缩小数组以设置项目数

Question

我有一个包含 300 个项目的 Ruby 数组。我想将该数组减少到一定数量的项目，从数组中均匀挑选。

数组中的项数不会每次都相同，所需的项数也不会相同。

像这样：

arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
num_of_items = 4
final_arr = [0, 5, 10, 15]

Answer 1

您可以使用 Enumerable#each_slice

arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
#=> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
num_of_items = 4
#=> 4
arr.each_slice(arr.size/num_of_items + 1).map(&:first)
#=> [0, 5, 10, 15] 
arr = (0..16).to_a
#=> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]
num_of_items = 5
#=> 5
arr.each_slice(arr.size/num_of_items + 1).map(&:first)
#=> [0, 4, 8, 12, 16]

或

数字#步长

arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
#=> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
num_of_items = 4
#=> 4
arr.first.step(arr.size, arr.size/num_of_items + 1).map { |i| arr[i] }
#=> [0, 5, 10, 15]
arr = (0..16).to_a
#=> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]
num_of_items = 5
#=> 5
arr.first.step(arr.size, arr.size/num_of_items + 1).map { |i| arr[i] }
#=> [0, 4, 8, 12, 16]

Answer 2

如果您从此数组中随机选择给定数量的项目，请使用 sample -

$ arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
$ arr.sample(5)
 => [1, 3, 5, 4, 12]
$ arr.sample(5)
 => [15, 6, 13, 5, 11]

Answer 3

如果你的算法目标与上面所示的完全一样，你可以使用 in_groups_of ，然后获取每个子数组的第一个元素：

1.9.3p484 :014 > num_of_items = 5
 => 5 
1.9.3p484 :011 > arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
 => [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15] 
1.9.3p484 :012 > arr.in_groups_of(num_of_items)
 => [[0, 1, 2, 3, 4], [5, 6, 7, 8, 9], [10, 11, 12, 13, 14], [15, nil, nil, nil, nil]] 
1.9.3p484 :013 > arr.in_groups_of(num_of_items).map(&:first)
 => [0, 5, 10, 15]