python_16

Posted 2021-01-12

字符串可使用sorted()函数

>> s = "huhongiang"
>> sorted(s)
[‘a‘, ‘g‘, ‘g‘, ‘h‘, ‘h‘, ‘i‘, ‘n‘, ‘n‘, ‘o‘, ‘u‘]
>> l = sorted(s)
>> l
[‘a‘, ‘g‘, ‘g‘, ‘h‘, ‘h‘, ‘i‘, ‘n‘, ‘n‘, ‘o‘, ‘u‘]

习题1："abcdefgh"里面挑出3个字母进行组合，一共有多少种组合，要求三个字母中不能有任何重复的字母，三个字母的同时出现次数，在所有组合中只能出现一次，例如出现abc了，不能出现cab和bca等。
方式1：

s = "abcdefgh"
result = []
temp_list = []
for k in s:
    for v in s:
        for j in s:
            if k != v and k != j and v !=j:
                #sorted("abc") ==》["a","b","c"]
                #[sorted(s) for s in result]每一个元素是一个包含三个字母的列表
                #if sorted(list(k+v+j))  not in [sorted(list(s)) for s in result]
                if sorted(list(k+v+j)) not in [sorted(s) for s in result]:

                    result.append(k+v+j)
                    #print(result)
print("组合数:",len(result))

方式2：

s = "abcdefgh"
result = []
temp_list = []
for k in s:
    for v in s:
        for j in s:
            if k != v and k != j and v !=j:
                #join后直接比较字符串
                if "".join(sorted(list(k+v+j)))  not in ["".join(sorted(list(s))) for s in result]:
                    result.append(k+v+j)
                    #print(result)
print("组合数:",len(result))

习题2：复制一个列表，不能使用切片和复制的函数进行赋值，尽可能使用少的内置函数

方式1:

lst = ["a","b","c","d","e"]

length = 0
i = 0
for v in lst:
    length += 1

result = [None]*length

while i< length:
    result[i] = lst[i]
    i += 1

print("复制后的列表为:",result)

方式2：

a = ["a","b","c","d","e"]

arr_length=0

for i in a:
    arr_length+=1

def iter_arr(n):
    arr = []
    i = 0
    while i<=n-1:
        arr+=[i]
        i+=1
    return arr

result = [""]*arr_length

for i in iter_arr(arr_length):
    result[i] = a[i]

print(result)

枚举：enumerate
可以遍历元素对应的行号 和元素
lst_1 = [1,2,3,["a","b"]]
for idx,v in enumerate(lst_1):
print(idx,v)
迭代器

可迭代列表、元组、字符串及字典，字典只迭代key
可使用next() next()进行输出元素

使用next()输出元素
列表

>> l = [5,6,7]
>> it = iter(l)

>> it.next()
5
>> it.next()
6
>> it.next()
7

没有元素时候迭代器会报错

>> it.next()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
StopIteration

字典:

>> it = iter({1:2,3:4})
>> it.next()
1
>> it.next()
3
>> it.next()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
StopIteration

>> it =iter({1:2,3:4}.values())
>> it.next()
2
>> it.next()
4
>> it.next()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
StopIteration

使用next()输出元素

>> l = [5,6,7]
>> it = iter(l)
>> next(it)
5
>> next(it)
6
>> next(it)
7
>> next(it)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
StopIteration

遍历迭代器

>> l = [1,2,3,4,6]
>> it = iter(l)
>> for i in it:
... print(i)
...
1
2
3
4
6
自定义迭代器
iter()和next()方法
这两个方法是迭代器最基本的方法：
? 一个用来获得迭代器对象
? 一个用来获取容器中的下一个元素

#encoding=utf-8
class MyIter(object):

    def __init__(self,n):
        self.idx = 0
        self.n = n

    def __iter__(self):
        return self

    def __next__(self):
        if self.idx <self.n:
            val = self.idx
            self.idx += 1
            return val
        else:
            raise  StopIteration()

myIter = MyIter(5)

print(next(myIter))
print(next(myIter))

for i in myIter:
print(i)