make.sparseDJB.R

# Copyright (C) 2010 Jelmer Ypma. All Rights Reserved.
# This code is published under the Eclipse Public License.
#
# File:   make.sparse.R
# Author: Jelmer Ypma
# Date:   18 April 2010
# Revised by Don Boyd May 18, 2015. This version is MUCH faster than the original, which is commented out below
#
# Input:  matrix with logical elements
# Output: list with as elements a vector of indices denoting non-zero (TRUE) elements of the matrix

make.sparse <- function(A) {
  f <- function(x) which(x!=0, arr.ind=TRUE)
  rownames(A) <- NULL # just to be safe
  S <- apply(A, 1, f)
  S <- as.list(S)
  return(S)
}




# make.sparse <- function( A ) {
#   
#   # start with empty list to append to
#   S <- list()    
#   
#   # loop over matrix by row 
#   for ( i in 1:nrow(A) ) {
#     indices <- c()
#     for ( j in 1:ncol(A) ) {
#       if (A[i,j]) {
#         indices <- c( indices, j )
#       }
#     }
#     S <- c( S, list(indices) )
#   }
#   
#   return( S )
# }