How to turn a list of lists to a sparse matrix in R without using lapply?

Question

I have a list of lists resulting from a bigsplit() operation (from package biganalytics, part of the bigmemory packages).

Each list represents a column in a matrix, and each list item is an index to a value of 1 in a binary matrix.

What is the best way to turn this list into a sparse binary (0/1) matrix? Is using lapply() within an lapply() the only solution? How do I keep the factors naming the lists as names for the columns?

Answer 1

You might also consider using the Matrix package which deals with large sparse matrices in a more efficient way than base R. You can build a sparse matrix of 0s and 1s by describing which rows and columns should be 1s.

library(Matrix)
Test <- list(
    col1=list(2,4,7),
    col2=list(3,2,6,8),
    col3=list(1,4,5,3,7)
)
n.ids <- sapply(Test,length)
vals <- unlist(Test)
out <- sparseMatrix(vals, rep(seq_along(n.ids), n.ids))

The result is

> out
8 x 3 sparse Matrix of class "ngCMatrix"

[1,] . . |
[2,] | | .
[3,] . | |
[4,] | . |
[5,] . . |
[6,] . | .
[7,] | . |
[8,] . | .