Thanks, Abdur for forwarding Steve's message. It is a very good explanation
with examples.
The following example does help:
-
py> s = "Nobody expects the Spanish Inquisition!"
py> s[-1:1:-2]
'!otsun snp h tex db'
is somewhat like:
for i in range(len(s)-1, 1, -2):
print s[i]
-
So my question is: how would you write "s[::-1]" in terms of a for loop for
illustration purpose?
Vikas
-Original Message-
From: Abdur-Rahmaan Janhangeer [mailto:arj.pyt...@gmail.com]
Sent: Monday, June 12, 2017 1:26 AM
To: Vikas YADAV
Cc: tutor
Subject: Re: [Tutor] string reversal using [::-1]
[QUOTED ENTIRELY FROM STEVE {st...@pearwood.info} IN ANSWER TO A SLICING
QUESTION]
The way to think about string indexing and slicing is that the index positions
mark *between* the characters. Take your string:
Machine learning is awesome!
For brevity, I'll just use the first word:
Machine
Imagine slicing it between the characters. I'll mark the cuts with a vertical
bar:
|M|a|c|h|i|n|e|
and add indexes. The indexes will only line correctly if you use a monspaced or
fixed width font like Courier, otherwise things may not line up correctly.
|M|a|c|h|i|n|e|
0 1 2 3 4 5 6 7
Here they are again starting from the right, I've spread things out a bit to
fit in the minus signs:
|M |a |c |h |i |n |e |
-7 -6 -5 -4 -3 -2 -1 0
Notice that 0 gets used twice. Of course, that's impossible, because it would
be ambiguous. If you give 0 as an index, how does Python know whether you mean
0 at the start or 0 or the end? So the simple rule Python uses is that 0
*always* means the start.
When you give a single index, Python always uses the character immediately to
the right of the cut:
s = "Machine"
s[0]
=> returns "M"
s[-1]
=> returns "e"
s[7]
=> raises an exception, because there is no character to the right
When you give two indexes, using slice notation, Python returns the characters
BETWEEN those cuts:
s[0:7]
=> returns "Machine"
s[1:-1]
=> returns "achin"
Because 0 always means the start of the string, how do you slice to the end?
You can use the length of the string (in this case, 7) or you can leave the
ending position blank, and it defaults to the length of the
string:
s[1:] # means the same as [1:len(s)]
You can leave the starting position blank too, it defaults to 0:
s[:] # means the same as [0:len(s)]
So remember that slices always cut *between* the index positions.
Things get complicated when you include a step (or stride), especially when the
step is negative. For step sizes other than 1, it is probably best to think of
looping over the string:
py> s = "Nobody expects the Spanish Inquisition!"
py> s[-1:1:-2]
'!otsun snp h tex db'
is somewhat like:
for i in range(len(s)-1, 1, -2):
print s[i]
--
Steve
[QUOTED ENTIRELY FROM STEVE {st...@pearwood.info} IN ANSWER TO A SLICING
QUESTION]
Abdur-Rahmaan Janhangeer,
Mauritius
abdurrahmaanjanhangeer.wordpress.com
On 10 Jun 2017 21:31, "Vikas YADAV" wrote:
> Question: Why does "123"[::-1] result in "321"?
>
>
>
> MY thinking is [::-1] is same as [0:3:-1], that the empty places
> defaults to start and end index of the string object.
>
> So, if we start from 0 index and decrement index by 1 till we reach 3,
> how many index we should get? I think we should get infinite infinite
> number of indices (0,-1,-2,-3.).
>
>
>
> This is my confusion.
>
> I hope my question is clear.
>
>
>
> Thanks,
>
> Vikas
>
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