[R] Using reduce to merge multiple files

[Kate Ignatius]

I have a list of files that I have called like so:

main_dir <- '/path/to/files/'
directories <- list.files(main_dir, pattern = '[[:alnum:]]', full.names=T)

filenames <- list.files(file.path(directories,"/tmpdir/"),  pattern =
'[[:alnum:][:punct:]]_eat.txt+$', recursive = TRUE, full.names=T)

This lists around 35 Files.  Each has multiple columns but they all
have three columns in common: Burger, Stall and Cost which I want to
merge on using:

m1 <- Reduce(function(a, b) { merge(a, b,
by=c("Burger",Stall","Cost")) }, filenames)

However, I get the error:

Error in fix.by(by.x, x) : 'by' must specify uniquely valid columns

Is there something that I have obviously overlooked here?

Thanks in advance!