minby.rb

def min_by(ary, &fn)
  first, *rest = ary;
  min_elem, _ = rest.reduce([first, fn.call(first)]) do |(min_elem, min), new_elem|
    if (new_min = fn.call(new_elem)) < min
      [ new_elem, new_min ]
    else
      [ min_elem, min ]
    end
  end
  min_elem
end

def min_by2(ary, &fun)
  tups = ary.map {|x| [x, fun.call(x)] } # Make 2 tuples with the original value, and the mapped _by value
  min = tups.inject(tups.first) { |(prev_x, prev_min), (x, candidate)| # Destructure the tuples
    prev_min < candidate ?
      [prev_x, prev_min] : [x, candidate] } # Rebuild the two tuple with the original value & the calculated min
  min[0] # Get the original value back out of the 2 tuple
end

a = ["4", "1", "4", "3", "-1"]
puts min_by(a) { |x| x.to_i }
puts min_by2(a) { |x| x.to_i }