Python數據結構與算法相關問題與解決技巧
１.如何在列表, 字典, 集合中根據條件篩選數據¶
In [1]:
from random import randint
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data = [randint(-10,10) for _ in range(10)]
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data
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# 篩選出列表中大於０的元素
# 使用filter函數
list(filter(lambda x:x>=0,data))
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In [5]:
# 列表解析
[x for x in data if x >= 0]
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In [6]:
timeit list(filter(lambda x:x>=0,data))
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timeit [x for x in data if x >= 0]
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# 字典解析
d = {x:randint(60,100) for x in range(1,21)}
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d
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In [10]:
{k:v for k,v in d.items() if v > 90 }
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In [11]:
# 集合解析
s = set(data)
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s
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In [13]:
{x for x in s if x % 2 == 0}
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2.如何爲元組中的每一個元素命名, 提升程序可讀性¶
In [14]:
#方案一,定義一系列數值常量
NAME,AGE,SEX,EMAIL = range(4)
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student = ('Tom','16','male','tom@gmail.com')
In [16]:
student[NAME]
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In [17]:
#方案二，使用namedtuple
from collections import namedtuple
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Student = namedtuple('Student',['name','age','sex','email'])
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#位置傳參
s = Student('Tom','16','male','tom@gmail.com')
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s
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In [21]:
#關鍵字傳參
s2 = Student(name='Tom',age='16',sex='male',email='tom@gmail.com')
In [22]:
s2
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In [23]:
s.name
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In [24]:
s.email
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In [25]:
isinstance(s,tuple)
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3.如何統計序列中元素的出現頻度¶
In [26]:
from random import randint
In [27]:
data = [randint(0,20) for _ in range(30)]
In [28]:
data
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In [29]:
#fromkeys：用於建立並返回一個新的字典。兩個參數：第一個是字典的鍵，第二個（可選）是傳入鍵的值，默認爲None。
c = dict.fromkeys(data,0)
In [30]:
c
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In [31]:
for x in data:
c[x] += 1
In [32]:
c
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In [33]:
# 使用Counter
from collections import Counter
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c2 = Counter(data)
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c2
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#找出頻率出現最高的３個元素
c2.most_common(3)
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In [37]:
import re
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txt = open('Alice.txt').read()
In [39]:
c3 = Counter(re.split('\W+',txt))
In [40]:
c3
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In [41]:
c3.most_common(10)
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4.如何根據字典中值的大小, 對字典中的項排序¶
In [42]:
sorted([9,3,1,67,6])
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In [43]:
from random import randint
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d = {x:randint(60,100) for x in 'xyzabc'}
In [45]:
d
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In [46]:
d.keys()
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In [47]:
d.values()
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In [48]:
#方案一：利用zip將字典轉換成元組進行排序
sorted(zip(d.values(),d.keys()))
Out[48]:
In [49]:
d.items()
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In [50]:
#方案二：傳遞sorted函數的key參數
sorted(d.items(),key=lambda x:x[1])
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5.如何快速找到多個字典中的公共鍵(key)¶
In [51]:
from random import randint,sample
In [52]:
from random import randint,sample
In [53]:
# 取樣
sample('abcdef',3)
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In [54]:
sample('abcdef',randint(3,6))
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In [55]:
# 找出s1,s2,s3中都存在的key
# 方案一:遍歷
s1 = {x:randint(1,4) for x in sample('abcdef',randint(3,6))}
In [56]:
s1
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In [57]:
s2 = {x:randint(1,4) for x in sample('abcdef',randint(3,6))}
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s3 = {x:randint(1,4) for x in sample('abcdef',randint(3,6))}
In [59]:
s2
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In [60]:
s3
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In [61]:
res = []
for k in s1:
if k in s2 and k in s3:
res.append(k)
In [62]:
res
Out[62]:
In [63]:
# 方案二：使用集合
s1
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In [64]:
s1.keys()
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In [65]:
s2.keys()
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In [66]:
s1.keys() & s2.keys() & s3.keys()
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In [67]:
# 方案三：使用map和reduce
# Python3裏，map返回的結果是迭代器(iterator)
list(map(dict.keys,[s1,s2,s3]))
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In [68]:
from functools import reduce
# reduce() 函數會對參數序列中元素進行累積。
reduce(lambda a,b:a & b,map(dict.keys,[s1,s2,s3]))
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6.如何讓字典保持有序¶
In [69]:
d = {}
d['Tom'] = (1,35)
d['Bob'] = (2,37)
d['Kate'] = (3,45)
In [70]:
d
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In [71]:
# python3.6以後字典有序,按照數據存入的順序
for k in d:
print(k)
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# 有序字典OrderedDict
from collections import OrderedDict
d = OrderedDict()
In [73]:
d['Tom'] = (1,35)
d['Bob'] = (2,37)
d['Kate'] = (3,45)
for k in d:
print(k)
In [74]:
from time import time
from random import randint
from collections import OrderedDict
d = OrderedDict()
players =list('ABCDEFGH')
start = time()
for i in range(8):
'''模擬選手比賽消耗時間並存入字典中'''
input()
p = players.pop(randint(0,7-i))
end = time()
print(i + 1, p, end - start,sep=' ')
d[p] = (i + 1, end - start)
print('-' * 30)
for k in d:
print(k, d[k])
7.如何實現用戶的歷史記錄功能(最多n條)¶
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from collections import deque
# 建立雙向隊列
q = deque([], 5)
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q
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In [77]:
q.append(1)
q.append(2)
q.append(3)
q.append(4)
q.append(5)
q
Out[77]:
In [78]:
q.append(6)
q
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In [79]:
from random import randint
from collections import deque
N = randint(0, 100)
history = deque([], 5)
def guess(k):
if k == N:
print('Right')
return True
if k < N:
print('%s is less-than N' % k)
else:
print('%s is greater-than N' % k)
return False
while True:
line = input('Please input a number:')
if line.isdigit():
k = int(line)
history.append(k)
if guess(k):
break
elif line == 'history' or line == 'h':
print(list(history))
In [80]:
import pickle
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q
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# 將數據存入文件
pickle.dump(q,open('history','wb'))
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# 讀取文件
q2 = pickle.load(open('history','rb'))
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q2
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