input: def find_index(arr, x):
      """
      Generate a Python function which takes an array and a value and returns the 
      index at which this value is present in the array. If the value is not found, 
      it should return -1.
  
      >>> find_index([2, 5, 9, 15], 5)
      1
      >>> find_index([1, 2, 3, 4], 5)
      -1
      """
      for i in range(len(arr)):
          if arr[i] == x:
              return i
      return -1
output:     for i in range(len(arr)):
          if arr[i] == x:
              return i
      return -1