a question about s[i:j] when i is negative

[dracula571]

s[i:j] slice of s from i to j (3), (4)

(3)
If i or j is negative, the index is relative to the end of the string:
len(s) + i or len(s) + j is substituted. But note that -0 is still 0.


(4)
The slice of s from i to j is defined as the sequence of items with
index k such that i <= k < j. If i or j is greater than len(s), use
len(s). If i is omitted or None, use 0. If j is omitted or None, use
len(s). If i is greater than or equal to j, the slice is empty.

i can't follow (3) very well.
for example:k = [1,2,3,4,5]
k[-1:2]=[]
k[-5:2]=[1,2]
but k[-6:2] = [1,2]
why k[-6:2] is [1,2]not [].i do follow (3),to make i positive by
plusing len(k) twice.But the result is not what i expect.why,or did i
misunderstand the rule of (3).

 

[Fredrik Lundh]

but k[-6:2] = [1,2]
why k[-6:2] is [1,2]not [].i do follow (3),to make i positive by
plusing len(k) twice.

twice?

</F>

 

[mensanator]

s[i:j] slice of s from i to j (3), (4)

(3)
If i or j is negative, the index is relative to the end of the string:
len(s) + i or len(s) + j is substituted. But note that -0 is still 0.


(4)
The slice of s from i to j is defined as the sequence of items with
index k such that i <= k < j. If i or j is greater than len(s), use
len(s). If i is omitted or None, use 0. If j is omitted or None, use
len(s). If i is greater than or equal to j, the slice is empty.

i can't follow (3) very well.
for example:k = [1,2,3,4,5]
k[-1:2]=[]
k[-5:2]=[1,2]
but k[-6:2] = [1,2]
why k[-6:2] is [1,2]not [].

Because 6 is greater then len(k), so when you ask
for i=-6, you get i=-5 instead. And relative -5 is actually
absolute 0 which is less than 2, so you do not get an
empty slice.

i do follow (3),to make i positive by
plusing len(k) twice.But the result is not what i expect.why,or did i
misunderstand the rule of (3).

See (4).

 

[dracula571]

Fredrik Lundh 写é“：

but k[-6:2] = [1,2]
why k[-6:2] is [1,2]not [].i do follow (3),to make i positive by
plusing len(k) twice.

twice?

</F>

that is -6 + len(k) + len(k) =4

 

[dracula571]

(e-mail address removed) 写é“：

s[i:j] slice of s from i to j (3), (4)

(3)
If i or j is negative, the index is relative to the end of the string:
len(s) + i or len(s) + j is substituted. But note that -0 is still 0.


(4)
The slice of s from i to j is defined as the sequence of items with
index k such that i <= k < j. If i or j is greater than len(s), use
len(s). If i is omitted or None, use 0. If j is omitted or None, use
len(s). If i is greater than or equal to j, the slice is empty.

i can't follow (3) very well.
for example:k = [1,2,3,4,5]
k[-1:2]=[]
k[-5:2]=[1,2]
but k[-6:2] = [1,2]
why k[-6:2] is [1,2]not [].

Because 6 is greater then len(k), so when you ask
for i=-6, you get i=-5 instead. And relative -5 is actually
absolute 0 which is less than 2, so you do not get an
empty slice.

i do follow (3),to make i positive by
plusing len(k) twice.But the result is not what i expect.why,or did i
misunderstand the rule of (3).

See (4).

when compared with len(k), the value of i is a absolute value?
in this example,i used -6 to compare with len(k).
I misunstood the rule (4),and used (3) wrong by plusing len(k) twice to
make it positive ,that is -6 + len(k) + len(k) = 4.
Thanks

 

[Fredrik Lundh]

but k[-6:2] = [1,2]
why k[-6:2] is [1,2]not [].i do follow (3),to make i positive by
plusing len(k) twice.

twice?

that is -6 + len(k) + len(k) =4

sure, but what makes you think you should add the length twice?

</F>