Retain row names for subsets of csv data - r

Question

I have some data in a csv file, which includes row names. I want to take a single column of the data, while retaining the row names. The csv file was produced in the following manner:
MAT <- matrix(nrow=5, ncol=2, c(1:10))
rownames(MAT) <- c("First","Second","Third","Fourth","Fifth")
write.csv(MAT, file='~/test.csv', row.names=TRUE)
The matrix MAT is given below. Ultimately I want the first column of this matrix (after loading the csv file), with the row names intact.
[,1] [,2]
First 1 6
Second 2 7
Third 3 8
Fourth 4 9
Fifth 5 10
If I now read the csv file,
MAT2 <- read.csv(file='~/test.csv')
MAT2 is given by
X V1 V2
1 First 1 6
2 Second 2 7
3 Third 3 8
4 Fourth 4 9
5 Fifth 5 10
The read.csv command seems to have created another row. In any case, if I do MAT3 <- MAT2[,2], I do not get a matrix like above. as.matrix(MAT2[,2]) does not retain the row names as I want.
Any ideas of how to proceed?

Answer 1

Perhaps a better starting point is:
read.csv(file='~/test.csv', row.names = 1)
V1 V2
First 1 6
Second 2 7
Third 3 8
Fourth 4 9
Fifth 5 10
You can also wrap this in as.matrix:
as.matrix(read.csv(file='~/test.csv', row.names = 1))
Compare their structures:
> str(read.csv(file='~/test.csv', row.names = 1))
'data.frame': 5 obs. of 2 variables:
$ V1: int 1 2 3 4 5
$ V2: int 6 7 8 9 10
> str(as.matrix(read.csv(file='~/test.csv', row.names = 1)))
int [1:5, 1:2] 1 2 3 4 5 6 7 8 9 10
- attr(*, "dimnames")=List of 2
..$ : chr [1:5] "First" "Second" "Third" "Fourth" ...
..$ : chr [1:2] "V1" "V2"
If all you are actually concerned about is how to extract a column while retaining the original structure, perhaps drop = FALSE is what you're after:
MAT2 <- as.matrix(read.csv(file='~/test.csv', row.names = 1))
# V1 V2
# First 1 6
# Second 2 7
# Third 3 8
# Fourth 4 9
# Fifth 5 10
MAT2[, 2]
# First Second Third Fourth Fifth
# 6 7 8 9 10
MAT2[, 2, drop = FALSE]
# V2
# First 6
# Second 7
# Third 8
# Fourth 9
# Fifth 10