I want to pull all rows from a boxed table that I've created in J, based
on specific values in certain columns. This is similar
to using WHERE <colvalue> = <value> to formulate SQL queries. In J I
can do this by hand using standard J operators. Now I would like to pull
it into a dyadic verb but I'm having trouble deciding the most common
way to formulate the arguments. The y value is easy it will the variable
that holds the table data (in this case I call it "dat"). But now having
used my y argument I now need to stuff into the x argument the column
numbers I want to base selection from and the strings that will be used
to test for equality. Just so you get a feeling for what I'm trying to
do the by hand formulation is as follows:
((2 }"1 dat) = <, 'Tom') *. (4}"1 dat) = <, 'McGuie'
The above giving me a boolean array where the ones represent the rows
where col 2 = Tom and Col4 =McGuire. Then I can just apply the answer
with copy (#) to give me a new array with just those rows that are
equal in col2 and col4.
So is there a better way to organize this? Do you make the table "dat"
global freeing up the y argument? I can certainly brute force this but
I'm hoping for a more elegant solution that will be applicable to future
problem spaces.
Tom McGuire
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