation: 4103

Array multiplication puzzle in ruby

I have an array:

x = [1,2,3,4,5]

I want to get:

output = [1,2,6,24,120]

Hope you see the pattern and what I want to accomplish here. x[1] is transformed into x[1]*x[0]. x[2] => x[2]*x[1]*x[0], etc.

What's the most efficient way to do this in Ruby?

Upvotes: 1

Views: 384

Answers (8)

poorva
poorva

Reputation: 1756

x.inject([]) do |result, arr_element| last_num = result.last result << (last_num.nil? ? arr_element : last_num * arr_element) result end

use inject and pass array as accumulator

Upvotes: 0

tokland
tokland

Reputation: 67850

This is a pretty general abstraction, usually called scanl in functional languages. A possible Ruby implementation:

module Enumerable
  def scanl(&block)
    self.inject([]) do |acc, x| 
      acc.concat([acc.empty? ? x : yield(acc.last, x)])
    end
  end
end

>> [1, 2, 3, 4, 5].scanl(&:*)
=> [1, 2, 6, 24, 120]

Upvotes: 1

fl00r
fl00r

Reputation: 83680

just for fun

[1,2,3,4,5].map.with_index{|a,i| ar[0,i+1].inject(:*)}

Upvotes: 0

lucapette
lucapette

Reputation: 20724

1.9.2 (main):0 > a
=> [1, 2, 3, 4, 5]
1.9.2 (main):0 > a.inject([]) {|arr, e| arr << a[0..arr.size].inject(:*) }

It's a nice way of doing it.

Upvotes: 1

Howard
Howard

Reputation: 39187

You can use e.g. the following code

a=1;output = x.map{|i|a*=i}

Upvotes: 6

jimbo
jimbo

Reputation: 11042

Here's how I'd do it

prev = 1
puts [1,2,3,4,5].map { |n| prev *= n }

Upvotes: 1

Jordan
Jordan

Reputation: 1238

arr = [1,2,3]
arr.each_with_index.map do |x,i|  
  arr[0..i].inject(1) { |total,y| total *= y } }
end

Upvotes: 1

Tejs
Tejs

Reputation: 41236

You'd probably want to make a recursive solution.

F(x) =  if x < 1 return 0
        else     return A(x) * F(x - 1)

In the simplest mathematical form, where F is your result array, and A is your source array (with indices as x).

Upvotes: 0

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