R:一个矩阵的子集，只有在某一列中具有一定值的行

I have a large matrix "dt" of emergency department visits over 2 months for a set of diagnosis codes. The columns are "age", "sex", "date", "county", "zip", "subjectid", "position", "diag", and "dt"; the dimensions are 872344 by 9. I have a large matrix "dt" of emergency departm