Index value for matrix in R?

r matrix filtering
Shane picture Shane · Dec 1, 2009 · Viewed 43k times · Source

Is there a function to get an index (row number and column number) for a matrix?

Suppose that I have a simple matrix:

a <- matrix(1:50, nrow=5)

Is there an easy way to get back something like c(3, 5) for the number "23", for instance? In this case, saying which(a==23) just returns 23.

This seems to work but I'm sure that there's a better way:

matrix.index <- function(a, value) {
  idx <- which(data.frame(a)==value)
  col.num <- ceiling(idx/nrow(a))
  row.num <- idx - (col.num-1) * nrow(a)
  return(c(row.num, col.num))
}
> matrix.index(a, 23)
[1] 3 5
> matrix.index(a, 50)
[1]  5 10

Answer

Shane picture Shane · Dec 2, 2009

Just looked at the help for which() after posting this and found the answer: the arr.ind parameter.

which(a==23, arr.ind=TRUE)
     row col
[1,]   3   5

Related questions

R memory management / cannot allocate vector of size n Mb

I am running into issues trying to use large objects in R. For example: > memory.limit(4000) > a = matrix(NA, 1500000, 60) > a = matrix(NA, 2500000, 60) > a = matrix(NA, 3500000, 60) Error: cannot allocate vector of size 801.1 Mb > a = matrix(NA, 2500000, 60) …

Read More
Subscript out of bounds - general definition and solution?

When working with R I frequently get the error message "subscript out of bounds". For example: # Load necessary libraries and data library(igraph) library(NetData) data(kracknets, package = "NetData") # Reduce dataset to nonzero edges krack_full_nonzero_edges <- …

Read More
Select rows of a matrix that meet a condition

In R with a matrix: one two three four [1,] 1 6 11 16 [2,] 2 7 12 17 [3,] 3 8 11 18 [4,] 4 9 11 19 [5,] 5 10 15 20 I want to extract the submatrix whose rows have column three = 11. That is: one two three four [1,] 1 6 11 16 [3,] 3 8 11 18 [4,] 4 9 11 19 I want to do this without looping. I am new to R so this …

Read More