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Array partition sums

I have an array of xs

numbers and I need to calculate the sums of the elements in three consecutive sections in it. sections xs[0]

, xs[1, N]

and xs[N + 1, N]

for some integer N

. For example, for N = 2

:

[1, 2, 3, 4, 5] #=> 1, 5, 9

my implementation is below. I couldn't make it more efficient in terms of memory / speed, but it still worries me, I guess I have nothing to do now, but maybe there is a more elegant and concise way to do it? I don't need to create an array, it can be any other structure.

xs.each_with_index.reduce([0, 0, 0]) do |m, x|
  v, i = x
  j = (i > N ? 2
       : i > 0 ? 1
       : 0)
  m[j] += v.to_i
  m
end
+3


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2 answers 


[xs[0..0],xs[1..N],xs[N+1..-1]].map{|x|x.reduce{|a,b|a+b}}

a.rb

:

N = (rand * 1E6).to_i
xs = 1.upto(1E6).to_a

xs.each_with_index.reduce([0, 0, 0]) do |m, x|
  v, i = x
  j = (i > N ? 2
       : i > 0 ? 1
       : 0)
  m[j] += v.to_i
  m
end

b.rb

:

N = (rand * 1E6).to_i
xs = 1.upto(1E6).to_a

[xs[0..0],xs[1..N],xs[N+1..-1]].map{|x|x.reduce{|a,b|a+b}}


Time:

$ time ruby a.rb 
ruby a.rb  0.85s user 0.01s system 99% cpu 0.860 total

$ time ruby b.rb 
ruby b.rb  0.29s user 0.01s system 99% cpu 0.296 total

Ruby version:

$ ruby -v
ruby 1.9.3p194 (2012-04-20 revision 35410) [x86_64-darwin12.0.0]
+2


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Small option:



[xs[0..0],xs[1..N],xs[N+1..-1]].map{|x|x.inject(:+)}
+2


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