You have drawn incorrect conclusions from your
experiments, or you did not do sufficient experiments.
x=: 4 (1000 [EMAIL PROTECTED] 2e9)} 1 $. 2e9
y=: 4 (1000 [EMAIL PROTECTED] 2e9)} 1 $. 2e9
3 $. x = y
1
3 $. x ~: y
0
3 $. 2 + x
2
3 $. x - 1
_1
3 $. 0 > x - 1
1
3 $. ^ x
1
3 $. ^. x
__
If x and y are sparse and f is atomic, then
(3 $. f y) = f 3$.y
(3 $. x f y) = (3$.x) f 3$.y
that is, the sparse element of f y is f applied
to the sparse element of y; likewise the sparse
element of x f y is (sparse element of x) f
(sparse element of y).
You can use (3;e)$.y to specify that the sparse
element should be e and 8$.y to "remove" the
sparse items. However, both of these are "after
the fact". You should avoid dense f sparse and
sparse f dense if you want to keep everything
sparse.
t=: (1e6 [EMAIL PROTECTED] 100) > $. 1e6{.1
+/t
989986
t
0 │ 1
1 │ 1
2 │ 1
3 │ 1
4 │ 1
...
q=: 8 $. (3;1) $. t
7!:5 ;:'t q'
5243072 320
----- Original Message -----
From: Michael Berry <[EMAIL PROTECTED]>
Date: Saturday, December 16, 2006 1:40 pm
Subject: [Jprogramming] Is there a way I can let J know what the sparse element
ought to be for the result of a boolean comparison?
> Dear J experts,
>
>
>
> In the course of trying to figure out why I sometimes get "out of
> memory"errors when I didn't think I had done anything to cause my
> sparse arrays to
> become dense, I became curious about how J knows what the right sparse
> element will be for the result of a Boolean function where one
> argument is a
> sparse integer array. By experiment, I see that if I compare my
> sparsearray to a scalar argument, the resulting Boolean array has
> the appropriate
> sparse element, namely the result of comparing the integer array's
> sparseelement with the scalar argument:
>
>
>
> v1 =.$.100{.3
>
> v2=.$._100{.3
>
> b1=.v1<3
>
> b2=.v2<3
>
> b1
>
> 0 | 0
>
> b2
>
> 99 | 0
>
> b3=.v1>0
>
> b4=.v2>0
>
> b3
>
> 0 | 1
>
> b4
>
> 99 | 1
>
>
>
> If, on the other hand, I have an array argument, the sparse
> element of the
> resulting array is always 0:
>
>
>
> v5=. ?. 100$10
>
> b1 =. v1>v5
>
> b2 =. v1<v5
>
> 3 $. b1
>
> 0
>
> 3 $. b2
>
> 0
>
> +/b1
>
> 0
>
> +/b2
>
> 94
>
>
>
> This explains my out of memory problem. b2 is mostly 1, but its
> sparseelement is 0. It is in the nature of the comparisons I am
> doing, that I
> expect my results to be either mostly 1 or mostly 0 and I know in
> advancewhich. I can, of course, rephrase my comparisons so that
> the "mostly" case
> is always 0, but this sometimes seems unnatural. I don't suppose
> there is a
> way I could whisper in J's ear to say I'm expecting mostly 1?
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