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[newbie] troubles with tuples

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  [newbie] troubles with tuples Jean Dupont <jeandupont314@gmail.com> - 2014-02-03 08:50 -0800
    Re: [newbie] troubles with tuples Asaf Las <roegltd@gmail.com> - 2014-02-03 08:59 -0800
    Re: [newbie] troubles with tuples Larry Martell <larry.martell@gmail.com> - 2014-02-03 12:00 -0500
    Re: [newbie] troubles with tuples Rustom Mody <rustompmody@gmail.com> - 2014-02-03 09:06 -0800
      Re: [newbie] troubles with tuples Jean Dupont <jeandupont314@gmail.com> - 2014-02-03 11:18 -0800
    Re: [newbie] troubles with tuples Terry Reedy <tjreedy@udel.edu> - 2014-02-03 16:52 -0500
    Re: [newbie] troubles with tuples Denis McMahon <denismfmcmahon@gmail.com> - 2014-02-04 00:50 +0000

#65343 — [newbie] troubles with tuples

I'm looking at the way to address tuples
e.g.
tup2 = (1, 2, 3, 4, 5, 6, 7 );

As I found out indices start with 0 in Python, so 
tup2[0] gives me 1, the first element in the tuple as expected
tup2[1] gives me 2, the second element in the tuple as expected
now here comes what surprises me:
tup2[0:1] does not give me the expected (1,2) but (2,)

what is the reason for this and how then should one get the first and the second element of a tuple? Or the 3rd until the 5th?

thanks in advance and kind regards,

jean

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#65345

On Monday, February 3, 2014 6:50:31 PM UTC+2, Jean Dupont wrote:
> I'm looking at the way to address tuples
> 
> e.g.
> tup2 = (1, 2, 3, 4, 5, 6, 7 );
> As I found out indices start with 0 in Python, so 
> tup2[0] gives me 1, the first element in the tuple as expected
> tup2[1] gives me 2, the second element in the tuple as expected
> now here comes what surprises me:
> tup2[0:1] does not give me the expected (1,2) but (2,)
> what is the reason for this and how then should one get the first and the second element of a tuple? Or the 3rd until the 5th?
> 
> thanks in advance and kind regards,
> 
> jean

Hi

from http://docs.python.org/3.3/library/stdtypes.html?highlight=tuple#tuple

" 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."

so in above  k < j but not equal so in your example slice will be of only one member. 

/Asaf

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#65346

On Mon, Feb 3, 2014 at 11:50 AM, Jean Dupont <jeandupont314@gmail.com> wrote:
> I'm looking at the way to address tuples
> e.g.
> tup2 = (1, 2, 3, 4, 5, 6, 7 );
>
> As I found out indices start with 0 in Python, so
> tup2[0] gives me 1, the first element in the tuple as expected
> tup2[1] gives me 2, the second element in the tuple as expected
> now here comes what surprises me:
> tup2[0:1] does not give me the expected (1,2) but (2,)
>
> what is the reason for this and how then should one get the first and the second element of a tuple? Or the 3rd until the 5th?
>
> thanks in advance and kind regards,

Some examples:

a[start:end]  # items start through end-1
a[start:]       # items start through the rest of the array
a[:end]        # items from the beginning through end-1
a[:]             # a copy of the whole array
a[-1]           # last item in the array
a[-2:]          # last two items in the array
a[:-2]          # everything except the last two items

HTH,
-larry

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#65347

On Monday, February 3, 2014 10:20:31 PM UTC+5:30, Jean Dupont wrote:
> I'm looking at the way to address tuples
> e.g.
> tup2 = (1, 2, 3, 4, 5, 6, 7 );

> As I found out indices start with 0 in Python, so 
> tup2[0] gives me 1, the first element in the tuple as expected
> tup2[1] gives me 2, the second element in the tuple as expected
> now here comes what surprises me:
> tup2[0:1] does not give me the expected (1,2) but (2,)


Python 2.7.6 (default, Jan 11 2014, 17:06:02) 
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> tup2=(1,2,3,4,5,6,7)
>>> tup2[0:1]
(1,)
>>> 

So assuming you meant (1,) and wrote (2,) :-)

> what is the reason for this and how then should one get the first and the second element of a tuple? Or the 3rd until the 5th?

