<font><font face="arial,helvetica,sans-serif">indeed, it makes sense now, as what is passed to the functi<font>on is indeed <font>a data<font>.table and not a <font>data.frame.<br><br><font>Thanks guys fo<font>r your help.<font> Now I'm a convinced data<font>.table user.<br>
<font>Best,<br>David</font><br></font></font></font></font></font></font></font></font></font></font><br><div class="gmail_quote">On Thu, Jan 17, 2013 at 5:25 PM, Akhil Behl <span dir="ltr"><<a href="mailto:akhil@igidr.ac.in" target="_blank">akhil@igidr.ac.in</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hey David,<br>
<br>
I thought your problem may have been a typo, but I realized that it is<br>
in fact a subtle difference between the way data.table and data.frame<br>
work.<br>
<br>
One must provide unquoted names in the `j' expression for a<br>
data.table, i.e. one can say x.dt[ , y] but not x.dt[ , "y"] (which<br>
will evaluate to just "y" and hence the error).<br>
<br>
There are tricks around it like using with=FALSE, or using the<br>
data.frame notation x.dt[["y"]]. But once again, you will find such<br>
examples and explanations of idiomatic data.table expressions in the<br>
vignettes.<br>
<br>
--<br>
ASB.<br>
<div class="HOEnZb"><div class="h5"><br>
On Thu, Jan 17, 2013 at 10:42 PM, David Bellot <<a href="mailto:david.bellot@gmail.com">david.bellot@gmail.com</a>> wrote:<br>
> Hi Matthew,<br>
><br>
> I read indeed the introduction but I wasn't sure about the way to write it.<br>
> Hence my question.<br>
><br>
> In fact, I do agree if the function would sum(sqrt(y)), but in my case, I<br>
> would like to do something like<br>
><br>
> f <- function(d) head(d,1)<br>
><br>
> It's a small example for the sake of simplicity, just to illustrate that I<br>
> really want to have access to the full sub data.frame (the d variable) and<br>
> not just one column.<br>
><br>
> Best,<br>
> David<br>
><br>
> On Thu, Jan 17, 2013 at 5:07 PM, Matthew Dowle <<a href="mailto:mdowle@mdowle.plus.com">mdowle@mdowle.plus.com</a>><br>
> wrote:<br>
>><br>
>><br>
>> Akhil,<br>
>><br>
>> Kind of, but defining :<br>
>><br>
>> my.func <- function (d) {<br>
>> sum(sqrt(d[["y"]]))<br>
>> }<br>
>><br>
>> followed by<br>
>><br>
>> x.dt[ , my.func(.SD), by=x]<br>
>><br>
>> isn't very data.table'ish. In fact the<br>
>> advice is to avoid .SD if possible, for speed.<br>
>><br>
>> We'd forget my.funct, and just do :<br>
>><br>
>> x.dt[, sum(sqrt(y)), by=x]<br>
>><br>
>> That is how we recommend it to be used, and<br>
>> allows data.table to optimize the query (which<br>
>> use of .SD may prevent).<br>
>><br>
>> David - have you read the introduction vignette and have<br>
>> you worked through example(data.table) at the prompt?<br>
>><br>
>> Matthew<br>
>><br>
>><br>
>><br>
>> On 17.01.2013 16:53, Akhil Behl wrote:<br>
>>><br>
>>> If I am not wrong, you are looking for `.SD'. In fact you can put in<br>
>>> the exact function you were throwing at ddply earlier. There are other<br>
>>> special names like .SD that you can find in the data.table FAQs.<br>
>>><br>
>>> Let's see:<br>
>>> R> require(plyr)<br>
>>> Loading required package: plyr<br>
>>><br>
>>> R> require(data.table)<br>
>>> Loading required package: data.table<br>
>>> data.table 1.8.7 For help type: help("data.table")<br>
>>><br>
>>> R> x.df <- data.frame(x=letters[1:2], y=1:10)<br>
>>> R> x.dt <- data.table(x.df)<br>
>>> R><br>
>>> R> my.func <- function (d) { # Define a function on the subset<br>
>>> + sum(sqrt(d[["y"]]))<br>
>>> + }<br>
>>> R><br>
>>> R> # The plyr way:<br>
>>> R> ddply(x.df, "x", my.func) -> ans.plyr<br>
>>> R><br>
>>> R> # The data.table way:<br>
>>> R> x.dt[ , my.func(.SD), by=x] -> ans.dt<br>
>>> R><br>
>>> R> ans.plyr<br>
>>> x V1<br>
>>> 1 a 10.61387<br>
>>> 2 b 11.85441<br>
>>><br>
>>> R> ans.dt<br>
>>> x V1<br>
>>> 1: a 10.61387<br>
>>> 2: b 11.85441<br>
>>><br>
>>> For more help, try this on an R prompt:<br>
>>><br>
>>> R> vignette('datatable-faq')<br>
>>><br>
>>> --<br>
>>> ASB.<br>
>>><br>
>>> On Thu, Jan 17, 2013 at 9:49 PM, David Bellot <<a href="mailto:david.bellot@gmail.com">david.bellot@gmail.com</a>><br>
>>> wrote:<br>
>>>><br>
>>>> Hi,<br>
>>>><br>
>>>> I've been looking all around the web without a clear answer to this<br>
>>>> trivial<br>
>>>> problem. I'm sure I'm not looking where I should:<br>
>>>><br>
>>>> in fact, I want to replace my use of ddply from the plyr package by<br>
>>>> data.table. One of my main use is to group a big data.frame by a group<br>
>>>> of<br>
>>>> variable and do something on this sub data.frame:<br>
>>>><br>
>>>> ddply( my_df, my_grouping_var, function (d) { do something with d } )<br>
>>>> ----> d is a data.frame again<br>
>>>><br>
>>>> and it's slow on big data.frame.<br>
>>>><br>
>>>><br>
>>>> However, I don't really understand how to redo the same thing with a<br>
>>>> data.table. Basically if "j" in a data.table is equivalent to the select<br>
>>>> clause in SQL, then how do I do SELECT * FROM etc...<br>
>>>><br>
>>>> I want to be able to pass a function like in ddply that will receive not<br>
>>>> only a few columns but the full subset that is selected by the "by"<br>
>>>> clause.<br>
>>>><br>
>>>> Thanks...<br>
>>>> Best,<br>
>>>> David<br>
>>>><br>
>>>> _______________________________________________<br>
>>>> datatable-help mailing list<br>
>>>> <a href="mailto:datatable-help@lists.r-forge.r-project.org">datatable-help@lists.r-forge.r-project.org</a><br>
>>>><br>
>>>><br>
>>>> <a href="https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help" target="_blank">https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help</a><br>
>>><br>
>>> _______________________________________________<br>
>>> datatable-help mailing list<br>
>>> <a href="mailto:datatable-help@lists.r-forge.r-project.org">datatable-help@lists.r-forge.r-project.org</a><br>
>>><br>
>>><br>
>>> <a href="https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help" target="_blank">https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help</a><br>
><br>
><br>
</div></div></blockquote></div><br>