# 直接计算概率: 10个小球，随机分到12个盒子。每个小球都有12个可能，事件总数是12^10。事件“恰好10个盒子为空”意味着：将10个球随机分到12个盒子的两个盒子中（这两个盒子不能有任何一个为空）。C(12,2)决定是哪两个盒子不为空；将10个球分到两个盒子中，有2^10种可能（不能确保两个盒子均为非空的），需要再减去两个盒子中有一个盒子为空的情况（共2种可能，设盒子为A、B,即“A为空并且B非空”和“A非空并且B为空”这两种可能）。所以最后结果为C(12,2)*(2^10-2)/(12^10)。
import random
random.seed(4)
n=100000
cnt=0
for i in range(n): 
    arr=[0]*12
    for j in range(10):
        rnd=random.randint(0,11)
        arr[rnd]+=1
    cnt0=0 
    for j in range(12):
        if arr[j]==0:
            cnt0+=1
    if cnt0==10:
        cnt+=1
print(cnt/n)

#比如4,4,5,6,7里面找4，我的代码返回了1，本该返回0。
def getPos(arr,val):
    n=len(arr)
    l=0
    r=n-1
    while l<r:
        mid=(l+r)//2
        if arr[mid]>=val:
            r=mid
        else:
            l=mid+1
    if arr[l]!=val:
        return -1
    return l

name = "Reiko"
f"She said her name is {name}." # => "She said her name is Reiko"

f"{name} is {len(name)} characters long." # => "Reiko is 5 characters long."

filled_dict.setdefault("five", 5)  # filled_dict["five"] is set to 5
filled_dict.setdefault("five", 6)  # filled_dict["five"] is still 5

{'a': 1, **{'b': 2}}  # => {'a': 1, 'b': 2}
{'a': 1, **{'a': 2}}  # => {'a': 2}

empty_set = set()

some_set = {1, 1, 2, 2, 3, 4}  # some_set is now {1, 2, 3, 4}

#set当中的元素也必须是不可变对象，因此list不能传入set。

invalid_set = {[1], 1}  # => Raises a TypeError: unhashable type: 'list'
valid_set = {(1,), 1}

#可以调用add方法为set插入元素：

filled_set = some_set
filled_set.add(5)  # filled_set is now {1, 2, 3, 4, 5}
# Sets do not have duplicate elements
filled_set.add(5)  # it remains as before {1, 2, 3, 4, 5}

#set还可以被认为是集合，所以它还支持一些集合交叉并补的操作。
# Do set intersection with &
# 计算交集
other_set = {3, 4, 5, 6}
filled_set & other_set  # => {3, 4, 5}

# Do set union with |
# 计算并集
filled_set | other_set  # => {1, 2, 3, 4, 5, 6}

# Do set difference with -
# 计算差集
{1, 2, 3, 4} - {2, 3, 5}  # => {1, 4}

# Do set symmetric difference with ^
# 这个有点特殊，计算对称集，也就是去掉重复元素剩下的内容
{1, 2, 3, 4} ^ {2, 3, 5}  # => {1, 4, 5}

# set还支持超集和子集的判断，我们可以用大于等于和小于等于号判断一个set是不是另一个的超集或子集：
# Check if set on the left is a superset of set on the right
{1, 2} >= {1, 2, 3} # => False

# Check if set on the left is a subset of set on the right
{1, 2} <= {1, 2, 3} # => True

# 和dict一样，我们可以使用in判断元素在不在set当中。用copy可以拷贝一个set。
# Check for existence in a set with in
2 in filled_set   # => True
10 in filled_set  # => False

# Make a one layer deep copy
filled_set = some_set.copy()  # filled_set is {1, 2, 3, 4, 5}
filled_set is some_set        # => False

#To loop over a list, and retrieve both the index and the value of each item in the list
#prints:
#    0 dog
#    1 cat
#    2 mouse
animals = ["dog", "cat", "mouse"]
for i, value in enumerate(animals):
    print(i, value)

# Python offers a fundamental abstraction called the Iterable.
# An iterable is an object that can be treated as a sequence.
# The object returned by the range function, is an iterable.

filled_dict = {"one": 1, "two": 2, "three": 3}
our_iterable = filled_dict.keys()
print(our_iterable)  # => dict_keys(['one', 'two', 'three']). This is an object that implements our Iterable interface.

# We can loop over it.
for i in our_iterable:
    print(i)  # Prints one, two, three

#我们不能使用下标来访问可迭代对象，但我们可以用iter将它转化成迭代器，使用next关键字来获取下一个元素。也可以将它转化成list类型，变成一个list。
# However we cannot address elements by index.
our_iterable[1]  # Raises a TypeError

# An iterable is an object that knows how to create an iterator.
our_iterator = iter(our_iterable)

# Our iterator is an object that can remember the state as we traverse through it.
# We get the next object with "next()".
next(our_iterator)  # => "one"

# It maintains state as we iterate.
next(our_iterator)  # => "two"
next(our_iterator)  # => "three"

# After the iterator has returned all of its data, it raises a StopIteration exception
next(our_iterator)  # Raises StopIteration

# We can also loop over it, in fact, "for" does this implicitly!
our_iterator = iter(our_iterable)
for i in our_iterator:
    print(i)  # Prints one, two, three

# You can grab all the elements of an iterable or iterator by calling list() on it.
list(our_iterable)  # => Returns ["one", "two", "three"]
list(our_iterator)  # => Returns [] because state is saved