input: Write a function to find the index of a given value in an array. If the value is not present in the array, 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], 7)==-1
  assert find_index([], 5)==-1
  assert find_index([2, 5, 9, 15], 15)==3
output: def find_index(arr, x):
      for i in range(len(arr)):
          if arr[i] == x:
              return i
      return -1