Extracting off-diagonal slice of large matrix

Question

I\'ve got a large nxn matrix and would like to take off-diagonal slices of varying sizes. For example:

1 2 3 4 5 6
1 2 3 4 5 6
1 2 3 4 5 6
1 2 3 4 5 6
1 2 3          


        

Answer 1

you can do:

matrix:

m<-
matrix(1:6,ncol = 6, nrow=6 ,byrow = T)

function:

n_diag <- function (x, n) {
    d <- dim(x)
    ndiag <- .row(d) - n >= .col(d)
    x[upper.tri(x) | ndiag] <- NA
    return(x)
}

call:

n_diag(m,3)

#     [,1] [,2] [,3] [,4] [,5] [,6]
#[1,]    1   NA   NA   NA   NA   NA
#[2,]    1    2   NA   NA   NA   NA
#[3,]    1    2    3   NA   NA   NA
#[4,]   NA    2    3    4   NA   NA
#[5,]   NA   NA    3    4    5   NA
#[6,]   NA   NA   NA    4    5    6

just for fun:

#lapply(1:6, n_diag, x = m)