Python编程绝对入门第5章挑战4

Question

挑战是改进之前的挑战“谁是你的爸爸”(参见我的成功代码：http://pastebin.com/AU2aRWHk)，添加一个选项，让用户输入一个名字并返回一个祖父。程序应该仍然只使用一个儿子-父亲对的字典。

我不能让它工作。到目前为止，我的所有代码都可以在以下位置看到：http://pastebin.com/33KrEMhT

显然，我已经让这件事变得比需要的更加困难，现在我被困在了一个复杂的世界里。下面是我编写的代码：

# create dictionary
paternal_pairs ={"a": "b",
                 "b": "c",
                 "c": "d"}

# initialize variables
choice = None

# program's user interface
while choice != 0:
print(
"""
   Who's Yo Daddy:

   2 - Look Up Grandfather of a Son
   """
)

choice = input("What would you like to do?: ")
print() 

    # look up grandfather of a son
    if choice == "2":
        son = input("What is the son's name?: ")
        # verify input exists in dictionary
        if son in paternal_pairs.values():
            for dad, boy in paternal_pairs.items():
                if dad == son:
                    temp_son = dad
                    for ol_man, kid in paternal_pairs.items():
                        if temp_son == kid:
                            print("\nThe grandfather of", son, "is", ol_man)
                        else:
                            print("\nNo grandfather listed for", son)
                else:
                    print("\nNo grandfather listed for", son)
        # if input does not exist in dictionary:
        else:
            print("Sorry, that son is not listed. Try adding a father-son pair.")

选择"2“后，我的输出：

What is the son's name?: d

No grandfather listed for d

No grandfather listed for d

No grandfather listed for d

No grandfather listed for d

No grandfather listed for d

No grandfather listed for d

No grandfather listed for d

No grandfather listed for d

显然，它暂时被困在一个小循环中，它不起作用。所有其他代码都按预期工作。帮助!

Answer 1

循环遍历字典中的每个条目，并匹配该值，如果不匹配，则对每个键-值对打印不匹配的值。

它相当于以下简化的循环：

>>> for i in range(3):
...     if i == 5:
...         print(i)
...     else:
...         print('Not 5')
... 
Not 5
Not 5
Not 5

请改用else: clause of the for loop，只有在完成了所有值的遍历后才会调用它；如果找到匹配的值，请使用break：

for ol_man, kid in paternal_pairs.items():
    if temp_son == kid:
        print("\nThe grandfather of", son, "is", ol_man)
        break
else:
    print("\nNo grandfather listed for", son)

下面的小演示演示了在使用for循环时else:子句是如何工作的：

>>> for i in range(3):
...     if i == 1:
...         print(i)
...         break
... else:
...     print('Through')
... 
1
>>> for i in range(3):
...     if i == 5:
...         print(i)
...         break
... else:
...     print('Through')
... 
Through

在第一个示例中，我们使用break打破了循环，但在第二个示例中，我们从未到达break语句(i从未等于5)，因此到达了else:子句并输出了Through。