I need to dynamically retrieve a slice of size 3 from an array (as part of a more complex method):
a = %w(a b c d e)
i = 0 # i is an argument, I need [nil, "a", "b"]
a[(i-1)..(i+1)]
=> [] # no luck
a[(i-1), 3]
=> ["e"]
I know that when code gets messed up, it's not ruby's fault, is mine. What I'm missing? Is there a better way to achieve this?
I need to clarify. What I want is a slice of a given size, around a given index, and maping to nil if the slice goes beyond offset.
Here's a compact way you can do this. It relies on the fact that indexing beyond an array returns nil.
>> i = 0; 3.times.map{|j| a[j+i]}
=> ["a", "b", "c"]
>> i = 4; 3.times.map{|j| a[j+i]}
=> ["e", nil, nil]
Is each_slice the method you are looking for? Otherwise try to provide a better example for what you actually want to achieve.
each_slice accepts no starting point and retrieves a smaller slice if a.size % 3 != 0, i.e.: (a.each_slice(3).to_a => [["a", "b", "c"], ["d", "e"]]), not what I'm looking for.Enumerable#each_cons will do the trick:
a.each_cons(3){|slice| p slice }
output:
["a", "b", "c"]
["b", "c", "d"]
["c", "d", "e"]
This is how I made it (having to go around ruby indexing negative values):
a = %w(a b c d e)
def slice_around(ary, index)
slice_3 = ->(x){ x == 0 ? [ary.size+1,0,1] : ((x-1)..(x+1)).to_a }
idxs = slice_3.call(index).map { |i| ary[i] }
end
> slice_around a, 0
=> [nil, "a", "b"]
> slice_around a, 2
=> ["b", "c", "d"]
It wont work if a negative index is passed though.
i=0, what do you expect? ['a', 'b','c'] ? or ['a', nil, nil]? why your expect is ['e', nil, nil]?ì), I want to return[a[i-1],a[i],a[i+1]], with nil if index goes offset or negative, that's what I mean with 'around'. The problem is that ruby doesnt return nil on negative indexes.