Finding the frequency of numbers in an array in R

Question

I need to find frequency of numbers say 0:8. in an every column of matrix A of order mxn.

How can I do that?

thanks in advance

Answer 1

You can use apply with MARGIN=2 to loop through the columns, subset the elements that are 0:8 ( x %in% 0:8 ), convert to factor with levels specified as 0:8 and use table to get the frequency of elements.

apply(A, 2, function(x) table(factor(x[x %in% 0:8], levels=0:8)))

Or another option would be to melt the matrix and convert to data.table using setDT , subset 0:8 ( J(0:8) ) from the "value" column after the setting the "value" column as key ( setkey ), group by "Var2", change the "value" column to "factor" class and get the frequency with table

library(data.table)
setkey(setDT(melt(A)), value)[J(0:8), 
      as.list(table(factor(value, levels=0:8))), by= Var2]

data

set.seed(24)
A <- matrix(sample(0:15, 10*20, replace=TRUE), ncol=10)