input: Write a function to return the index of the first occurrence of a given value in an array, and return -1 if the value is not found in the array. The function should take two parameters: an array and a value.
  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, value):
      for i in range(len(arr)):
          if arr[i] == value:
              return i
      return -1