Search code examples
rmatrixvectorfrequency

Find frequency of vector elements in a matrix

I have a matrix in R, here is a small example:

set.seed(1)
n.columns<-6
mat <- matrix(, nrow = 5, ncol = n.columns)
for(column in 1:n.columns){
  mat[, column] <- sample(1:10,5)
}
mat

The matrix looks like this:

     [,1] [,2] [,3] [,4] [,5] [,6]
[1,]    3    9    3    5   10    4
[2,]    4   10    2    7    2    1
[3,]    5    6    6    8    6   10
[4,]    7    5   10    3    1    7
[5,]    2    1    5   10    9    3

I also have a vector v of integers, v<-c(1,3,6), whose elements could theoretically appear in the matrix mat above.

What I am looking for is an overview of the number of times that each element in v appears in mat per column. For the current example this overview is

1: 0 1 0 0 1 1
3: 1 0 1 1 0 1
6: 0 1 1 0 1 0

It is fairly straightforward to do this using for-loops and if-statements, but this solution is not very pretty.

Is there a professional way to do this?

Solution

  • One option using sapply:

    t(sapply(v, function(a) colSums(mat==a)))

#     [,1] [,2] [,3] [,4] [,5] [,6]
#[1,]    0    1    0    0    1    1
#[2,]    1    0    1    1    0    1
#[3,]    0    1    1    0    1    0