input: Write a 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 in the array, the function should 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], 10)==-1
  assert find_index([], 5)==-1
  assert find_index([1, 2, 3, 4, 5], 5)==4
output: def find_index(arr, x):
      for i in range(len(arr)):
          if arr[i] == x:
              return i
      return -1