ARFF+(book+version)

toc An ARFF (Attribute-Relation File Format) file is an ASCII text file that describes a list of instances sharing a set of attributes.

= Overview = 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: code format="text" % 1. Title: Iris Plants Database %   % 2. Sources: %     (a) Creator: R.A. Fisher %     (b) Donor: Michael Marshall (MARSHALL%PLU@io.arc.nasa.gov) %     (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} code The **Data** of the ARFF file looks like the following: code format="text" @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 code Lines that begin with a are comments. The **@RELATION**, **@ATTRIBUTE** and **@DATA** declarations are case insensitive.

= Examples = Several well-known machine learning datasets are distributed with Weka in the directory as ARFF files.

The ARFF Header Section
The ARFF Header section of the file contains the relation declaration and attribute declarations.

The @relation Declaration
The relation name is defined as the first line in the ARFF file. The format is: code format="text" @relation  code where //// is a string. The string must be quoted if the name includes spaces.

The @attribute Declarations
Attribute declarations take the form of an ordered sequence of **@attribute** statements. Each attribute in the data set has its own **@attribute** statement which uniquely defines the name of that attribute and its data type. The order the attributes are declared indicates the column position in the data section of the file. For example, if an attribute is the third one declared then Weka expects that all that attributes values will be found in the third comma delimited column.

The format for the **@attribute** statement is: code format="text" @attribute  code where the //// must start with an alphabetic character. If spaces are to be included in the name then the entire name must be quoted.

The can be any of the four types supported by Weka: where //// and //// are defined below. The keywords **numeric**, **real**, **integer**, **string** and **date** are case insensitive.
 * //numeric//
 * //integer// is treated as //numeric//
 * //real// is treated as //numeric//
 * 
 * //string//
 * //date// []

Numeric attributes
Numeric attributes can be real or integer numbers.

Nominal attributes
Nominal values are defined by providing an  listing the possible values: {, , , ...}

For example, the class value of the Iris dataset can be defined as follows: code format="text" @ATTRIBUTE class       {Iris-setosa,Iris-versicolor,Iris-virginica} code Values that contain spaces must be quoted.

String attributes
String attributes allow us to create attributes containing arbitrary textual values. This is very useful in text-mining applications, as we can create datasets with string attributes, then write Weka Filters to manipulate strings (like ). String attributes are declared as follows: code format="text" @ATTRIBUTE LCC   string code

Date attributes
Date attribute declarations take the form: code format="text" @attribute date [] code where is the name for the attribute and //// is an optional string specifying how date values should be parsed and printed (this is the same format used by ). The default format string accepts the ISO-8601 combined date and time format:. Check out the Javadoc of the class for supported character patterns.

Dates must be specified in the data section as the corresponding string representations of the date/time (see example below).

The ARFF Data Section
The ARFF Data section of the file contains the data declaration line and the actual instance lines.

The @data Declaration
The **@data** declaration is a single line denoting the start of the data segment in the file. The format is: code format="text" @data code

The instance data
Each instance is represented on a single line, with carriage returns denoting the end of the instance.

Attribute values for each instance can be delimited by commas or tabs. A comma/tab may be followed by zero or more spaces. Attribute values must appear in the order in which they were declared in the header section (i.e., the data corresponding to the nth **@attribute** declaration is always the nth field of the attribute).

A missing value is represented by a single question mark, as in: code format="text" @data 4.4,?,1.5,?,Iris-setosa code Values of string and nominal attributes are case sensitive, and any that contain space must be quoted, as follows: code format="text" @relation LCCvsLCSH @attribute LCC string @attribute LCSH string @data AG5,  'Encyclopedias and dictionaries.;Twentieth century.' AS262, 'Science -- Soviet Union -- History.' AE5,  'Encyclopedias and dictionaries.' AS281, 'Astronomy, Assyro-Babylonian.;Moon -- Phases.' AS281, 'Astronomy, Assyro-Babylonian.;Moon -- Tables.' code Dates must be specified in the data section using the string representation specified in the attribute declaration. For example: code format="text" @RELATION Timestamps @ATTRIBUTE timestamp DATE "yyyy-MM-dd HH:mm:ss" @DATA "2001-04-03 12:12:12"   "2001-05-03 12:59:55" code

= Sparse ARFF files = Sparse ARFF files are very similar to ARFF files, but data with value 0 are not be explicitly represented.

Sparse ARFF files have the same header (i.e **@relation** and **@attribute** tags) but the data section is different. Instead of representing each value in order, like this: code format="text" @data 0, X, 0, Y, "class A"   0, 0, W, 0, "class B" code the non-zero attributes are explicitly identified by attribute number and their value stated, like this: code format="text" @data {1 X, 3 Y, 4 "class A"} {2 W, 4 "class B"} code Each instance is surrounded by curly braces, and the format for each entry is:  where index is the attribute index (starting from 0).

Note that the omitted values in a sparse instance are **0**, they are not "missing" values! If a value is unknown, you must explicitly represent it with a question mark (?).


 * Warning:** There is a known problem saving SparseInstance objects from datasets that have string attributes. In Weka, string and nominal data values are stored as numbers; these numbers act as indexes into an array of possible attribute values (this is very efficient). However, the first string value is assigned index 0: this means that, internally, this value is stored as a 0. When a SparseInstance is written, string instances with internal value 0 are not output, so their string value is lost (and when the arff file is read again, the default value 0 is the index of a different string value, so the attribute value appears to change). To get around this problem, add a dummy string value at index 0 that is never used whenever you declare string attributes that are likely to be used in SparseInstance objects and saved as Sparse ARFF files.

= See also =
 * ARFF Syntax Highlighting for various editors

= Links =
 * [|ISO-8601]
 * Javadoc of (lists the supported character patterns)