Iterate Array and add each consecutive array value

Question

I have an amount. I want to iterate the array starting from the first value and if I add the next value and it equals amount then those two values in the array should be re

Answer 1

Try this:

array = [1,2,3,4,7]
amount = 17

master_array = (1..array.size).map { |i| array.each_slice(i).to_a }

master_array.each.with_index do |e,i| 
  p e[0] if master_array[i][0].inject(:+) == amount
end

Answer 2

arr = [1,2,3,4,7]
tot = 6

Case 1: The elements to be summed are the first n elements of the array

Not very efficient, but reads well:

idx = arr.size.times.find { |i| arr[0..i].reduce(:+) == tot }
idx ? arr[0..idx] : nil
  #=> [1, 2, 3]

More efficient:

t = 0
idx = arr.size.times.find { |i| t += arr[i]; t == tot }
idx ? arr[0..idx] : nil
  #=> [1, 2, 3]

Use recursion

def partial_sum(arr, remaining)
  return nil if arr.empty?
  first, *rest = arr
  if remaining == first
    [first]
  else
    ret = partial_sum(rest, remaining-first)
    ret ? [first, *ret] : nil
  end
end

partial_sum(arr, 6)
  #=> [1, 2, 3]
partial_sum(arr, 7)
  #=> nil

Case 2: the elements to be summed can be any elements of the array

def sum_subarrays(arr, tot)
  (0...arr.size).each do |n| 
    b = arr.combination(n).find { |a| a.reduce(:+) == tot }
    b ? (return b) : nil
  end
  nil
end

(1..17).each do |tot|
  v = sum_subarrays(arr, tot)
  puts "sum_subarrays(arr, #{tot}) = #{v ? v : 'nil'}"
end
sum_subarrays(arr, 1)  = [1]
sum_subarrays(arr, 2)  = [2]
sum_subarrays(arr, 3)  = [3]
sum_subarrays(arr, 4)  = [4]
sum_subarrays(arr, 5)  = [1, 4]
sum_subarrays(arr, 6)  = [2, 4]
sum_subarrays(arr, 7)  = [7]
sum_subarrays(arr, 8)  = [1, 7]
sum_subarrays(arr, 9)  = [2, 7]
sum_subarrays(arr, 10) = [3, 7]
sum_subarrays(arr, 11) = [4, 7]
sum_subarrays(arr, 12) = [1, 4, 7]
sum_subarrays(arr, 13) = [2, 4, 7]
sum_subarrays(arr, 14) = [3, 4, 7]
sum_subarrays(arr, 15) = [1, 3, 4, 7]
sum_subarrays(arr, 16) = [2, 3, 4, 7]
sum_subarrays(arr, 17) = nil