Fast way to create a binary matrix with known number of 1 each row in R

Question

I have a vector that provides how many \"1\" each row of a matrix has. Now I have to create this matrix out of the vector.

For example, let say I want to create a 4 x 9

Answer 1

vapply is usually faster than sapply. This assigns the desired number of ones to a length-9 vector and then transposes.

> t( vapply( c(2,6,3,9), function(y) { x <- numeric( length=9); x[1:y] <- 1;x}, numeric(9) ) )
     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9]
[1,]    1    1    0    0    0    0    0    0    0
[2,]    1    1    1    1    1    1    0    0    0
[3,]    1    1    1    0    0    0    0    0    0
[4,]    1    1    1    1    1    1    1    1    1

Less than 5 seconds on an old Mac.

 system.time( M <- t( vapply( sample(1:500, 100000, rep=TRUE), function(y) { x <- numeric( length=500); x[1:y] <- 1;x}, numeric(500) ) ) )
   user  system elapsed 
  3.531   1.208   4.676