I want to move the element at index 2 to the start of the array [1, 2, 3, 4], the resulting array should look like [3, 1, 2, 4].
My solution was to do the following
[3] + ([1, 2, 3, 4] - [3])
Is there a better way to do this?
A method that takes the first n elements from an array and rotates them by one, then adds back the remaining elements.
def rotate_first_n_right(arr, n)
arr[0...n].rotate(-1) + arr[n..-1]
end
rotate_first_n_right([1,2,3,4], 3)
# => [3, 1, 2, 4]
This does fail if we try to use it on an array that is too short, as the arr[n..-1] slice will yield nil which will cause an error when we try to add it to the first array.
We can fix this by expanding both slices into a list.
def rotate_first_n_right(arr, n)
[*arr[0...n].rotate(-1), *arr[n..-1]]
end
To see why this works, a very simple example:
[*[1, 2, 3], *nil]
# => [1, 2, 3]
A problem with your example is what happens if 3 occurs in the array more than once. E.g.
[1,2,3,3,3,4] - [3]
# => [1, 2, 4]