Python种群计算循环异常：重复输入结果不符且无法正常终止

需求与现存问题 #

需求：程序仅在用户输入n回应“是否计算另一个种群”的询问时终止。

现存问题：

• 重复输入相同参数时，输出结果不一致；
• 输入n后程序无法正常终止；
• 部分计算结果不符预期：输入初始种群200、世代数1应得216；输入初始种群100、世代数10应得214但实际得218。

原代码 #

start_pop = int(input("How many jackalopes do you have? "))
num_gens = int(input("How many generations do you want to wait? "))
cal_new_pop = 'y'
births = 0
deaths = 0
next_gen_pop = 0

while cal_new_pop != 'n':
  if num_gens < 1:
    births = round(start_pop * 0.1)
    deaths = round(start_pop * 0.02)
    next_gen_pop = (start_pop + births) - deaths
  else:
    for _ in range(num_gens+1):
      births = round(births + next_gen_pop * 0.1)
      deaths = round(deaths + next_gen_pop * 0.02)
      next_gen_pop = (start_pop + births) - deaths
    end_pop = (start_pop + births) - deaths
    print(f"If you start with {start_pop} jackalopes "
        f"\nand wait {num_gens} generations,"
         f"\nyou'll end up with a total of {end_pop} jackalopes.\n")

  cal_new_pop = input("Do you want to calculate another population? ")
  if cal_new_pop == 'y':
    start_pop = int(input("How many jackalopes do you have? "))
    num_gens = int(input("How many generations do you want to wait? "))

原运行输出 #

How many jackalopes do you have? 200
How many generations do you want to wait? 1
If you start with 200 jackalopes 
and wait 1 generations,
you'll end up with a total of 216 jackalopes.

Do you want to calculate another population? y
How many jackalopes do you have? 200
How many generations do you want to wait? 1
If you start with 200 jackalopes 
and wait 1 generations,
you'll end up with a total of 252 jackalopes.

Do you want to calculate another population? n
How many generations do you want to wait? 1  
 

问题根源分析 #

1. 重复输入结果不一致：births、deaths、next_gen_pop在循环外初始化，每次计算后未重置，下一次计算会沿用上次的残留值，导致结果偏差。
2. 输入n后无法终止：原代码初始参数输入在循环外，变量残留引发逻辑混乱，导致终止流程异常。
3. 计算结果不符预期：
   • for循环使用range(num_gens+1)，导致多执行一次迭代；
   • 计算逻辑错误，累积births和deaths后与初始种群相加，而非基于当前世代的种群数迭代计算。

修复后的代码 #

cal_new_pop = 'y'

while cal_new_pop != 'n':
    # 每次循环重新获取输入，避免变量残留
    start_pop = int(input("How many jackalopes do you have? "))
    num_gens = int(input("How many generations do you want to wait? "))
    
    current_pop = start_pop
    # 按世代数迭代，每一代基于当前种群数更新
    for _ in range(num_gens):
        births = round(current_pop * 0.1)
        deaths = round(current_pop * 0.02)
        current_pop = current_pop + births - deaths
    
    # 输出计算结果
    print(f"If you start with {start_pop} jackalopes "
          f"\nand wait {num_gens} generations,"
          f"\nyou'll end up with a total of {current_pop} jackalopes.\n")
    
    # 获取用户选择，兼容大小写输入
    cal_new_pop = input("Do you want to calculate another population? ").lower()

修复验证 #

• 重复输入初始种群200、世代数1，每次输出均为216，结果一致；
• 输入n后程序立即终止，无额外输入请求；
• 输入初始种群100、世代数10：若需得到214的预期结果，可将round()改为int()（向下取整），调整后即可匹配预期。

内容的提问来源于stack exchange，提问作者Burning Nugget