The ARFF data format

Weka wants its input data in ARFF format. This data format is described in section 2.4 of Witten and Frank's book. As probably not everyone has access to this book, I give here a short description:

• A dataset has to start with a declaration of its name:

  @relation name

• This is followed by a list of all the attributes in the dataset (including the predicted attribute). These declarations have the form:

  @attribute attribute_name specification

  • If an attribute is nominal, specification contains a list of the possible attribute values in curly brackets:

    @attribute nominal_attribute {first_value, second_value, third_value}

  • If an attribute is numeric, specification is replaced by the keyword numeric: (Integer values are treated as real numbers in WEKA.)

    @attribute numeric_attribute numeric

  • In addition to these two types of attributes, there also exists a string attribute type. This attribute provides the possibility to store a comment or ID field for each of the instances in a dataset:

    @attribute string_attribute string

• After the attribute declarations, the actual data is introduced by a tag:

  @data

• This is followed by a list of all the instances. The instances are listed in comma-separated format, with a question mark representing a missing value.

• Comments are lines starting with %

% The weather data
% This example was copied from Witten and Frank's data mining book

@relation weather

@attribute outlook { sunny, overcast, rainy }
@attribute temperature numeric
@attribute humidity numeric
@attribute windy { TRUE, FALSE }
@attribute play { yes, no }

@data
sunny, 85, 85, FALSE, no
sunny, 80, 90, TRUE, no
overcast, 83, 86, FALSE, yes
rainy, 70, 96, FALSE, yes
rainy, 68, 80, FALSE, yes
rainy, 65, 70, TRUE, no
overcast, 64, 65, TRUE, yes
sunny, 72, 95, FALSE, no
sunny, 69, 70, FALSE, yes
rainy, 75, 80, FALSE, yes
sunny, 75, 70, TRUE, yes
overcast, 72, 90, TRUE, yes
overcast, 81, 75, FALSE, yes
rainy, 71, 91, TRUE, no

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