# What's the difference between these 2 statements?

Discussion in 'Python' started by ATSkyWalker, Apr 20, 2005.

1. ### ATSkyWalkerGuest

What's the difference between these 2 statements?

If you have a String s="12345"

s[len(s)::-1] = "54321"

But

s[len(s):0:-1] = "5432"

Why? What's the difference? What number then can I use as the end of
the slice if I were to supply all 3 parameters?

Thanks,
AT

ATSkyWalker, Apr 20, 2005

2. ### Reinhold BirkenfeldGuest

ATSkyWalker wrote:
> What's the difference between these 2 statements?
>
> If you have a String s="12345"
>
> s[len(s)::-1] = "54321"
>
> But
>
> s[len(s):0:-1] = "5432"
>
> Why? What's the difference? What number then can I use as the end of
> the slice if I were to supply all 3 parameters?

-1.

Reinhold

Reinhold Birkenfeld, Apr 20, 2005

3. ### tiissaGuest

Reinhold Birkenfeld wrote:
> ATSkyWalker wrote:
>
>>What's the difference between these 2 statements?
>>
>>If you have a String s="12345"
>>
>>s[len(s)::-1] = "54321"
>>
>>But
>>
>>s[len(s):0:-1] = "5432"
>>
>>Why? What's the difference? What number then can I use as the end of
>>the slice if I were to supply all 3 parameters?

>
>
> -1.

-len(s) or less.
-1 will return an empty string.

Actually you start from len(s)-1 (len(s) is not an index in s) and you
stop when you reach the index specified (or the end). Since -1 is the
same index as the starting one (-1~>len(s)-1, -2~>len(s)-2,
-len(s)+1~>0), you end up with an empty string.

Therefore you have to try to reach indices lower (due to the negative
step) than the minimum valid index of your list in order to reverse it
fully.

tiissa, Apr 20, 2005
4. ### Guest

s[len(s):-1:-1] yields an empty list !

Test code :

s = "12345"
print s[len(s)::-1] -> prints "54321"
print s[len(s):-1:-1] -> prints "" (nothing)

, Apr 20, 2005
5. ### Reinhold BirkenfeldGuest

tiissa wrote:
> Reinhold Birkenfeld wrote:
>> ATSkyWalker wrote:
>>
>>>What's the difference between these 2 statements?
>>>
>>>If you have a String s="12345"
>>>
>>>s[len(s)::-1] = "54321"
>>>
>>>But
>>>
>>>s[len(s):0:-1] = "5432"
>>>
>>>Why? What's the difference? What number then can I use as the end of
>>>the slice if I were to supply all 3 parameters?

>>
>>
>> -1.

>
> -len(s) or less.
> -1 will return an empty string.
>
> Actually you start from len(s)-1 (len(s) is not an index in s) and you
> stop when you reach the index specified (or the end). Since -1 is the
> same index as the starting one (-1~>len(s)-1, -2~>len(s)-2,
> -len(s)+1~>0), you end up with an empty string.
>
> Therefore you have to try to reach indices lower (due to the negative
> step) than the minimum valid index of your list in order to reverse it
> fully.

Right, sorry.

Well, I guess that's why one can leave out the index...

Reinhold

Reinhold Birkenfeld, Apr 20, 2005
6. ### Guest

I'm sorry, I'm not really following your logic. Can you supply the
statement with the three parameters ?

so if I want to reverse it fully using s[len(s)-1:x:-1] what would x be
or is it impossible to express it in this way ?

Thanks,
AT

, Apr 20, 2005
7. ### Peter OttenGuest

wrote:

> so if I want to reverse it fully using s[len(s)-1:x:-1] what would x be
> or is it impossible to express it in this way ?

This does not work for integers, because the theoretically correct value
x = -1 already has another interpretation as the gap between the last and
the last but one character. Here are two workarounds:

1. Set x to None

>>> s = "12345"
>>> s[len(s):None:-1]

'54321'

2. Separate slicing operation and reversal:

>>> s = "12345"
>>> s[0:len(s)][::-1]

'54321'

Peter

Peter Otten, Apr 20, 2005
8. ### ATSkyWalkerGuest

Peter,

I like the way you put it "the gap between the last and
the last but one character" .

I guess this is a side effect of of python's asymetric slice indexing
approach which takes a little getting used to.

AT

ATSkyWalker, Apr 20, 2005
9. ### tiissaGuest

wrote:
> I'm sorry, I'm not really following your logic. Can you supply the
> statement with the three parameters ?
>
> so if I want to reverse it fully using s[len(s)-1:x:-1] what would x be
> or is it impossible to express it in this way ?

Contrary to what I said above x should be _strictly_ less than -len(s).
You stop when you reach in the list the given end index (and don't take
the item there) or if you leave the index range.

But -1 as an index is the same as (len(s)-1).
Therefore going from len(s)-1 down to -1 is the same as going from
len(s)-1 to len(s)-1 hence an empty list.

And -len(s) is the same as 0 (my mistake above)
But -len(s)-1 is not in the list thus you won't discard any limit.

The example:
In [1]: s='12345'

In [2]: s[len(s)-1:0:-1]
Out[2]: '5432'

In [3]: s[len(s)-1:-1:-1]
Out[3]: ''

In [4]: s[-1],s[len(s)-1]
Out[4]: ('5', '5')

In [5]: s[len(s)-1:-len(s)-1:-1]
Out[5]: '54321'

In [6]: s[len(s)-1:-len(s):-1]
Out[6]: '5432'

tiissa, Apr 20, 2005
10. ### tiissaGuest

Peter Otten wrote:
> wrote:
>
>
>>so if I want to reverse it fully using s[len(s)-1:x:-1] what would x be
>>or is it impossible to express it in this way ?

>
>
> This does not work for integers, because the theoretically correct value
> x = -1 already has another interpretation as the gap between the last and
> the last but one character.

AFAIK, it is not an issue of integer (what else can an slice index be in
python?) but simply of index aliasing.

For x=-len(s)-1, you get the whole reversed list:

In [5]: s[len(s)-1:-len(s)-1:-1]
Out[5]: '54321'

tiissa, Apr 20, 2005
11. ### Peter OttenGuest

tiissa wrote:

> Peter Otten wrote:
>> wrote:
>>
>>
>>>so if I want to reverse it fully using s[len(s)-1:x:-1] what would x be
>>>or is it impossible to express it in this way ?

>>
>>
>> This does not work for integers, because the theoretically correct value
>> x = -1 already has another interpretation as the gap between the last and
>> the last but one character.

> AFAIK, it is not an issue of integer (what else can an slice index be in
> python?) but simply of index aliasing.
>
> For x=-len(s)-1, you get the whole reversed list:
>
> In [5]: s[len(s)-1:-len(s)-1:-1]
> Out[5]: '54321'

Clever. I didn't think of that.
Still, for practical purposes you have to test for slicelen >= stringlen, so
whether you choose None, -len(s)-1, or -sys.maxint as the second slice
parameter doesn't matter much.

Peter

Peter Otten, Apr 20, 2005
12. ### tiissaGuest

Peter Otten wrote:
> Still, for practical purposes you have to test for slicelen >= stringlen, so
> whether you choose None, -len(s)-1, or -sys.maxint as the second slice
> parameter doesn't matter much.

Sure, for practical purposes you don't bother to write extra characters
and leave it void.
But we knew it from the start of the thread.

tiissa, Apr 20, 2005