Python的切片表示法是如何工作的?也就是说:当我编写[x:y:z]、a[:]、a]::2]等代码时,我如何理解哪些元素最终会出现在切片中?请在适当的地方附上参考资料。
另请参见:为什么切片和范围上限是互斥的?
Python的切片表示法是如何工作的?也就是说:当我编写[x:y:z]、a[:]、a]::2]等代码时,我如何理解哪些元素最终会出现在切片中?请在适当的地方附上参考资料。
另请参见:为什么切片和范围上限是互斥的?
当前回答
在Python 2.7中
Python中的切片
[a:b:c]
len = length of string, tuple or list
c -- default is +1. The sign of c indicates forward or backward, absolute value of c indicates steps. Default is forward with step size 1. Positive means forward, negative means backward.
a -- When c is positive or blank, default is 0. When c is negative, default is -1.
b -- When c is positive or blank, default is len. When c is negative, default is -(len+1).
理解索引分配非常重要。
In forward direction, starts at 0 and ends at len-1
In backward direction, starts at -1 and ends at -len
当你说[a:b:c]时,你是说根据c的符号(向前或向后),从a开始,到b结束(不包括bth索引中的元素)。使用上面的索引规则,并记住您只能找到此范围内的元素:
-len, -len+1, -len+2, ..., 0, 1, 2,3,4 , len -1
但这一范围在两个方向上无限延伸:
...,-len -2 ,-len-1,-len, -len+1, -len+2, ..., 0, 1, 2,3,4 , len -1, len, len +1, len+2 , ....
例如:
0 1 2 3 4 5 6 7 8 9 10 11
a s t r i n g
-9 -8 -7 -6 -5 -4 -3 -2 -1
如果在使用上面的a、b、c的规则进行遍历时,a、b和c的选择允许与上面的范围重叠,则会得到一个包含元素的列表(在遍历过程中被触摸),或者得到一个空列表。
最后一件事:如果a和b相等,那么也会得到一个空列表:
>>> l1
[2, 3, 4]
>>> l1[:]
[2, 3, 4]
>>> l1[::-1] # a default is -1 , b default is -(len+1)
[4, 3, 2]
>>> l1[:-4:-1] # a default is -1
[4, 3, 2]
>>> l1[:-3:-1] # a default is -1
[4, 3]
>>> l1[::] # c default is +1, so a default is 0, b default is len
[2, 3, 4]
>>> l1[::-1] # c is -1 , so a default is -1 and b default is -(len+1)
[4, 3, 2]
>>> l1[-100:-200:-1] # Interesting
[]
>>> l1[-1:-200:-1] # Interesting
[4, 3, 2]
>>> l1[-1:-1:1]
[]
>>> l1[-1:5:1] # Interesting
[4]
>>> l1[1:-7:1]
[]
>>> l1[1:-7:-1] # Interesting
[3, 2]
>>> l1[:-2:-2] # a default is -1, stop(b) at -2 , step(c) by 2 in reverse direction
[4]
其他回答
如果我们可以将切片与范围联系起来,这很容易理解,因为范围给出了索引。我们可以将切片分为以下两类:
1.无台阶或台阶>0。例如,[i:j]或[i:j:k](k>0)
假设序列为s=[1,2,3,4,5]。
如果0<i<len(s)和0<j<len,则[i:j:k]->范围(i,j,k)
例如,[0:3:2]->范围(0,3,2)->0,2
如果i>len或j>len,则i=len或j=len
例如,[0:100:2]->范围(0,len(s),2)->范围(0,5,2)->0,2,4
如果i<0或j<0,则i=max(0,len(s)+i)或j=max
例如,[0:-3:2]->范围(0,len(s)-3,2)->范围(0,2,2)->0
例如,[0:-1:2]->范围(0,len(s)-1,2)->范围(0,4,2)->0,2
如果未指定i,则i=0
例如,[:4:2]->范围(0,4,2)->范围(4,2)->0,2
如果未指定j,则j=len(s)
例如,[0::2]->范围(0,len(s),2)->范围(0,5,2)->0,2,4
2.步骤<0。例如,[i:j:k](k<0)
假设序列为s=[1,2,3,4,5]。
如果0<i<len(s)和0<j<len,则[i:j:k]->范围(i,j,k)
例如,[5:0:-2]->范围(5,0,-2)->5,3,1
如果i>len或j>len,则i=len(s)-1或j=len(s)-1
例如,[100:0:-2]->范围(len(s)-1,0,-2)->范围(4,0,-2)->4,2
如果i<0或j<0,则i=max(-1,len(s)+i)或j=max(-1len(s)+j)
例如,[-2:-10:-2]->range(len(s)-2,-1,-2)->range(3,-1,-1)->3,1
如果未指定i,则i=len(s)-1
