Specifying column names from a list in the data.frame command

Question

I have a list called cols with column names in it:

cols <- c('Column1','Column2','Column3')

I'd like to reproduce this command, but with a call to the list:

data.frame(Column1=rnorm(10))

Here's what happens when I try it:

> data.frame(cols[1]=rnorm(10))

Error: unexpected '=' in "data.frame(I(cols[1])="

The same thing happens if I wrap cols[1] in I() or eval().

How can I feed that item from the vector into the data.frame() command?

Update:

For some background, I have defined a function calc.means() that takes a data frame and a list of variables and performs a large and complicated ddply operation, summarizing at the level specified by the variables.

What I'm trying to do with the data.frame() command is walk back up the aggregation levels to the very top, re-running calc.means() at each step and using rbind() to glue the results onto one another. I need to add dummy columns with 'All' values in order to get the rbind to work properly.

I'm rolling cast-like margin functionality into ddply, basically, and I'd like to not retype the column names for each run. Here's the full code:

cols <- c('Col1','Col2','Col3')
rbind ( calc.means(dat,cols),
    data.frame(cols[1]='All', calc.means(dat, cols[2:3])),
    data.frame(cols[1]='All', cols[2]='All', calc.means(dat, cols[3]))
)

Answer 1

Use can use structure:

cols <- c("a","b")

foo <- structure(list(c(1, 2 ), c(3, 3)), .Names = cols, row.names = c(NA, -2L), class = "data.frame")

I don't get why you are doing this though!