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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