Why Python does *SLICING* the way it does??

[seberino]

Many people I know ask why Python does slicing the way it does.....

Can anyone /please/ give me a good defense/justification?? ?

I'm referring to why mystring[:4] gives me
elements 0, 1, 2 and 3 but *NOT* mystring[4] (5th element).

Many people don't like idea that 5th element is not invited.

(BTW, yes I'm aware of the explanation where slicing
is shown to involve slices _between_ elements. This
doesn't explain why this is *best* way to do it.)

Chris

[Ganesan Rajagopal]

>>>>> "seberino@spawa r" == seberino@spawar navy mil <se******@spawa r.navy.mil> writes:

Many people I know ask why Python does slicing the way it does.....
Can anyone /please/ give me a good defense/justification?? ?

Try http://www.everything2.com/index.pl?node_id=1409551

Ganesan

--
Ganesan Rajagopal (rganesan at debian.org) | GPG Key: 1024D/5D8C12EA
Web: http://employees.org/~rganesan | http://rganesan.blogspot.com

[Raymond Hettinger]

<se******@spawa r.navy.mil>

Many people I know ask why Python does slicing the way it does.....

Half open intervals are just one way of doing things. Each approach has its own
merits and issues.

Python's way has some useful properties:

* s == s[:i] + s[i:]

* len(s[i:j]) == j-i # if s is long enough
OTOH, it has some aspects that bite:

* It is awkward with negative strides such as with s[4:2:-1]. This was the
principal reason for introducing the reversed() function.

* It makes some people cringe when they first see it (you're obviously in that
group).
I suspect that whether it feels natural depends on your previous background and
whether you're working in an environment with arrays indexed from one or from
zero. For instance, C programmers are used to seeing code like: for(i=0 ;
i<n; i++) a[i]=f(i); In contrast, a BASIC programmer may be used to FOR I = 1
to N: a[i]=f(I); NEXT. Hence, the C coders may find Python's a[:n] to be
more natural than BASIC programmers.

As long as a language is consistent about its approach, you just get used to it
and it stops being an issue after a few days.
Raymond Hettinger

[John Bokma]

Raymond Hettinger wrote:

to seeing code like: for(i=0 ; i<n; i++) a[i]=f(i); In contrast, a
BASIC programmer may be used to FOR I = 1 to N: a[i]=f(I); NEXT.

Afaik, at least BBC BASIC uses zero based arrays :-) Maybe ZX Spectrum
Basic too (too long ago to remember).

--
John MexIT: http://johnbokma.com/mexit/
personal page: http://johnbokma.com/
Experienced programmer available: http://castleamber.com/
Happy Customers: http://castleamber.com/testimonials.html

[Nick Efford]

se******@spawar .navy.mil <se******@spawa r.navy.mil> wrote:

Many people I know ask why Python does slicing the way it does..... Can anyone /please/ give me a good defense/justification?? ? I'm referring to why mystring[:4] gives me
elements 0, 1, 2 and 3 but *NOT* mystring[4] (5th element).

mystring[:4] can be read as "the first four characters of mystring".
If it included mystring[4], you'd have to read it as "the first
five characters of mystring", which wouldn't match the appearance
of '4' in the slice.

Given another slice like mystring[2:4], you know instantly by
looking at the slice indices that this contains 4-2 = 2 characters
from the original string. If the last index were included in the
slice, you'd have to remember to add 1 to get the number of
characters in the sliced string.

It all makes perfect sense when you look at it this way!
Nick

[Torsten Bronger]

Hallöchen!

nd*@no.spam.org (Nick Efford) writes:

se******@spawar .navy.mil <se******@spawa r.navy.mil> wrote:

Many people I know ask why Python does slicing the way it does.....

Can anyone /please/ give me a good defense/justification?? ?

I'm referring to why mystring[:4] gives me elements 0, 1, 2 and 3
but *NOT* mystring[4] (5th element).

mystring[:4] can be read as "the first four characters of
mystring". If it included mystring[4], you'd have to read it as
"the first five characters of mystring", which wouldn't match the
appearance of '4' in the slice.

[...]

It all makes perfect sense when you look at it this way!

Well, also in my experience every variant has its warts. You'll
never avoid the "i+1" or "i-1" expressions in your indices or loops
(or your mind ;).

It's interesting to muse about a language that starts at "1" for all
arrays and strings, as some more or less obsolete languages do. I
think this is more intuitive, since most people (including
mathematicians) start counting at "1". The reason for starting at
"0" is easier memory address calculation, so nothing for really high
level languages.

