SPMF documentation > Mining Minimal Rare Itemsets using the AprioriRare Algorithm

This example explains how to run the AprioriRare algorithm using the SPMF open-source data mining library.

How to run this example?

What is AprioriRare?

AprioriRare is an algorithm for mining minimal rare itemsets from a transaction database. It is an Apriori-based algorithm. It was proposed by Szathmary et al. (2007).

What is the input ?

The input is a transaction database (aka binary context) and a threshold named minsup (a value between 0 and 100 %).

A transaction database is a set of transactions. Each transaction is a set of items. For example, consider the following transaction database. It contains 5 transactions (t1, t2, ..., t5) and 5 items (1,2, 3, 4, 5). For example, the first transaction represents the set of items 1, 2, 4 and 5. This database is provided as the file contextZart.txt in the SPMF distribution. It is important to note that an item is not allowed to appear twice in the same transaction and that items are assumed to be sorted by lexicographical order in a transaction.

Transaction id Items
t1 {1, 2, 4, 5}
t2 {1, 3}
t3 {1, 2, 3, 5}
t4 {2, 3, 5}
t5 {1, 2, 3, 5}

What is the output?

The output of AprioriRare is the set of minimal rare itemsets. To explain what it a minimal rare itemset, it is necessary to review a few definitions. An itemset is a unordered set of distinct items. The support of an itemset is the number of transactions that contain the itemset divided by the total number of transactions. For example, the itemset {1, 2} has a support of 60% because it appears in 3 transactions out of 5 in the previous database (it appears in t1, t2 and t5). A frequent itemset is an itemset that has a support no less than the minsup parameter. A minimal rare itemset is an itemset that is not a frequent itemset and that all its subsets are frequent itemsets.

For example, if we run AprioriRare algorithm with minsup = 60 % and the previous transaction database, we obtain the following set of minimal rare itemsets:

Minimal Rare Itemsets Support
{4} 20 %
{1, 3, 5} 40 %
{1, 2, 3} 40 %

Input file format

The input file format of AprioriRARE is defined as follows. It is a text file. An item is represented by a positive integer. A transaction is a line in the text file. In each line (transaction), items are separated by a single space. It is assumed that all items within a same transaction (line) are sorted according to a total order (e.g. ascending order) and that no item can appear twice within the same line.

For example, for the previous example, the input file is defined as follows:

1 2 4 5
1 3
1 2 3 5
2 3 5
1 2 3 5

Note that it is also possible to use the ARFF format as an alternative to the default input format. The specification of the ARFF format can be found here. Most features of the ARFF format are supported except that (1) the character "=" is forbidden and (2) escape characters are not considered. Note that when the ARFF format is used, the performance of the data mining algorithms will be slightly less than if the native SPMF file format is used because a conversion of the input file will be automatically performed before launching the algorithm and the result will also have to be converted. This cost however should be small.

Output file format

The output file format of AprioriRARE is defined as follows. It is a text file, where each line represents a maximal rare itemset. On each line, the items of the itemset are first listed. Each item is represented by an integer and it is followed by a single space. After, all the items, the keyword "#SUP:" appears, which is followed by an integer indicating the support of the itemset, expressed as a number of transactions. For example, we show below the output file for this example.

4 #SUP: 1
1 2 #SUP: 2
1 5 #SUP: 2

The output file here consists of three lines which indicates that the itemsets {4}, {1, 2} {1, 5} are perfectly rare itemsets having respectively a support of 1 transaction, 2 transactions and 2 transactions.

Note that if the ARFF format is used as input instead of the default input format, the output format will be the same except that items will be represented by strings instead of integers.

Optional parameter(s) :

There is an alternative implementation of AprioriRare in SPMF called "AprioriRare_TID". This implementation is based on AprioriTID instead of the standard Apriori algorithm. The key difference is that the identifiers of transactions where patterns are found are kept in memory to avoid scanning the database. This can be faster on some datasets. Beside, this implementation offers an additional parameter:

"show transaction ids?" (true/false) This parameter allows to specify that transaction ids of transactions containing a pattern should be output for each pattern found. For example, if the parameter is set to true, each pattern in the output file will be followed by the keyword #TID followed by a list of transaction ids (integers separated by space). For example, a line terminated by "#TID: 0 2" means that the pattern on this line appears in the first and the third transactions of the transaction database (transactions with ids 0 and 2).

Optional feature: giving names to items

Some users have requested the feature of given names to items instead of using numbers. This feature is offered in the user interface of SPMF and in the command line of SPMF. To use this feature, your file must include @CONVERTED_FROM_TEXT as first line and then several lines to define the names of items in your file. For example, consider the example database "contextZart.txt". Here we have modified the file to give names to the items: 

1 2 4 5
1 3
1 2 3 5
2 3 5
1 2 3 5

In this file, the first line indicates, that it is a file where names are given to items. Then, the second line indicates that the item 1 is called "apple". The third line indicates that the item 2 is called "orange". Then the following lines define four sequences in the SPMF format.

Then, if we apply a sequential pattern mining algorithm using this file using the user interface of SPMF or the command line, the output file contains several patterns, including the following ones:

milk #SUP: 1
apple orange tomato #SUP: 2
apple tomato bread #SUP: 2

Note that this feature could be also used from the source code of SPMF using the ResultConverter class. However, there is currently no example provided for using it from the source code.


AprioriRare is the only algorithm for minimal rare itemset mining offered in SPMF. Since it is based on Apriori, it suffers from the same fundamental limitations (it may generate too much candidates and it may generate candidates that do not appear in the database).

Where can I get more information about this algorithm?

The AprioriRare algorithm is described in this paper:

Laszlo Szathmary, Amedeo Napoli, Petko Valtchev: Towards Rare Itemset Mining. ICTAI (1) 2007: 305-312

Also, for a good overview of frequent itemset mining algorithms, you may read this survey paper.

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