How to delete specific rows and columns from a matrix in a smarter way?

You can do:

t1<- t1[-4:-6,-7:-9]

You can use

t1<- t1[-4:-6,-7:-9]  

or

t1 <- t1[-(4:6), -(7:9)]

or

t1 <- t1[-c(4, 5, 6), -c(7, 8, 9)]

You can pass vectors to select rows/columns to be deleted. First two methods are useful if you are trying to delete contiguous rows/columns. Third method is useful if You are trying to delete discrete rows/columns.

> t1 <- array(1:20, dim=c(10,10));

> t1[-c(1, 4, 6, 7, 9), -c(2, 3, 8, 9)]

     [,1] [,2] [,3] [,4] [,5] [,6]
[1,]    2   12    2   12    2   12
[2,]    3   13    3   13    3   13
[3,]    5   15    5   15    5   15
[4,]    8   18    8   18    8   18
[5,]   10   20   10   20   10   20