But most programmers are used to do it the Python (and most other
languages) way, so this opportunity has been missed for good.

Tschö,
Torsten.

--
Torsten Bronger, aquisgrana, europa vetus

[Antoon Pardon]

Op 2005-04-20, Torsten Bronger schreef <br*****@physik .rwth-aachen.de>:

Hallöchen!

nd*@no.spam.org (Nick Efford) writes:

se******@spawar .navy.mil <se******@spawa r.navy.mil> wrote:

Many people I know ask why Python does slicing the way it does.....

Can anyone /please/ give me a good defense/justification?? ?

I'm referring to why mystring[:4] gives me elements 0, 1, 2 and 3
but *NOT* mystring[4] (5th element).

mystring[:4] can be read as "the first four characters of
mystring". If it included mystring[4], you'd have to read it as
"the first five characters of mystring", which wouldn't match the
appearance of '4' in the slice.

[...]

It all makes perfect sense when you look at it this way!

Well, also in my experience every variant has its warts. You'll
never avoid the "i+1" or "i-1" expressions in your indices or loops
(or your mind ;).

It's interesting to muse about a language that starts at "1" for all
arrays and strings, as some more or less obsolete languages do. I
think this is more intuitive, since most people (including
mathematicians) start counting at "1". The reason for starting at
"0" is easier memory address calculation, so nothing for really high
level languages.

Personnaly I would like to have the choice. Sometimes I prefer to
start at 0, sometimes at 1 and other times at -13 or +7.

--
Antoon Pardon

[Terry Hancock]

On Wednesday 20 April 2005 01:36 am, Raymond Hettinger wrote:

<se******@spawa r.navy.mil>

Many people I know ask why Python does slicing the way it does.....

[...] Python's way has some useful properties: [...] OTOH, it has some aspects that bite: [...] I suspect that whether it feels natural depends on your previous background and
whether you're working in an environment with arrays indexed from one or from
zero. For instance, C programmers are used to seeing code like: for(i=0 ;
i<n; i++) a[i]=f(i); In contrast, a BASIC programmer may be used to FOR I = 1
to N: a[i]=f(I); NEXT. Hence, the C coders may find Python's a[:n] to be
more natural than BASIC programmers.

Well, I learned Basic, Fortran, C, Python --- more or less. And I first found
Python's syntax confusing as it didn't follow the same rules as any of the
previous ones.

However, I used to make "off by one" errors all the time in both C and Fortran,
whereas I hardly ever make them in Python.

So I like Python's slicing because it "bites *less*" than intervals in C or Fortran.

Cheers,
Terry
--
Terry Hancock ( hancock at anansispacework s.com )
Anansi Spaceworks http://www.anansispaceworks.com

[Sion Arrowsmith]

Raymond Hettinger <py****@rcn.com > wrote:

<se******@spaw ar.navy.mil>

Many people I know ask why Python does slicing the way it does.....

Python's way has some useful properties:

* s == s[:i] + s[i:]

* len(s[i:j]) == j-i # if s is long enough

The latter being particularly helpful when i = 0 -- the first n
elements are s[:n] . (Similarly elegantly, although of no
practical significance, s == s[0:len(s)] .)

--
\S -- si***@chiark.gr eenend.org.uk -- http://www.chaos.org.uk/~sion/
___ | "Frankly I have no feelings towards penguins one way or the other"
\X/ | -- Arthur C. Clarke
her nu becomeþ se bera eadward ofdun hlæddre heafdes bæce bump bump bump

[Torsten Bronger]

Hallöchen!

Antoon Pardon <ap*****@forel. vub.ac.be> writes:

Op 2005-04-20, Torsten Bronger schreef <br*****@physik .rwth-aachen.de>:

[...]

It's interesting to muse about a language that starts at "1" for
all arrays and strings, as some more or less obsolete languages
do. I think this is more intuitive, since most people (including
mathematicians) start counting at "1". The reason for starting
at "0" is easier memory address calculation, so nothing for
really high level languages.

Personnaly I would like to have the choice. Sometimes I prefer to
start at 0, sometimes at 1 and other times at -13 or +7.

In HTBasic you have the choice between 0 and 1; there is a global
source code directive for it. However, hardly anybody really wants
to use HTBasic.

Tschö,
Torsten.

--
Torsten Bronger, aquisgrana, europa vetus