EdX平台Python猜数字习题判错求助：二分查找代码排查

问题背景 #

你需要实现一个电脑通过二分查找猜数字的程序：用户心里想一个0（包含）到100（不包含）之间的整数，电脑给出猜测，用户输入h（猜高了）、l（猜低了）或c（猜对了）反馈，直到电脑猜中。你测试了83、8、42等数字都正常，但EdX平台判定代码错误，需要排查缺陷。

原始代码 #

high = 100
low = 0
correct = False
response = ""
user_number = input("Please think of a number between 0 and 100!")
while (response != "c"):
    guess = int((high + low)/2)
    print("Is your secret number", guess, "?")
    response = input("Enter 'h' to indicate the guess is too high. Enter 'l' to indicate the guess is too low. Enter 'c' to indicate I guessed correctly")
    if not (response == "h" or response == "c" or response == "l"):
        print("Sorry, I did not understand your input.")
    elif (response is "h"):
        high = guess
    elif (response is "l"):
        low = guess
print ("Game over. Your secret number was:", guess)

代码缺陷分析 #

我找到了几个可能导致EdX判错的关键点：

• 字符串比较误用is运算符：在Python中，判断字符串内容相等应该用==，is是用来判断两个对象是否为同一个内存实例的。虽然短字符串有时会因为Python的字符串驻留机制看起来能用is判断，但这不是可靠的写法——EdX的测试环境可能刚好触发了is判断不成立的场景，导致逻辑分支错误。你代码里的response is "h"和response is "l"必须改成response == "h"和response == "l"。

• 冗余的输入与变量：你定义了correct变量但全程没用到，属于无效代码；另外user_number = input(...)这一行完全多余——题目要求用户心里想数字，不需要输入，这行输入会干扰EdX的测试流程（比如测试脚本预期没有这个输入步骤，或者会错误处理这个输入内容）。

• 浮点数转整数的不严谨性：int((high + low)/2)在Python3中会先得到浮点数再转整数，虽然结果没问题，但更规范的做法是用整数除法(high + low) // 2，既避免不必要的浮点数转换，也更符合二分查找的整数操作逻辑。

修正后的代码 #

high = 100
low = 0
response = ""
print("Please think of a number between 0 and 100!")
while response != "c":
    guess = (high + low) // 2
    print(f"Is your secret number {guess}?")
    response = input("Enter 'h' to indicate the guess is too high. Enter 'l' to indicate the guess is too low. Enter 'c' to indicate I guessed correctly: ")
    if response not in ["h", "c", "l"]:
        print("Sorry, I did not understand your input.")
    elif response == "h":
        high = guess
    elif response == "l":
        low = guess
print(f"Game over. Your secret number was: {guess}")

修正说明 #

1. 替换is为==，保证字符串比较的可靠性；
2. 移除冗余的user_number输入和correct变量，让流程完全贴合题目要求；
3. 用整数除法//优化猜测值的计算；
4. 用f-string优化输出格式，提升代码可读性。

这些修改应该能通过EdX的所有测试用例，你可以运行这个版本试试。

内容的提问来源于stack exchange，提问作者Aries