Convert a vector into logical matrix

Question

Is there a native R function that will take an input vector and return the corresponding binary matrix where the matrix has the same number of columns as unique values in the in

Answer 1

model.matrix() might help here, but you need to suppress the intercept:

> model.matrix(~ factor(1:3) - 1)
  factor(1:3)1 factor(1:3)2 factor(1:3)3
1            1            0            0
2            0            1            0
3            0            0            1
attr(,"assign")
[1] 1 1 1
attr(,"contrasts")
attr(,"contrasts")$`factor(1:3)`
[1] "contr.treatment"

Something slightly more complex:

> set.seed(1)
> fac <- factor(sample(1:3, 10, replace = TRUE))
> model.matrix(~ fac - 1)
   fac1 fac2 fac3
1     1    0    0
2     0    1    0
3     0    1    0
4     0    0    1
5     1    0    0
6     0    0    1
7     0    0    1
8     0    1    0
9     0    1    0
10    1    0    0
attr(,"assign")
[1] 1 1 1
attr(,"contrasts")
attr(,"contrasts")$fac
[1] "contr.treatment"

Answer 2

Actually, contrasts is what you want.

contrasts(as.factor(1:3), contrasts=FALSE)

  1 2 3
1 1 0 0
2 0 1 0
3 0 0 1