How to delete columns in a dataset that have binomial variables - r

Question

The dataset named data has both categorical and continuous variables. I would like to the delete categorical variables.
I tried:
data.1 <- data[,colnames(data)[[3L]]!=0]
No error is printed, but categorical variables stay in data.1. Where are problems ?
The summary of "head(data)" is
id 1,2,3,4,...
age 45,32,54,23,...
status 0,1,0,0,...
...
(more variables like as I wrote above)
All variables are defined as "Factor".

Answer 1

What are you trying to do with that code? First of all, colnames(data) is not a list so using [[]] doesn't make sense. Second, The only thing you test is whether the third column name is not equal to zero. As a column name can never start with a number, that's pretty much always true. So your code translates to :
data1 <- data[,TRUE]
Not what you intend to do.
I suppose you know the meaning of binomial. One way of doing that is defining your own function is.binomial() like this :
is.binomial <- function(x,na.action=c('na.omit','na.fail','na.pass'){
FUN <- match.fun(match.arg(na.action))
length(unique(FUN(x)))==2
}
in case you want to take care of NA's. This you can then apply to your dataframe :
data.1 <- data[!sapply(data,is.binomial)]
This way you drop all binomial columns, i.e. columns with only two distinct values.

Answer 2

#Shimpei Morimoto,
I think you need a different approach.
Are the categorical variables defines in the dataframe as factors?
If so you can use:
data.1 <- data[,!apply(data,2,is.factor)]
The test you perform now is if the colname number 3L is not 0.
I think this is not the case.
Another approach is
data.1 <- data[,-3L]
works only if 3L is a number and the only column with categorical variables

Answer 3

I think you're getting there, with your last comment to #Mischa Vreeburg. It might make sense (as you suggest) to reformat your original data file, but you should also be able to solve the problem within R. I can't quite replicate the undefined columns error you got.
Construct some data that look as much like your data as possible:
X <- read.csv(textConnection(
"id,age,pre.treat,status
1,'27', 0,0
2,'35', 1,0
3,'22', 0,1
4,'24', 1,2
5,'55', 1,3
, ,yes(vs)no,"),
quote="\"'")
Take a look:
str(X)
'data.frame': 6 obs. of 4 variables:
$ id : int 1 2 3 4 5 NA
$ age : int 27 35 22 24 55 NA
$ pre.treat: Factor w/ 3 levels " 0"," 1","yes(vs)no": 1 2 1 2 2 3
$ status : int 0 0 1 2 3 NA
Define #Joris Mey's function:
is.binomial <- function(x,na.action=c('na.omit','na.fail','na.pass')) {
FUN <- match.fun(match.arg(na.action))
length(unique(FUN(x)))==2
}
Try it out: you'll see that it does not detect pre.treat as binomial, and keeps all the variables.
sapply(X,is.binomial)
X1 <- X[!sapply(X,is.binomial)]
names(X1)
## keeps everything
We can drop the last row and try again:
X2 <- X[-nrow(X),]
sapply(X2,is.binomial)
It is true in general that R does not expect "extraneous" information such as level IDs to be in the same column as the data themselves. On the one hand, you can do even better in the R world by simply leaving the data as their original, meaningful values ("no", "yes", or "healthy", "sick" rather than 0, 1); on the other hand the data take up slightly more space if stored as a text file, and, more important, it becomes harder to incorporate other meta-data such as units in the file along with the data ...