Hi,
In general you need to convert the data.frame to a data.table before you
can use data.table syntax and features on it :
DT = as.data.table(DF)
But that's a copy of the whole objects so it's easier and faster to
start off with a data.table in the first place; e.g., using fread as you
said. Its development status means that fread's current argument names,
types and order might possibly change a bit in a backwards incompatible
way. It isn't intended to convey that it's too flaky for end user use.
Quite a few people are already using it routinely.
If you have a list of many data.frame's, then that's difficult work
with. My first thought would be to pass that to rbindlist to create one
large data.table, then := by group to add the column.
Matthew
On 05/08/13 16:36, iembry wrote:
Hi all, I am new to data.table and I have some questions.
Since fread is still in development stage, I have not used it to read my
data files stored on the disk. I have used read.table and the objects are
stored as data.frame.
How do I use data.table on those existing data.frame objects?
This is a reproducible example:
dput(df)
structure(list(a = c(5, 6, 6), b = c(20.2, 32.9, 0.99), cdo = c(0.2,
32, 90.34)), .Names = c("a", "b", "cdo"), row.names = c(NA, -3L
), class = "data.frame")
In my real data set, I have a list of many data frames where I will need to
change an existing column.
For example, in the "df" data.frame, how would I make a revised "a" = "a" +
"b"?
The code below represents what I am attempting to do with my real data set:
getratings2 <- lapply(seq_along(dailyvaluesneednew$site_no), function(u)
getratings[[u]]$y <- getratings[[u]]$y + getratings[[u]]$shift)
In this case, I would like to make the column named "y" = "y" + "shift" and
perform that operation for each data.frame in the list.
Thank you.
Irucka Embry
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