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Re: [Bug-apl] Supporting negative ranks for ⍤ operator

From: Elias Mårtenson
Subject: Re: [Bug-apl] Supporting negative ranks for ⍤ operator
Date: Wed, 27 Apr 2016 16:17:51 +0800

Thanks, that's interesting.

However, I'm still a big confused about the 3-value case. When is P, Q and R all used?

Secondly, do you agree that that negative ranks would be useful?


On 27 April 2016 at 16:02, Jay Foad <address@hidden> wrote:
You're reading section 9.3.4 "Rank operator deriving monadic
function". You also need to look at 9.3.5 "Rank operator deriving
dyadic function".

Given g ← f⍤P Q R:
P is the monadic rank
Q is the left rank
R is the right rank

g Y applies g to the P-cells of Y
X g Y applies g to the Q-cells of X and the R-cells of Y

The ⌽3⍴⌽y1 stuff is just a too-cute way of saying that you can specify
fewer than 3 values in the right operand, and:
R is shorthand for R R R
Q R is shorthand for R Q R


On 27 April 2016 at 08:28, Elias Mårtenson <address@hidden> wrote:
> About the ISO specification of ⍤
> In writing the above message, I was reading the ISO specification for the
> rank operator, and I find it incredibly confusing. I have quoted the
> description below, and based on my reading of this text, the rank parameter
> is not just a single value, but can be up to three values. However, no
> matter how I read it, I still can't see how any but the the very first value
> is actually every used.
> Also, the case where LENGTH ERROR is supposed to be raised does not happen
> in GNU APL.
> It seems as the specification for the rank operator is just broken on
> several levels in the spec. That seems to me to be reason enough to not pay
> attention to the spec in this case and just adapt the way Dyalog does it.
> Here's the spec for the rank operator from the ISO spec:
> Informal Description:
> The result of f⍤y is a function which, when applied to B, returns Z, the
> result of applying the function f to the rank-y cells of B.
> Evaluation Sequence:
> If y is a scalar, set y1 to ,y. Otherwise set y1 to y.
> If y1 is not a vector, signal domain-error.
> If y1 has more than three elements, signal length-error.
> If any element of y1 is not a near-integer, signal domain-error.
> Set y2 to ⌽3⍴⌽y1.
> Set y3 to the first-item in y2.
> Set y4 to the integer-nearest-to y3.
> If y4 exceeds the rank of B, set y5 to the rank of B, otherwise set y5 to
> y4.
> If y5 is negative, set y6 to 0⌈y5 plus the rank of B, otherwise set y6 to
> y5.
> Apply f to the rank-y6 cells of B.
> Conform the individual result cells. Let their common shape after conforming
> be q, and let p be the frame of B with respect to f, that is, (rank of B)
> minus y6, and
> return the overall result with shape p,q.

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