例如,[:0:-2]->范围(len(s)-1,0,-2)->范围(4,0,-2)->4,2
如果未指定j,则j=-1
例如,[2::-2]->范围(2,-1,-2)->2,0
例如,[::-1]->range(len(s)-1,-1,-1)->range(4,-1,1)->4,3,2,1,0
总而言之
#!/usr/bin/env python
def slicegraphical(s, lista):
if len(s) > 9:
print """Enter a string of maximum 9 characters,
so the printig would looki nice"""
return 0;
# print " ",
print ' '+'+---' * len(s) +'+'
print ' ',
for letter in s:
print '| {}'.format(letter),
print '|'
print " ",; print '+---' * len(s) +'+'
print " ",
for letter in range(len(s) +1):
print '{} '.format(letter),
print ""
for letter in range(-1*(len(s)), 0):
print ' {}'.format(letter),
print ''
print ''
for triada in lista:
if len(triada) == 3:
if triada[0]==None and triada[1] == None and triada[2] == None:
# 000
print s+'[ : : ]' +' = ', s[triada[0]:triada[1]:triada[2]]
elif triada[0] == None and triada[1] == None and triada[2] != None:
# 001
print s+'[ : :{0:2d} ]'.format(triada[2], '','') +' = ', s[triada[0]:triada[1]:triada[2]]
elif triada[0] == None and triada[1] != None and triada[2] == None:
# 010
print s+'[ :{0:2d} : ]'.format(triada[1]) +' = ', s[triada[0]:triada[1]:triada[2]]
elif triada[0] == None and triada[1] != None and triada[2] != None:
# 011
print s+'[ :{0:2d} :{1:2d} ]'.format(triada[1], triada[2]) +' = ', s[triada[0]:triada[1]:triada[2]]
elif triada[0] != None and triada[1] == None and triada[2] == None:
# 100
print s+'[{0:2d} : : ]'.format(triada[0]) +' = ', s[triada[0]:triada[1]:triada[2]]
elif triada[0] != None and triada[1] == None and triada[2] != None:
# 101
print s+'[{0:2d} : :{1:2d} ]'.format(triada[0], triada[2]) +' = ', s[triada[0]:triada[1]:triada[2]]
elif triada[0] != None and triada[1] != None and triada[2] == None:
# 110
print s+'[{0:2d} :{1:2d} : ]'.format(triada[0], triada[1]) +' = ', s[triada[0]:triada[1]:triada[2]]
elif triada[0] != None and triada[1] != None and triada[2] != None:
# 111
print s+'[{0:2d} :{1:2d} :{2:2d} ]'.format(triada[0], triada[1], triada[2]) +' = ', s[triada[0]:triada[1]:triada[2]]
elif len(triada) == 2:
if triada[0] == None and triada[1] == None:
# 00
print s+'[ : ] ' + ' = ', s[triada[0]:triada[1]]
elif triada[0] == None and triada[1] != None:
# 01
print s+'[ :{0:2d} ] '.format(triada[1]) + ' = ', s[triada[0]:triada[1]]
elif triada[0] != None and triada[1] == None:
# 10
print s+'[{0:2d} : ] '.format(triada[0]) + ' = ', s[triada[0]:triada[1]]
elif triada[0] != None and triada[1] != None:
# 11
print s+'[{0:2d} :{1:2d} ] '.format(triada[0],triada[1]) + ' = ', s[triada[0]:triada[1]]
elif len(triada) == 1:
print s+'[{0:2d} ] '.format(triada[0]) + ' = ', s[triada[0]]
if __name__ == '__main__':
