RE: [Jprogramming] Choose Operators

[Leigh J. Halliwell]

Björn, that’s a nice solution to my first question; however, I want to know
if there’s a primitive operator for it, something like X SYMBOL Y. And can
anyone help with my second question?  Thanks.
 
Leigh
 
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Björn Helgason
Sent: Tuesday, October 24, 2006 11:39 AM
To: Programming forum
Subject: Re: [Jprogramming] Choose Operators
 
Is
(0 0;2 1;1 2;0 3){Y
 
What you are looking for?
 
2006/10/24, Leigh J. Halliwell <[EMAIL PROTECTED]>:
> 
> Dear J Forum:
> 
> I have two questions about selecting, or choosing.
> 
> 1) There is a m-element vector X whose elements belong to i. n.  And there
> is an (mxn) matrix Y.  I want the n-element vector Z whose jth element is
> (X(j), j).  In other words, X tells the rows of the elements to choose
> from
> the columns of Y.  As an example:
> 
> X =. 0 2 1 0
> Y =. i. 3 4
> Then Z =. 0 9 6 3
> 
> I believe that I once saw a J primitive that does this; but I can't find
> it
> now.
> 
> 2) X0 and X1 are numeric vectors, and B is a Boolean vector.  They all
> have
> the same length.  I'd like to select from X0 where B is 0, and from X1
> where
> B is 1.  I try the expression:
> 
> X1 [^:B X0
> 
> But the adverb ^:B wants to get two-dimensional, i.e., to apply each B to
> every pair of X1 and X2.  How can I make J to apply the adverb one-to-one
> with the arguments?  Thanks.
> 
> Sincerely,
> 
> Leigh
> ----------------------------------------------------------------------
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> 
 
 
 
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Björn Helgason, Verkfræðingur
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