function to assign value in a matrix (j-programming)

Question

I have two vectors (say, X and Y) which correspond to row and columns numbers. I want to write a function (a verb, in j-programming) that takes these and assign 1 in a n x n zero matrix. Here's for a simple case.

I have these vectors:

X=:1 2 1 5
Y=:0 3 3 9

and a zeros matrix:

mat=: 10 10$0 

and I wrote the following function (I used boxing):

   1(|:(,./<"0(|:(X,:Y)))) } 10 10$0

but the problem is it takes these vectors and assigns 1 to every column. So if I take (1,0) it assigns 1 to rows number 1 and 0 in all the columns (like this in Matlab (1,:) ). how can I overcome this problem?

Answer 1

I understand you to want to amend a boolean noun to put 1 at designated coordinates. You start with the coordinate pairs as separate lists. I recommend stitching those lists together like this:

   Y,.X
0 1
3 2
3 1
9 5

Y comes before X because in J axes are naturally arranged in decreasing sequence (that is, most fine-grained to the right.) To use these as coordinate pairs with Amend, they'll need to be boxed:

   <"1 Y,.X
+---+---+---+---+
|0 1|3 2|3 1|9 5|
+---+---+---+---+

Those will work with Amend to set 1 at those particular coordinates, so:

   1 (<"1 Y,.X)} 10 10$0
0 1 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0
0 1 1 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 1 0 0 0 0

If I've understood your question, this is the matrix you were looking to produce.