问题
I've been trying to slice two characters out of a string using a loop, but instead of grabbing two characters, it only grabs one.
I've tried:
input[i:i+1]
and
input[i:(i+1)]
but neither seems to work.
How do I use a variable for slicing?
The full routine:
def StringTo2ByteList(input):
# converts data string to a byte list, written for ascii input only
rlist = []
for i in range(0, len(input), 2):
rlist.append(input[i:(i+1)])
return rlist
回答1:
The slice values aren't the start and end characters of the slice, they're the start and end points. If you want to slice two elements then your stop must be 2 greater than your start.
input[i:i+2]
回答2:
A nice way to remember slice indexing is to think of the numbers as labeling the positions between the elements.
So for example to slice ba you would use fruit[0:2]. When thinking of it this way, things no longer seem counter-intuitive, and you can easily avoid making off-by-one errors in your code.
回答3:
Reasoning on the basis of the in-between positions works when we do a left-to-right iteration:
li = [10, 20, 55, 65, 75, 120, 1000]
print 'forward slicing li[2:5] ->',li[2:5]
# prints forward slicing li[2:5] -> [55, 65, 75]
because pos 2 is between 20 / 55 and pos 5 is between 75 / 120
.
But it doesn't work when we do right-to-left iteration:
li = [10, 20, 55, 65, 75, 120, 1000]
print li
print 'reverse slicing li[5:2:-1] ->',li[5:2:-1]
# prints reverse slicing li[5:2:-1] -> [120, 75, 65]
We must think of li[ 5 : 2 : -1 ] as :
from element li[5] (which is 120) as far as uncomprised element li[2] (which is 55)
that is to say
from element li[5] (which is 120) only as far as li[3] included (which is 65) .
.
That makes dangerous reverse slicing from the very end:
li = [10, 20, 55, 65, 75, 120, 1000]
print li[ -1 : 2 : -1 ]
# prints [1000, 120, 75, 65] not [120, 75, 65, 55]
# the same as:
print li[ None : 2 : -1 ]
# prints [1000, 120, 75, 65]
print li[ : 2 : -1 ]
# prints [1000, 120, 75, 65]
and
li = [10, 20, 55, 65, 75, 120, 1000]
print li[ 4 : 0 : -1]
# prints [75, 65, 55, 20] , not [65, 55, 20, 10]
.
In case of difficulty to think like that, a manner to prevent errors is to write
print list(reversed(li[2:5]))
# prints [75, 65, 55]
print list(reversed(li[2:-1]))
# prints [120, 75, 65, 55]
print list(reversed(li[0:4]))
# prints [65, 55, 20, 10]
.
Note that the only way to obtain a reverse slicing as far as the first element INCLUDED is
print li[ 4 : : -1]
that stands for
print li[ 4 : None : -1]
来源：https://stackoverflow.com/questions/7625326/how-do-you-use-a-variable-as-an-index-when-slicing-strings-in-python