[R] Adding new column to data frame and filling some rows of it - classes

[Gorjanc Gregor]

Hello!

Few days ago I was asking on a list about adding a column and filling
it in some rows. I was satisfied, but one thing raised my attention. I
will show itthrough an example:

tmp <- data.frame(y1=1:4, f1=factor(c("A", "B", "C", "D")))
f2 <- factor(c("Z", "Y"))

# I would like to add f2 to tmp and I know that values in f2 fit to
# first two values in tmp. I used

tmp$f2 <- NA
tmp[1:2, "f2"] <- f2
tmp
  y1 f1 f2
1  1  A  2
2  2  B  1
3  3  C NA
4  4  D NA

# However tmp$f2 is not factor. How can I make it to be a factor? I tried
# with 
class(tmp$f2) <- "factor"

# but I get this
tmp
  y1 f1   f2
1  1  A NULL
2  2  B <NA>
3  3  C <NA>
4  4  D <NA>
Warning message: 
corrupt data frame: columns will be truncated or padded with NAs in: format.data.frame(x, digits = digits) 

# I tried the other approach
tmp <- data.frame(y1=1:4, f1=factor(c("A", "B", "C", "D")))
f2 <- factor(c("Z", "Y"))
tmp$f2 <- f2
tmp[3:4, "f2"] <- NA
tmp
  y1 f1   f2
1  1  A    Z
2  2  B    Y
3  3  C <NA>
4  4  D <NA>

class(tmp$f2)
[1] "factor"

# And I have now the same class for tmp$f2 as in f2. Is this the
# only way?

--
Lep pozdrav / With regards,
    Gregor GORJANC

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University of Ljubljana
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