input: Write a function to find the index of a specified value in an array. If the value is present, return its index; otherwise, return -1.
  def find_index(arr, x):
      for i in range(len(arr)):
          if arr[i] == x:
              return i
      return -1
  
  assert find_index([2, 5, 9, 15], 5) == 1
  assert find_index([2, 5, 9, 15], 9) == 2
  assert find_index([2, 5, 9, 15], 15) == 3
  assert find_index([2, 5, 9, 15], 2) == 0
  assert find_index([], 5) == -1
output: def find_index(arr, x):
      for i in range(len(arr)):
          if arr[i] == x:
              return i
      return -1