With data.table, the following is routine : DT[order(a)] # ascending DT[order(-a)] # descending, if a is numeric DT[a>5,sum(z),by=c][order(-V1)] # sum of z group by c, just where a>5, then show me the largest first DT[order(-a,b)] # order by a descending then by b ascending, if a and b are both numeric
It avoids peppering your code with $, and becomes quite natural after a short while; especially compound queries such as the 3rd example. Matthew http://datatable.r-forge.r-project.org/ "Ivan Calandra" <[email protected]> wrote in message news:[email protected]... I was wondering whether it would be possible to make a method for data.frame with sort(). I think it would be more intuitive than using the complex construction of df[order(df$a),] Is there any reason not to make it? Ivan Le 5/12/2011 15:40, Marc Schwartz a écrit : > On May 12, 2011, at 8:09 AM, John Kane wrote: > >> Argh. I knew it was at least partly obvious. I never have been able to >> read the order() help page and understand what it is saying. >> >> THanks very much. >> >> By the way, to me it is counter-intuitive that the the command is >> >>> df1[order(df1[,2],decreasing=TRUE),] >> For some reason I keep expecting it to be >> order( , df1[,2],decreasing=TRUE) >> >> So clearly I don't understand what is going on but at least I a lot >> better off. I may be able to get this graph to work. > > John, > > Perhaps it may be helpful to understand that order() does not actually > sort() the data. > > It returns a vector of indices into the data, where those indices are the > sorted ordering of the elements in the vector, or in this case, the > column. > > So you want the output of order() to be used within the brackets for the > row *indices*, to reflect the ordering of the column (or columns in the > case of a multi-level sort) that you wish to use to sort the data frame > rows. > > set.seed(1) > x<- sample(10) > >> x > [1] 3 4 5 7 2 8 9 6 10 1 > > > # sort() actually returns the sorted data >> sort(x) > [1] 1 2 3 4 5 6 7 8 9 10 > > > # order() returns the indices of 'x' in sorted order >> order(x) > [1] 10 5 1 2 3 8 4 6 7 9 > > > # This does the same thing as sort() >> x[order(x)] > [1] 1 2 3 4 5 6 7 8 9 10 > > > set.seed(1) > df1<- data.frame(aa = letters[1:10], bb = rnorm(10)) > >> df1 > aa bb > 1 a -0.6264538 > 2 b 0.1836433 > 3 c -0.8356286 > 4 d 1.5952808 > 5 e 0.3295078 > 6 f -0.8204684 > 7 g 0.4874291 > 8 h 0.7383247 > 9 i 0.5757814 > 10 j -0.3053884 > > > # These are the indices of df1$bb in sorted order >> order(df1$bb) > [1] 3 6 1 10 2 5 7 9 8 4 > > > # Get df1$bb in increasing order >> df1$bb[order(df1$bb)] > [1] -0.8356286 -0.8204684 -0.6264538 -0.3053884 0.1836433 0.3295078 > [7] 0.4874291 0.5757814 0.7383247 1.5952808 > > > # Same thing as above >> sort(df1$bb) > [1] -0.8356286 -0.8204684 -0.6264538 -0.3053884 0.1836433 0.3295078 > [7] 0.4874291 0.5757814 0.7383247 1.5952808 > > > You can't use the output of sort() to sort the data frame rows, so you > need to use order() to get the ordered indices and then use that to > extract the data frame rows in the sort order that you desire: > >> df1[order(df1$bb), ] > aa bb > 3 c -0.8356286 > 6 f -0.8204684 > 1 a -0.6264538 > 10 j -0.3053884 > 2 b 0.1836433 > 5 e 0.3295078 > 7 g 0.4874291 > 9 i 0.5757814 > 8 h 0.7383247 > 4 d 1.5952808 > > >> df1[order(df1$bb, decreasing = TRUE), ] > aa bb > 4 d 1.5952808 > 8 h 0.7383247 > 9 i 0.5757814 > 7 g 0.4874291 > 5 e 0.3295078 > 2 b 0.1836433 > 10 j -0.3053884 > 1 a -0.6264538 > 6 f -0.8204684 > 3 c -0.8356286 > > > Does that help? > > Regards, > > Marc Schwartz > > ______________________________________________ > [email protected] mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide > http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > -- Ivan CALANDRA PhD Student University of Hamburg Biozentrum Grindel und Zoologisches Museum Abt. Säugetiere Martin-Luther-King-Platz 3 D-20146 Hamburg, GERMANY +49(0)40 42838 6231 [email protected] ********** http://www.for771.uni-bonn.de http://webapp5.rrz.uni-hamburg.de/mammals/eng/1525_8_1.php
______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
