[R] why it returns list level number instead of its content?

[Richard M. Heiberger]

The class "factor" is defined for vectors, not matrices.
The attempt to use a factor in a matrix setting coerces it to numeric.
See the documentation for factor where it says
"In particular, as.numeric applied to a factor is meaningless,
and may happen by implicit coercion."

> tmp <- matrix(1:8, 4)
> tmp
     [,1] [,2]
[1,]    1    5
[2,]    2    6
[3,]    3    7
[4,]    4    8
> factor(tmp)
[1] 1 2 3 4 5 6 7 8
Levels: 1 2 3 4 5 6 7 8
> 


## A simplified example based on yours:

> tmp <- data.frame(a=factor(letters[1:4]), b=factor(letters[5:8]))
> tmp
  a b
1 a e
2 b f
3 c g
4 d h

> cbind(tmp$a, tmp$b)  ## coerced to numeric, both factors went to 1:4
     [,1] [,2]
[1,]    1    1
[2,]    2    2
[3,]    3    3
[4,]    4    4

> tmp[, c("a","b")]  ## in your example, subscripting is probably the right method
  a b
1 a e
2 b f
3 c g
4 d h

> cbind.data.frame(tmp$a, tmp$b)  ## explicit use of cbind.data.frame works.
  tmp$a tmp$b
1     a     e
2     b     f
3     c     g
4     d     h

Rich