Generally ranges in python are lower-inclusive upper-exclusive
What some math texts write as [lo, hi)

So if you want from index 1 to 2-inclusive it is 1 to 3 exclusive
tup2[0:2]


See for motivations
http://www.cs.utexas.edu/~EWD/transcriptions/EWD08xx/EWD831.html

And one more surprising thing to note is that negatives count from the end

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#65368

Op maandag 3 februari 2014 18:06:46 UTC+1 schreef Rustom Mody:
> On Monday, February 3, 2014 10:20:31 PM UTC+5:30, Jean Dupont wrote:
> > I'm looking at the way to address tuples
> > e.g.
> > tup2 = (1, 2, 3, 4, 5, 6, 7 );
> > As I found out indices start with 0 in Python, so 
> > tup2[0] gives me 1, the first element in the tuple as expected
> > tup2[1] gives me 2, the second element in the tuple as expected
> > now here comes what surprises me:
> > tup2[0:1] does not give me the expected (1,2) but (2,)
>
> Python 2.7.6 (default, Jan 11 2014, 17:06:02) 
> [GCC 4.8.2] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
> >>> tup2=(1,2,3,4,5,6,7)
> >>> tup2[0:1]
> (1,)
> >>> 
> So assuming you meant (1,) and wrote (2,) :-)
> > what is the reason for this and how then should one get the first and the second element of a tuple? Or the 3rd until the 5th?
> Generally ranges in python are lower-inclusive upper-exclusive
> What some math texts write as [lo, hi)
> So if you want from index 1 to 2-inclusive it is 1 to 3 exclusive
> tup2[0:2]
>
> See for motivations
> http://www.cs.utexas.edu/~EWD/transcriptions/EWD08xx/EWD831.html
> And one more surprising thing to note is that negatives count from the end
Thank you (and the others) for making this "logical"

kind regards,
jean

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#65380

On 2/3/2014 11:50 AM, Jean Dupont wrote:
> I'm looking at the way to address tuples
> e.g.
> tup2 = (1, 2, 3, 4, 5, 6, 7 );
>
> As I found out indices start with 0 in Python, so
> tup2[0] gives me 1, the first element in the tuple as expected
> tup2[1] gives me 2, the second element in the tuple as expected
> now here comes what surprises me:
> tup2[0:1] does not give me the expected (1,2) but (2,)
>
> what is the reason for this and how then should one get the first and the second element of a tuple? Or the 3rd until the 5th?
>
> thanks in advance and kind regards,

This should be covered in the tutorial, which you should read if you 
have not already.

-- 
Terry Jan Reedy

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#65395

On Mon, 03 Feb 2014 08:50:31 -0800, Jean Dupont wrote:

> I'm looking at the way to address tuples e.g.
> tup2 = (1, 2, 3, 4, 5, 6, 7 );
> 
> As I found out indices start with 0 in Python, so tup2[0] gives me 1,
> the first element in the tuple as expected tup2[1] gives me 2, the
> second element in the tuple as expected now here comes what surprises
> me:
> tup2[0:1] does not give me the expected (1,2) but (2,)
> 
> what is the reason for this and how then should one get the first and
> the second element of a tuple? Or the 3rd until the 5th?
> 
> thanks in advance and kind regards,
> 
> jean

>>> tup = (0,1,2,3,4,5,6,7)
>>> tup[0:1]
(0,)
>>> tup[1:2]
(1,)
>>> tup[2:3]
(2,)
>>> tup[0:2]
(0, 1)
>>> tup[2:5]
(2, 3, 4)

This is as I'd expect, as the notation [0:1] means:

starting from the 0th element, up to and including the element preceding 
the first element

Or [m:n] means starting from the mth element, everything up to and 
including the element preceding the nth element

Are you sure you got (2,) for [0:1] and not for [2:3]? Are you sure your 
initial tuple is what you thought it was?

-- 
Denis McMahon, denismfmcmahon@gmail.com

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