Re: Is subscripting a slice useful
> Date: Tue, 12 Oct 2004 02:03:10 +0100 From: Pete Lomax <petelomax at
> blueyonder.co.uk> Subject: Re: Is subscripting a slice useful On 11 Oct 2004
> 11:56:47 +0200, Christian Cuvier <Christian.CUVIER at agriculture.gouv.fr> wrote:
>
>>>You are using a typo from the original message to make your point
>
> Would that be the same typo mercilessly repeated no less than NINE
> times in a subsequent post? 
)
>
>>>about your personal dislike of this construct.
>
> I'm entitled to an opinion, last time I checked.
>
Of course, but using methods that rely on an error borders on the unfair.
Well, that's not the main point abyway.
>>>
>>
>>>>> There is *no* way that asdf[1..2] should return anything other than a
>>>>> sequence of length 2,
>>
>>>
>>>True
>>>
>>
>>>>> and hence a nested subscript of that could not
>>>>> possibly yield a sequence of length 4.
>>
>>>
>>>False, if the intermediate sequence is made of sequences itself.
>
> I don't understand what you mean. Please give me an example.
Let's call this question Q0, see below.
>>>
>>
>>>>> This whole idea (which has been extensively discussed before) is so
>>>>> open to misinterpretation it should definitely NOT be "standard".
>>>>> Every application needs something different.
>>
>>>
>>>Really? Most of them need vertical slicing anyway, so it is desirable for it
>>>to become a standard. Could you give some examples of different schemes that
>>>might be concurrently needed?
>
> The complexities this introduces worry me, and I fear it is not
> flexible enough to justify it. Suppose that
> c =
> {{{2,4,6,20}, {8,10,12,22}, {14,16,18,24}},
> {{3,6,9,30}, {12,15,18,33}, {21,24,27,36}},
> {{24,25,26,33}, {27,28,29,34}, {30,31,32,35}}}
>
> How do you get {8,14,12,21}?
>
You need the [] construct, which would mean "skip this level of sequencing".
Then you can go to this point by using c[1..2][2..3][][][1].
Not flexible enough, you said?
Slicing just restricts the range of available indexes; you need another
operator to reduce depth.
> For that matter, and to spoil your fun, what is c[1..2][2..3][1..1]?
> Can you explain to me why that should _not_ be
> {{{8,10,12,22}}, {{12,15,18,33}}} ?
Well, I can undestand how you got there, but it's not consistent:
c[1..2] is {{{2,4,6,20}, {8,10,12,22}, {14,16,18,24}},
{{3,6,9,30}, {12,15,18,33}, {21,24,27,36}}}
Apply [2..3] to it -> {{{8,10,12,22}, {14,16,18,24}},
{{12,15,18,33}, {21,24,27,36}}}
(answers Q0 btw)
Apply [1..1] to it -> {{{8}, {14}}, {{12}, {21}}}
Explanation is: c[1..2][2..3][1..1] restricts the first index level to lie
between 1 and 2, the second one between 2 and 3 and the third one between 1 and
1.
{{{8,10,12,22}}, {{12,15,18,33}}} would be (c[1..2][2..3])[1..2][1..1]
by considering the intermediate result above as a 2x2 matrix and operating on
it *as such*. Forgetting the parens would be an error, since there are 3 index
levels only, not 4.
> Also, do you believe that eg
>
> c[2..3][2][3..4]+=1
>
> should be supported?
No, but c[2..3][2..2][3..4]+=1 would. Result would be
{{{2,4,6,20}, {8,10,12,22}, {14,16,18,24}},
{{3,6,9,30}, {12,15,19,34}, {21,24,28,37}},
{{24,25,26,33}, {27,28,30,35}, {30,31,33,36}}}
c[2..3][2][3..4] is {{18,33},{29,34}}. You can add 1 to this, but then how to
plug it back in?
> If so, what should the result be?
>
>>>
>>>[snipped remainder]
>>>
>>>Consider a sequence with at least two levels, and look at how you access it.
>>>Each index specification (like [3]) just selects one element. Each slice
>>>specification (like [1..3]) selects a *range* of elements.
>>>Vertical slicing is not ambiguous if you consider that [3] and [3..3] are
>>>unambiguously referring to different objects. Slices operate on matrices
>>>(well, tensors in dimensions >2), and slices selectt individual elements as
>>>probably expected.
>>>
>>>To come back to the original example above:
>>>
>>>-asdf[1..2][1] would print {97,97}: select element #1 in all subsequences of
>>>asdf[1..2].
>>>-asdf[1..2][3..4] would print {{100,102},{100,102}}: slice [3..4] of all
>>>elements of asdf[1..2].
>>>-asdf[1][1..2] would print {97,115}, as usual.
>
> Yes, that's fine.
>
>>>
>>>Could you elaborate an example where this extended slicing, compatible with
>>>the current EU syntax, could lead to two different interpretations?
>
> Someone once posted that a[x..y][z] would just return a[x+z-1], and
> there is certainly some logical reasoning to that,
I hope so, but I can't grasp it atm.
> though of course it
> would be utterly pointless to make a change which works that way.
>
> Maybe something like:
> c =
> {{{1,1}, {1,1}, {1,1}}, {{1,1}, {1,1}, {1,1}}, {{1,1}, {1,1}, {1,1}}}
>
> c[2..3][2..3]+={2,3}
>
> could give
>
> {{{1,1}, {1,1}, {1,1}}, {{1,1}, {3,3}, {4,4}}, {{1,1}, {3,3}, {4,4}}}
>
> or
>
> {{{1,1}, {1,1}, {1,1}}, {{1,1}, {3,3}, {3,3}}, {{1,1}, {4,4}, {4,4}}}
>
> or
>
> {{{1,1}, {1,1}, {1,1}}, {{1,1}, {3,4}, {3,4}}, {{1,1}, {3,4}, {3,4}}}
>
> To be honest I have no idea which would be correct.
>
I'd say the second one. You get the third one by c[2..3][2..3]+={{2,3}}, and
the first one by c[2..3][2..3]+={{2},{3}}.
> Regards,
> Pete
Regards
CChris
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