input: Write a function that takes an array and a value, and returns the index at which the value is present in the array. If the value is not found, 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], 15) == 3
  assert find_index([2, 5, 9, 15], 3) == -1
  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