Pad an array to be a certain size

Question

There's probably a more efficient and more Ruby-ish way to do this:

# Pad array to size n by adding x's. Don't do anything if n <= a.length.
def padleft(a, n, x)
  return a if n <= a.length
  return padleft([x] + a, n, x)
end

What would you suggest?

Answer 1

Edited due to my misunderstanding of the question. Pervious version of my answer padded from the right side, but the question was asking to do it from the left side. I corrected it accordingly. This is due to naming convention. ljust, rjust are builtin methods for String, and I extended that convention to Array, but that corresponds to padright and padleft, respectively, in the terminology of the question.

Destructive methods

def padleft!(a, n, x)
  a.insert(0, *Array.new([0, n-a.length].max, x))
end
def padright!(a, n, x)
  a.fill(x, a.length...n)
end

It would be more natural to have it defined on Array class:

class Array
  def rjust!(n, x); insert(0, *Array.new([0, n-length].max, x)) end
  def ljust!(n, x); fill(x, length...n) end
end

Non-destructive methods

def padleft(a, n, x)
  Array.new([0, n-a.length].max, x)+a
end
def padright(a, n, x)
  a.dup.fill(x, a.length...n)
end

or

class Array
  def rjust(n, x); Array.new([0, n-length].max, x)+self end
  def ljust(n, x); dup.fill(x, length...n) end
end