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列表解析
根据已有列表，高效创建新列表的方式。
列表解析是Python迭代机制的一种应用，它常用于实现创建新的列表，因此用在[]中。
[expression for iter_val in iterable]
[expression for iter_val in iterable if cond_expr]
from pythonds.basic.stack import Stack
def parChecker(symbolString):
s = Stack()
balanced = True
index = 0
while index < len(symbolString) and balanced:
symbol = symbolString[index]
if symbol in "([{":
s.push(symbol)
else:
if s.isEmpty():
balanced = False
else:
top = s.pop()
if not matches(top,symbol):
balanced = False
for i in range(s.size()):
print(s.items[i],end='')
# print([s.items[i] for i in range(s.size())])
index = index + 1
if balanced and s.isEmpty():
return True
else:
return False
def matches(open,close):
opens = "([{"
closers = ")]}"
return opens.index(open) == closers.index(close)
print(parChecker('{{([][])}()}'))
print(parChecker('[{)(]'))
from pythonds.basic.stack import Stack
def parChecker(symbolString):
s = Stack()
balanced = True
index = 0
while index < len(symbolString) and balanced:
symbol = symbolString[index]
if symbol in "([{":
s.push(symbol)
else:
if s.isEmpty():
balanced = False
else:
top = s.pop()
if not matches(top,symbol):
balanced = False
print([s.items[i] for i in range(s.size())])
index = index + 1
if balanced and s.isEmpty():
return True
else:
return False
def matches(open,close):
opens = "([{"
closers = ")]}"
return opens.index(open) == closers.index(close)
print(parChecker('{{([][])}()}'))
print(parChecker('[{)(]'))
上述代码段可以看出列表解析的优势
同时
print(s.items[i] for i in range(s.size()))
print([s.items[i] for i in range(s.size())])
是有很大区别的
实例展示
转载地址：https://www.cnblogs.com/liu-shuai/p/6098227.html
要求：列出1~10所有数字的平方
1、普通方法：
>>> L = []
>>> for i in range(1,11):
... L.append(i**2)
...
>>> print(L)
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
2、列表解析
>>>L = [ i**2 for i in range(1,11)]
>>> print(L)
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
要求：列出1~10中大于等于4的数字的平方
1、普通方法：
>>> L = []
>>> for i in range(1,11):
... if i >= 4:
... L.append(i**2)
...
>>> print(L)
[16, 25, 36, 49, 64, 81, 100]
2、列表解析
>>>L = [ i**2 for i in range(1,11) if i >= 4 ]
>>>print(L)
[16, 25, 36, 49, 64, 81, 100]
要求：列出1~10所有数字的平方除以2的值
1、普通方法
>>> L = []
>>> for i in range(1,11):
... L.append(i**2/2)
...
>>> print(L)
[0, 2, 4, 8, 12, 18, 24, 32, 40, 50]
2、列表解析
>>> L = [i**2/2 for i in range(1,11) ]
>>> print(L)
[0, 2, 4, 8, 12, 18, 24, 32, 40, 50]
要求：列出"/var/log"中所有已’.log’结尾的文件
1、普通方法
import os
file = []
for file in os.listdir('/var/log'):
if file.endswith('.log'):
file.append(file)
print(file)
>>>['anaconda.ifcfg.log', 'Xorg.0.log', 'anaconda.storage.log', 'Xorg.9.log', 'yum.log', 'anaconda.log', 'dracut.log', 'pm-powersave.log', 'anaconda.yum.log', 'wpa_supplicant.log', 'boot.log', 'spice-vdagent.log', 'anaconda.program.log']
2.列表解析
import os
file = [ file for file in os.listdir('/var/log') if file.endswith('.log') ]
print(file)
>>>
['anaconda.ifcfg.log', 'Xorg.0.log', 'anaconda.storage.log', 'Xorg.9.log', 'yum.log', 'anaconda.log', 'dracut.log', 'pm-powersave.log', 'anaconda.yum.log', 'wpa_supplicant.log', 'boot.log', 'spice-vdagent.log', 'anaconda.program.log']
要求：实现两个列表中的元素逐一配对
1、普通方法:
L1 = ['x','y','z']
L2 = [1,2,3]
L3 = []
for a in L1:
for b in L2:
L3.append((a,b))
print(L3)
>>>[('x', 1), ('x', 2), ('x', 3), ('y', 1), ('y', 2), ('y', 3), ('z', 1), ('z', 2), ('z', 3)]
2、列表解析:
L1 = ['x','y','z']
L2 = [1,2,3]
L3 = [ (a,b) for a in L1 for b in L2 ]
print(L3)
>>>[('x', 1), ('x', 2), ('x', 3), ('y', 1), ('y', 2), ('y', 3), ('z', 1), ('z', 2), ('z', 3)]
使用列表解析生成 9*9 乘法表
print('\n'.join([''.join(['%s*%s=%-2s '%(y,x,x*y)for y in range(1,x+1)])for x in range(1,10)]))