# Change "s" to what ever string you like, make it 9 characters for
# better representation.
s = 'COMPUTERS'
# add to this list different lists to experement with indexes
# to represent ex. s[::], use s[None, None,None], otherwise you get an error
# for s[2:] use s[2:None]
lista = [[4,7],[2,5,2],[-5,1,-1],[4],[-4,-6,-1], [2,-3,1],[2,-3,-1], [None,None,-1],[-5,None],[-5,0,-1],[-5,None,-1],[-1,1,-2]]
slicegraphical(s, lista)
你可以运行这个脚本并进行实验,下面是我从脚本中获得的一些示例。
+---+---+---+---+---+---+---+---+---+
| C | O | M | P | U | T | E | R | S |
+---+---+---+---+---+---+---+---+---+
0 1 2 3 4 5 6 7 8 9
-9 -8 -7 -6 -5 -4 -3 -2 -1
COMPUTERS[ 4 : 7 ] = UTE
COMPUTERS[ 2 : 5 : 2 ] = MU
COMPUTERS[-5 : 1 :-1 ] = UPM
COMPUTERS[ 4 ] = U
COMPUTERS[-4 :-6 :-1 ] = TU
COMPUTERS[ 2 :-3 : 1 ] = MPUT
COMPUTERS[ 2 :-3 :-1 ] =
COMPUTERS[ : :-1 ] = SRETUPMOC
COMPUTERS[-5 : ] = UTERS
COMPUTERS[-5 : 0 :-1 ] = UPMO
COMPUTERS[-5 : :-1 ] = UPMOC
COMPUTERS[-1 : 1 :-2 ] = SEUM
[Finished in 0.9s]
当使用否定步骤时,请注意答案向右移动1。
我想加一个你好,世界!为初学者解释切片基础知识的示例。这对我帮助很大。
让我们列出六个值[“P”、“Y”、“T”、“H”、“O”、“N”]:
+---+---+---+---+---+---+
| P | Y | T | H | O | N |
+---+---+---+---+---+---+
0 1 2 3 4 5
现在,该列表中最简单的部分是其子列表。符号是[<index>:<index>],关键是这样读:
[ start cutting before this index : end cutting before this index ]
现在,如果你从上面的列表中选择一个片段[2:5],就会发生这种情况:
| |
+---+---|---+---+---|---+
| P | Y | T | H | O | N |
+---+---|---+---+---|---+
0 1 | 2 3 4 | 5
在索引为2的元素之前进行了一次切割,在索引为5的元素之前又进行了一个切割。因此,结果将是这两个剪辑之间的一个片段,一个列表['T','H','O']。
简单易懂:
在Python中,切片符号a[start:stop:step]可以用于从序列中选择一系列元素(例如列表、元组或字符串)。
起始索引是包括在切片中的第一个元素,
停止索引是从切片中排除的第一个元素,也是最后一个元素
步长值是切片元素之间的索引数。
例如,考虑以下列表:
a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
如果要选择a的所有元素,可以使用切片符号a[:]:
>>> a[:]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
如果我们想选择a的所有元素,但跳过其他元素,我们可以使用切片符号a[::2]:
>>> a[::2]
[0, 2, 4, 6, 8]
如果我们想选择从第三个元素(索引2)到第七个元素(索引号6)的所有元素,我们可以使用切片符号a[2:7]:
>>> a[2:7]
[2, 3, 4, 5, 6]
如果我们想选择从第三个元素(索引2)到第七个元素(索引号6)的所有元素,但跳过其他元素,我们可以使用切片符号a[2:7:2]:
>>> a[2:7:2]
[2, 4, 6]
如果我们想选择从第三个元素(索引2)到列表末尾的所有元素,我们可以使用切片符号a[2:]:
>>> a[2:]
[2, 3, 4, 5, 6, 7, 8, 9]
如果我们想选择从列表开头到第七个元素(索引6)的所有元素,我们可以使用切片符号a[:7]:
>>> a[:7]
[0, 1, 2, 3, 4, 5, 6]
如果您想了解有关切片表示法的更多信息,可以参考Python官方文档:链接1链接2
如果你觉得切片中的负指数令人困惑,这里有一个非常简单的方法来考虑:用len-index替换负指数。例如,用len(list)-3替换-3。
说明切片在内部做什么的最佳方法是在实现此操作的代码中显示它:
def slice(list, start = None, end = None, step = 1):
# Take care of missing start/end parameters
start = 0 if start is None else start
end = len(list) if end is None else end
# Take care of negative start/end parameters
start = len(list) + start if start < 0 else start
end = len(list) + end if end < 0 else end
# Now just execute a for-loop with start, end and step
return [list[i] for i in range(start, end, step)]