Search code examples
rmatrixsparse-matrixkronecker-product

Create matrix of 1s from array of counts

Given an array of length 6 of 3 unique numbers, say:

c(1,1,1,2,3,3)

How can I construct the following 6x3 matrix:

[1, 0 , 0]  
[1, 0 , 0]  
[1, 0 , 0]  
[0, 1 , 0]  
[0, 0, 1]  
[0, 0, 1]

A naive way would be:

V = c(1,1,1,2,3,3)
Z = matrix(0,6,3)
for (i in 1:6) {Z[i, V[i]] = 1}

but is there a nice one liner for this type of operation?

Solution

  • We may use model.matrix in base R

    model.matrix(~ . - 1, data.frame(v1 = factor(v1)))

    -output

       v11 v12 v13
1   1   0   0
2   1   0   0
3   1   0   0
4   0   1   0
5   0   0   1
6   0   0   1

    Or use dummy_cols from fastDummies

    library(fastDummies)
dummy_cols(v1)[-1]
  .data_1 .data_2 .data_3
1       1       0       0
2       1       0       0
3       1       0       0
4       0       1       0
5       0       0       1
6       0       0       1

    data

    v1 <- c(1,1,1,2,3,3)