How do I stack only some columns in a data frame?

Question

I have some data in a data frame in the following form:

A  B  C  V1  V2  V3
1  1  1  x   y   z
1  1  2  a   b   c
...

Where A,B,C are facto

Answer 1

Using reshape2 package

dat <- read.table(text = 'A  B  C  V1  V2  V3
1  1  1  x   y   z
1  1  2  a   b   c',header= T)
library(reshape2)
melt(dat,id.vars = c('A','B','C'))
 A B C variable value
1 1 1 1       V1     x
2 1 1 2       V1     a
3 1 1 1       V2     y
4 1 1 2       V2     b
5 1 1 1       V3     z
6 1 1 2       V3     c