Are you thinking of the monadic case of amend?
]x0=:i.5
0 1 2 3 4
]x1=:10*x0
0 10 20 30 40
]b=:?.5#2
0 1 1 0 0
b}x0,:x1
0 10 20 3 4
Leigh J. Halliwell wrote:
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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--
Clifford A. Reiter
Mathematics Department, Lafayette College
Easton, PA 18042 USA, 610-330-5277
http://www.lafayette.edu/~reiterc
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