string[i:j:k]

[konstantin]

Hello,
I'm not a newbie in python, but recently faced a problem in simple
expression:
some_string[i:j:k]
What does it mean? I believe this grammar (http://docs.python.org/ref/
slicings.html) describes the syntax. But I can't grasp it.
Thanks!

[alex23]

Re: string[i:j:k]

On Jul 22, 3:10 pm, konstantin <konstantin.sel iva...@gmail.co mwrote:

some_string[i:j:k]
What does it mean?

i = start position, j = end position, k = step size

>>s = "ABABABABABABAB "
>>s[0:6:2]

'AAA'

>>s = "ABCABCABCABCAB C"
>>s[0:6:3]

'AA'

Hope this helps.

- alex23

[konstantin]

Re: string[i:j:k]

On Jul 22, 9:18 am, alex23 <wuwe...@gmail. comwrote:

On Jul 22, 3:10 pm, konstantin <konstantin.sel iva...@gmail.co mwrote:
>

some_string[i:j:k]
What does it mean?

>
i = start position, j = end position, k = step size
>

>s = "ABABABABABABAB "
>s[0:6:2]

'AAA'

>s = "ABCABCABCABCAB C"
>s[0:6:3]

>
'AA'
>
Hope this helps.
>
- alex23

Thanks!
It seems that negative step leads in reverse direction.
But logic isn't completely clear for me.

>>s = '123456789'
>>s[::-2]

'97531'

but

>>s[:-1:-2]

''
though I expected something like '8642'
What did i missed?

[Gary Herron]

Re: string[i:j:k]

konstantin wrote:

On Jul 22, 9:18 am, alex23 <wuwe...@gmail. comwrote:
>

>On Jul 22, 3:10 pm, konstantin <konstantin.sel iva...@gmail.co mwrote:
>>
>>

>>some_string[i:j:k]
>>What does it mean?
>>>

>i = start position, j = end position, k = step size
>>
>>

>>>>s = "ABABABABABABAB "
>>>>s[0:6:2]
>>>>>

>'AAA'
>>

>>>>s = "ABCABCABCABCAB C"
>>>>s[0:6:3]
>>>>>

>'AA'
>>
>Hope this helps.
>>
>- alex23
>>

>
Thanks!
It seems that negative step leads in reverse direction.
But logic isn't completely clear for me.
>

>>>s = '123456789'
>>>s[::-2]
>>>>

'97531'
>
but
>

>>>s[:-1:-2]
>>>>

The slice s[:-1]
means start at zero and go to n-1(where n-len(s))
(it does not mean start at zero and go to -1)

So since the indexing is counting upward, the step size had better be
positive. Thus:

>>s = '123456789'
>>s[:-1:2]

'1357'

>>>

Gary Herron

''
though I expected something like '8642'
What did i missed?
>
--

http://mail.python.org/mailman/listinfo/python-list

>

[John McMonagle]

Re: string[i:j:k]

konstantin wrote:

>
Thanks!
It seems that negative step leads in reverse direction.
But logic isn't completely clear for me.

>>>s = '123456789'
>>>s[::-2]

'97531'
>
but

>>>s[:-1:-2]

''
though I expected something like '8642'
What did i missed?
>
--

You need to *start* at the second from last index:

s[-2::-2]

Regards,

John

[konstantin]

Re: string[i:j:k]

Now it's clear.
Thanks.

[Terry Reedy]

Re: string[i:j:k]



konstantin wrote:

Hello,
I'm not a newbie in python, but recently faced a problem in simple
expression:
some_string[i:j:k]
What does it mean? I believe this grammar (http://docs.python.org/ref/
slicings.html) describes the syntax. But I can't grasp it.

When you post a link, please put it on one line by itself, like this:

http://docs.python.org/ref/slicings.html

so it will be clickable as a whole by modern newsreaders.

Experimenting with bits of code in the interactive interpreter (or IDLE,
which is easier to cut from), like the responders did, is a great way to
learn.