R: List of indices, including empty ones, to binary matrix -

say have list of indices, including elements empty, like:

l <- list(c(1,2,3), c(1), c(1,5), null, c(2, 3, 5)) 

which specify non-zero elements in matrix, like:

(m <- matrix(c(1,1,1,0,0, 1,0,0,0,0, 1,0,0,0,5, 0,0,0,0,0, 0,1,1,0,1), nrow=5, byrow=true))      [,1] [,2] [,3] [,4] [,5] [1,]    1    1    1    0    0 [2,]    1    0    0    0    0 [3,]    1    0    0    0    5 [4,]    0    0    0    0    0 [5,]    0    1    1    0    1 

what fastest way, using r, make m l, giving matrix big, 50.000 rows , 2000 columns?

you can filter non-null list elements 'l' , use melt reshape 'data.frame' 'key/value' columns or `row/column' index columns.

library(reshape2) d2 <- melt(filter(negate(is.null), setnames(l, seq_along(l)))) un1 <- unlist(l) m1 <- matrix(0, nrow=length(l), ncol=max(un1)) m1[cbind(as.numeric(d2[,2]), d2[,1])] <- 1 m1 #     [,1] [,2] [,3] [,4] [,5] #[1,]    1    1    1    0    0 #[2,]    1    0    0    0    0 #[3,]    1    0    0    0    1 #[4,]    0    0    0    0    0 #[5,]    0    1    1    0    1 

or

 library(matrix)  as.matrix(sparsematrix(as.numeric(d2[,2]), d2[,1], x=1))  #     [,1] [,2] [,3] [,4] [,5] #[1,]    1    1    1    0    0 #[2,]    1    0    0    0    0 #[3,]    1    0    0    0    1 #[4,]    0    0    0    0    0 #[5,]    0    1    1    0    1