input: Write a python function to find the minimum value in an array.
  assert find_min([5, 7, -1, 4]) == -1
  assert find_min([10, 20, 30, 40, 50]) == 10
  assert find_min([-10, -20, -30, -40, -50]) == -50
output: def find_min(arr):
      min_val = arr[0]
      for num in arr:
          if num < min_val:
              min_val = num
      return min_val