上Python课程，学习二分查找，疑惑为何我的语法不正确

Question

我们的任务是写一段代码，来猜一个从0到100之间的秘密数字。
这是我写的代码：

low = 0
mid = 50
high = 100
secretnum = "Is your secret number " + str(mid) + "?"
print"Please think of a number between 0 and 100!"
print secretnum
herp = raw_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. ")
while herp != 'c':
    if herp == 'h':
        high = mid
        mid = int((mid + low)/2)
    elif herp == 'l':
        low = mid
        mid = int((mid + high)/2)
    else:
        print"Sorry, I did not understand your input."
    print secretnum
    herp = raw_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 herp == 'c':
    print "Game over. Your secret number was: " + str(mid)

这是运行后的结果：

Is your secret number 50?
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. h
Is your secret number 50?
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. l
Is your secret number 50?
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. c
Game over. Your secret number was: 37

Answer 1

在最后一行，你只需要写：

print "游戏结束。你的秘密数字是: " + str(mid)

这是因为Python想确保你真的知道自己在做什么——也就是说，它要你把两个字符串加在一起，而不是把一个整数和一个字符串加在一起，这样是禁止的。str()这个函数的作用就是把你给它的任何东西都转换成字符串。关于你提到的语义问题，我觉得这个版本的代码表现得很正常：

low = 0  
mid = 50  
high = 100  
print "Please think of a number between 0 and 100!"  
herp = 50  
while herp != 'c':  
    print "Is your secret number " + str(mid) + "?"
    herp = raw_input("Enter 'h' to indicate the guess is too high. Enter 'l' to indicate the guess is to low. Enter 'c' to indicate the guess is correct")   
    if herp == 'h':  
        high = mid  
        mid = int((mid + low)/2)  
    elif herp == 'l':  
        low = mid  
        mid = int((mid + high)/2)  
    else:  
        print"Sorry, I did not understand your input."  
if herp == 'c':  
    print "Game over. Your secret number was: " + str(mid) 

Answer 2

对于这个错误，把它改成：

print "Game over. Your secret number was:",mid

关于一直输出50的问题，把while循环里的 print secretnum 改成：

print "Is your secret number " + str(mid) + "?"

当你在一开始设置 secretnum="Is your secret number " + str(mid) + "?" 时，它会生成一个字符串，这个字符串和mid是完全分开的。所以如果你改变了mid，字符串里的内容不会跟着改变。

在Python中，字符串是不可变的，这意味着一旦创建了字符串，它就不能被修改。你不能直接改变字符串的内容，必须重新创建一个新的字符串。str(mid) 的作用是把 mid 转换成字符串。在这个例子中，会生成字符串 "50" 并放入到字符串中，这个字符串是不会被修改的。因此，当你要显示一个字符串时，需要确保它显示的是最新的值，这就需要再次调用 str(mid)。

Answer 3

正如Raufio提到的，Python中的字符串是不可改变的。这意味着一旦你创建了一个字符串，就不能直接修改它。如果你想解决50这个数字重复出现的问题，你需要在打印问题的时候再次调用str(mid)。比如：

low = 0
mid = 50
high = 100
secretnum = "Is your secret number: " 
print"Please think of a number between 0 and 100!"
print secretnum + str(mid) + "?"
herp = raw_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. ")
while herp != 'c':
    if herp == 'h':
        high = mid
        mid = int((mid + low)/2)
    elif herp == 'l':
        low = mid
        mid = int((mid + high)/2)
    else:
        print"Sorry, I did not understand your input."
    print secretnum + str(mid) + "?"
    herp = raw_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 herp == 'c':
    print "Game over. Your secret number was:", mid