input: Write a function to find the index of a specified value in an array. If the value is present, the function should return its index; otherwise, it should return -1. If the array is empty, return -1.
  assert find_index([2, 5, 9, 15], 5)==1
  assert find_index([2, 5, 9, 15], 9)==2
  assert find_index([1, 3, 5, 7], 2)==-1
  assert find_index([], 4)==-1
output: def find_index(arr, value):
      if not arr:
          return -1
      for i, v in enumerate(arr):
          if v == value:
              return i
      return -1