Create a matrix based on names

Question

I would like to create a matrix using characters. The characters are names of vectors in a data frame.

df <- data.frame(rnorm(100),rnorm(100),rnorm(100),rnorm(100))
colnames(df) <- c("a1","b1","c1","d1")
a <- paste("df$",names(df), sep = "")

How can I use these characters to reference the data frame and take the data in the vector to a matrix? Something like:

as.matrix(cbind(df$a1,df$b1))

but instead of me writing df$a1,df$b1 these names come from a

In the real case I don't want all of the vectors in `df` only a select few — dpel, Jul 30 '15 at 11:37
Just use `as.matrix(df[,vec])` where `vec` is either a numeric vector whose values are the column indices you want to keep or a character vector with the names of the columns. — nicola, Jul 30 '15 at 11:41
@nicola please post an answer, as the situation here is getting worse by the minute. — David Arenburg, Jul 30 '15 at 13:22

score 5 · Accepted Answer · answered Jul 30 '15 at 13:40

5

Just use

as.matrix(df[,vec])

where vec is either a numeric vector whose values are the column indices you want to keep or a character vector with the names of the columns.

answered Jul 30 '15 at 13:40

nicola

1 Answers1