Let's say I would like to create a new array of arrays
arr = Array.new(5,[])
=> [[], [], [], [], []]
How can I specifically push an element to one of those arrays?
When I try to push to only one of the arrays, the value is always added to all of them:
arr[3].push("foo")
=> ["foo"]
arr
=> [["foo"], ["foo"], ["foo"], ["foo"], ["foo"]]
The problem isn't the way you're pushing, it's the way you're creating the array.
The array is initialised with 5 references to the same array, so when you modify one of them, all of the other copies change too.
You need to create the array using a different method, so that you create five different arrays rather than five copies of the same array. This can be done like so:
arr = Array.new(5) { [] }
The block (the bit between {}) is executed to create all 5 array items, so you end up with 5 different arrays.
Your code then works as expected:
arr = Array.new(5) { [] }
=> [[], [], [], [], []]
arr[3].push("foo")
=> ["foo"]
arr
=> [[], [], [], ["foo"], []]
Array.new.