input: Write a function to find the index of a given value in an array. If the value is found, return the index; otherwise, 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], 15)==3
  assert find_index([2, 5, 9, 15], 7)==-1
  assert find_index([], 5)==-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