Function for querying a data.table

Question

I have a data.table called prices that has 3 columns: ticker, date and price. I have run

setkey(prices,ticker,date)

If I do this from the code, it works

prices[list("MSFT",as.Date("2013-01-15")]

returning only the row for MSFT in 2013-01-15 However, if I write this function

getPrice <- function(ticker,date) {
  prices[list(ticker,date)]
}

it returns the whole data.table I suspect that it has something to do with scoping in the i parameter, but I can't get it to work. How do I query the data.table if I don't know the parameters in advance?

Without your data I cannot confirm, but using a toy example this works fine for me. — Justin
– Justin, Commented May 21, 2013 at 21:30

eddi · Accepted Answer · 2013-05-21 21:44:11Z

3

Your problem is the variable names in your function. Change them to be e.g. x and y (so that they are not the same as column names in your data.table) and everything will work. What you're doing now is constructing a data.table with all of ticker and date columns and then joining that, thus recovering the original data.

Another (more robust) option is to do smth like this in your function:

getPrice <- function(ticker,date) {
  tmp = list(ticker, date)
  prices[tmp]
}

See FAQs 2.12 and 2.13 for more on this - http://datatable.r-forge.r-project.org/datatable-faq.pdf

edited May 21, 2013 at 21:44

answered May 21, 2013 at 21:33

eddi

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mnel Over a year ago

There has been mention of plans (In the NEWS for version 1.8.2) to add ..() syntax so that .() and ..() are analogous to the file system's ./ and ../; i.e., .() evaluates within the frame of DT and ..() in the parent scope.

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