From simon.urbanek at r-project.org  Tue Dec 15 23:15:36 2015
From: simon.urbanek at r-project.org (Simon Urbanek)
Date: Tue, 15 Dec 2015 17:15:36 -0500
Subject: [Rd] How to efficiently share data (a dataframe) between R and
	Java
In-Reply-To: <1d7e60b7eb54fcbb94bfdde18942a22b@imap.fit.cvut.cz>
References: <466cd25ef41dbefc2ee18c725a581e78@imap.fit.cvut.cz>
	<8F33B5D6-5D3D-4FD1-9830-9575094D4890@r-project.org>
	<1d7e60b7eb54fcbb94bfdde18942a22b@imap.fit.cvut.cz>
Message-ID: <630C80A0-9799-4F70-82B3-A3216AC6813F@r-project.org>

You can pass the entire df, example:

> data(iris)
> iris$sp = as.character(iris$Species)
> o=.jarray(lapply(iris, .jarray))
> .jcall("C",,"df",o)
df, 6 variables
[0]: double[150]
[1]: double[150]
[2]: double[150]
[3]: double[150]
[4]: int[150]
[5]: String[150]


Java code:

public class C {
       static void df(Object df[]) {
       	      int n;
       	      System.out.println("df, " + (n = df.length) + " variables");
	      int i = 0;
	      while (i < n) {
	      	    if (df[i] instanceof double[]) {
		    	double d[] = (double[]) df[i];
		        System.out.println("["+i+"]: double["+d.length+"]");
		    } else if (df[i] instanceof int[]) {
		    	int d[] = (int[]) df[i];
		        System.out.println("["+i+"]: int["+d.length+"]");
		    } else if (df[i] instanceof String[]) {
		        String s[] = (String[]) df[i];
			System.out.println("["+i+"]: String["+s.length+"]");
		    } else {
		        System.out.println("["+i+"]: some other type...");
		    }
		    i++;
	      }
        }
}

Normally, you wouldn't pass the entire df but instead have methods for the types you care about as the modeling function - that's more Java-like approach, but either is valid and there is no difference in efficiency.

Cheers,
Simon



> On Dec 15, 2015, at 12:50 PM, Ing. Jaroslav Kucha? <jaroslav.kuchar at fit.cvut.cz> wrote:
> 
> Dear all,
> 
> thank you for your hints. I would prefer to do not use Rserve as Dirk
> mentioned. 
> 
> @Simon
> I have full control over the Java implementation - I can adapt the code
> that I use for the communication R <-> Java. 
> 
>> You can natively access structures on each side. The fastest way is to
>> use R representation (column-oriented) in Java - that is much faster
>> than any kind of serialization or anything you mention above since you
>> pass the variables as a whole.
> 
> Could you please send any reference to more examples or documentation
> that can help me? 
> The main goal is to copy a full dataframe from R to Java.
> 
> Best regards,
> Jaroslav
> 
> On 2015-12-07 03:19, Simon Urbanek wrote:
>> On Dec 6, 2015, at 12:36 PM, Ing. Jaroslav Kucha?
>> <jaroslav.kuchar at fit.cvut.cz> wrote:
>> 
>>> Dear all,
>>> 
>>> in our ongoing project we use Java implementations of several
>>> algorithms. We also provide a ?wrapper? implemented as an R package
>>> using rJava (https://github.com/jaroslav-kuchar/rCBA). Based on our
>>> recent experiments, the significant portion of time is spent on copying
>>> a dataframe from R to Java. The Java implementation needs access to the
>>> source dataframe.
>>> 
>>> I have tested several approaches: calling Java method row-by-row;
>>> serialize the whole data-frame to a temp file and parsing in Java; or
>>> row binding to a single vector and calling a single Java method. Each
>>> approach has its limitations e.g. time-consuming row-by-row copying,
>>> serialization and parsing performance or memory limitations of a single
>>> vector.
>>> 
>>> Is there an efficient approach how to copy a dataframe from R to Java
>>> and another one from Java to R?
>>> 
>>> Thanks for any help you can provide...
>>> 
>> 
>> You can natively access structures on each side. The fastest way is to
>> use R representation (column-oriented) in Java - that is much faster
>> than any kind of serialization or anything you mention above since you
>> pass the variables as a whole.
>> 
>> Typically, the bottleneck are Java applications which may require very
>> inefficient data structures. If you have control over the algorithms,
>> you can simply use proper data structures and avoid that problem. If
>> you don't have control, you'll have to add Java code that converts to
>> whatever structure is needed by the Java code form the data frame
>> pushed to the Java side. The main point here is that you do NOT want
>> to do any conversion on the R side.
>> 
>> Cheers,
>> ?imon
> 



[Rd] How to efficiently share data (a dataframe) between R and Java
Simon Urbanek