[R] Accessing values of a matrix

[statquant]

hello all,
thank you for taking the time

I have a matrix A that have column names (let say n columns), I want to
reduce the matrix to have just a few of those column (p colums, this is
trivial), but for the lines I want only the lines such that A(i,J) is part
of a list (J is fixed and known)

I am sure it is very easy but I don't find it (I tryed which but it doesn't
seem to work)
Surely I could do a loop but I want to learn how to do things without time
consuming loops.

Thanks
Colin
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