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]

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 = "ABCABCABCABCABC"
s[0:6:3]

'AA'

Hope this helps.

- alex23

 

[konstantin]

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 = "ABCABCABCABCABC"
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?

 

[John McMonagle]

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

 

[Gary Herron]

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 = "ABCABCABCABCABC"
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

 

[konstantin]

Now it's clear.
Thanks.

 

[Terry Reedy]

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.