Python list comprehension => Ruby select / reject on index rather than element

Question

Given:

arr=[10, 20, 30, 40, 50, 60, 70, 80, 90, 100]
      0   1   2   3   4   5   6   7   8    9    # index of arr

In Python, one can select or reject elements of a list given the index of that element by combining enumerate and an if clause in a list comprehension:

Every item except the third:

>>> [e for i,e in enumerate(arr) if i%3]
[20, 30, 50, 60, 80, 90]

Every third item in a list:

>>> [e for i,e in enumerate(arr) if not i%3]
[10, 40, 70, 100]

Or, even easier, with a slice:

>>> arr[::3]
[10, 40, 70, 100]   

In Ruby, we have .select and .reject

> arr.each_with_index.reject { |e,i| i%3==0 }
=> [[20, 1], [30, 2], [50, 4], [60, 5], [80, 7], [90, 8]]
> arr.each_with_index.select { |e,i| i%3==0 }
=> [[10, 0], [40, 3], [70, 6], [100, 9]]

And then apply .collect to that:

> arr.each_with_index.select { |e,i| i%3==0 }.collect{|e,i| e}
=> [10, 40, 70, 100]

For the slice, the rough Ruby equivalent might be:

> (0..arr.length).step(3).each { |e| p arr[e] }
10
40
70
100
=> 0..10

But I can't figure out how toe collect those into a new array other than:

> new_arr=[]
=> []
> (0..arr.length).step(3).each { |e| new_arr.push(arr[e]) }
=> 0..10
> new_arr
=> [10, 40, 70, 100]

Questions:

1. Are these the best Ruby idioms for what I am trying to do (.select or .reject with .collect)?
2. Is there a way to do something along these lines with a slice:
   new_arr=(0..arr.length).step(3).each { |e| arr[e] }.some_method?

Answer 1

The method order is relevant:

arr.each_with_index.select { |e, i| i % 3 == 0 }
#=> [[10, 0], [40, 3], [70, 6], [100, 9]]

versus:

arr.select.each_with_index { |e, i| i % 3 == 0 }
#=> [10, 40, 70, 100]

Since select returns an enumerator, you could also use Enumerator#with_index:

arr.select.with_index { |e, i| i % 3 == 0 }
#=> [10, 40, 70, 100]

Regarding your slice equivalent, you can use map (or its alias collect) to collect the items in an array:

(0..arr.length).step(3).map { |e| arr[e] }
#=> [10, 40, 70, 100]

or values_at to fetch the items at the given indices:

arr.values_at(*(0..arr.length).step(3))
#=> [10, 40, 70, 100]

* turns the argument into an array (via to_a) and then into an argument list, i.e.:

arr.values_at(*(0..arr.length).step(3))
arr.values_at(*(0..arr.length).step(3).to_a)
arr.values_at(*[0, 3, 6, 9])
arr.values_at(0, 3, 6, 9)

Slightly shorter:

arr.values_at(*0.step(arr.size, 3))
#=> [10, 40, 70, 100]

Answer 2

You could use each_slice to iterate over the arr and to get the elements "in threes", using each_slice(3):

p arr.each_slice(3).to_a
# => [[10, 20, 30], [40, 50, 60], [70, 80, 90], [100]]

So, having that array you can see that each first element is an element in the output you want to get, so you can use map and to get the first element:

arr = [10, 20, 30, 40, 50, 60, 70, 80, 90, 100]
p arr.each_slice(3).map(&:first)
# => [10, 40, 70, 100]

You could do it as .map{|e| e.first}, or .map{|e| e[0]}, but for a shorter version you can use just .map(&:first).