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([1, 3, 5, 7, 9], 7)==3
  assert find_index([4, 6, 8, 10], 5)==-1
  assert find_index([], 3)==-1
output: def find_index(arr, x):
      for i in range(len(arr)):
          if arr[i] == x:
              return i
      return -1