find indices of non zero elements in matrix

Question

I want to get the indices of non zero elements in a matrix.for example

X <- matrix(c(1,0,3,4,0,5), byrow=TRUE, nrow=2); 

should give me something like this

row col 1    1 1    3 2    1 2    3 

Can any one please tell me how to do that?

Answer 1

which(X!=0,arr.ind = T)      row col [1,]   1   1 [2,]   2   1 [3,]   1   3 [4,]   2   3 

If arr.ind == TRUE and X is an array, the result is a matrix whose rows each are the indices of the elements of X

Answer 2

There's an error in your example code - True is not defined, use TRUE.

X <-matrix(c(1,0,3,4,0,5), byrow = TRUE, nrow = 2) 

which should do it:

which(!X == 0) X[ which(!X == 0)] #[1] 1 4 3 5 

to get the row/col indices:

 row(X)[which(!X == 0)]  col(X)[which(!X == 0)] 

to use those to index back into the matrix:

   X[cbind(row(X)[which(!X == 0)], col(X)[which(!X == 0)])]    #[1] 1 4 3 5