为什么我们要移动"if“语句？

Question

我不明白为什么要搬家

if cipher_direction == "decode":
      shift_amount *= -1

在end_text下工作，但它不能在下面所示的位置工作？

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完整的代码：

alphabet = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm',
            'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z',
            'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm',
            'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']

direction = input("Type 'encode' to encrypt, type 'decode' to decrypt:\n")
text = input("Type your message:\n").lower()
shift = int(input("Type the shift number:\n"))

#TODO-1: Combine the encrypt() and decrypt() functions into a single function
# called caesar().

def caesar(start_text, shift_amount, cipher_direction):
  end_text = ""
  if cipher_direction == "decode":
      shift_amount *= -1
  for letter in start_text:
    position = alphabet.index(letter)
    new_position = position + shift_amount
    end_text += alphabet[new_position]
  print(f"Here's the {direction}d result: {end_text}")


#TODO-2: Call the caesar() function, passing over the 'text', 'shift' and
# 'direction' values.
caesar(start_text=text, shift_amount=shift, cipher_direction=direction)

Answer 1

因为变量赋值(*=)经常在循环中被覆盖。要在循环中使用它，需要分配一次。

例如：

def caesar(start_text, shift_amount, cipher_direction):
  end_text = ""
  #if cipher_direction == "decode":
  #  shift_amount *= -1
  for letter in start_text:
    if cipher_direction == "decode" and shift_amount > 0:
      shift_amount *= -1
    position = alphabet.index(letter)
    new_position = position + shift_amount
    end_text += alphabet[new_position]
    print(f"Here's the {direction}d result: {end_text}")

注意:程序在带有空格的单词中抛出一个错误。