The Full Form of ARFF is‍ Attribute-Relation File Format.

An ARFF (Attribute-Relation File Format) file is an ASCII text file that describes a list of instances sharing a set of attributes. ARFF files were developed by the Machine Learning Project at the Department of Computer Science of The University of Waikato for use with the Weka machine learning software. This document descibes the version of ARFF used with Weka versions 3.2 to 3.3; this is an extension of the ARFF format as described in the data mining book written by Ian H. Witten and Eibe Frank (the new additions are string attributes, date attributes, and sparse instances).

This explanation was cobbled together by Gordon Paynter (gordon.paynter at ucr.edu) from the Weka 2.1 ARFF description, email from Len Trigg (lenbok at myrealbox.com) and Eibe Frank (eibe at cs.waikato.ac.nz), and some datasets. It has been edited by Richard Kirkby (rkirkby at cs.waikato.ac.nz). Contact Len if you’re interested in seeing the ARFF 3 proposal.

ARFF files have two distinct sections. The first section is the Header information, which is followed the Data information.

The Header of the ARFF file contains the name of the relation, a list of the attributes (the columns in the data), and their types. An example header on the standard IRIS dataset looks like this:

   % 1. Title: Iris Plants Database
   % 
   % 2. Sources:
   %      (a) Creator: R.A. Fisher
   %      (b) Donor: Michael Marshall (MARSHALL%[email protected])
   %      (c) Date: July, 1988
   % 
   @RELATION iris

   @ATTRIBUTE sepallength  NUMERIC
   @ATTRIBUTE sepalwidth   NUMERIC
   @ATTRIBUTE petallength  NUMERIC
   @ATTRIBUTE petalwidth   NUMERIC
   @ATTRIBUTE class        {Iris-setosa,Iris-versicolor,Iris-virginica}
  

The Data of the ARFF file looks like the following:

   @DATA
   5.1,3.5,1.4,0.2,Iris-setosa
   4.9,3.0,1.4,0.2,Iris-setosa
   4.7,3.2,1.3,0.2,Iris-setosa
   4.6,3.1,1.5,0.2,Iris-setosa
   5.0,3.6,1.4,0.2,Iris-setosa
   5.4,3.9,1.7,0.4,Iris-setosa
   4.6,3.4,1.4,0.3,Iris-setosa
   5.0,3.4,1.5,0.2,Iris-setosa
   4.4,2.9,1.4,0.2,Iris-setosa
   4.9,3.1,1.5,0.1,Iris-setosa
  

Lines that begin with a % are comments. The @RELATION, @ATTRIBUTE and @DATA declarations are case insensitive.