LCM: Linear time Closed itemset Miner

Coded by Takeaki Uno, e-mail:uno@nii.jp,
homepage: http://research.nii.ac.jp/~uno/index.html

This program is available for only academic use, basically. Anyone can modify this program, but he/she has to write down the change of the modification on the top of the source code. Neither contact nor appointment to Takeaki Uno is needed. If one wants to re-distribute this code, do not forget to refer the newest code, and show the link to homepage of Takeaki Uno, to notify the news about codes for the users. For the commercial use, please make a contact to Takeaki Uno.

ProblemDefinition
Usage
InputFileFormat
UseGeneralNamesforVariablesandOtherFormats
BatchFilesforSimpleuse
OutputFormat
Performance
SolvingOtherProblems
IntroductionstoFrequentItemsetMining
AlgorithmsandImplementationIssue
Acknowledgments
References

Problem Definition

Let I be a set of items. An itemset is a subset of I. Let D be a transaction database such that each record (called transaction) is an itemset. Frequency of an itemset is the number of transactions including the itemset. For a given number t (called support), an itemset is said to be frequent if its frequency is no less than t. A frequent itemset is called maximal if it is included in no other frequent itemset, and called closed if it is included in no other itemset of the same frequency. The task of this program, LCM, is to enumerate (output, or count) all frequent itemsets, all maximal frequent itemsets, or all frequent closed itemsets in a given transaction database for given a support.

Usage

==== How to Compile ====

Unzip the file into arbitrary directory, and execute "make". Then you can see "lcm" (or lcm.exe) in the same directory. In the case of ver. 2xx and 3, you see fim_all, fim_closed, and fim_maximal.

==== Command Line Options for ver.2xx and ver.3 ====

The parameter format follows the FIMI implementations. (http://fimi.cs.helsinki.fi/)
To enumerate frequent itemsets, execute fim_all. For frequent closed itemsets, execute fim_closed, and for maximal frequent itemsets, execute fim_maximal. The first parameter is the input file name, the second is minimum support, and the third is the output file name. We can omit the third parameter, then no output file will be generated, and the program counts the number of solutions.

Example)
% fim_closed input-filename support [output-filename]

==== Command Line Options for ver.4, and ver.5 ====

To execute LCM, just type lcm and give some parameters as follows.

% lcm [command] [options] input-filename support [output-filename]

"%" is not needed to type. It is a symbol to represent command line. To see a simple explanation, just execute "lcm" without parameters.

"input-filename" is the filename of the input transaction database. The 1st letter of input-filename must not be '-'. Otherwise it is regarded as an option. The input file format is written below. "support" is the support, which is for given the threshold of frequency. "output-filename" is the name of file to write the itemsets the program finds. You can omit the output file to see only the number of frequent itemsets in the database. If the output file name is "-", the solutions will be output to the standard output.

The first parameter [command] is composed of some letters, and given to the program to indicate the task.

F: enumerate frequent itemsets,
C: enumerate closed frequent itemsets
M: enumerate maximal frequent itemsets

For example, if you want to enumerate maximal frequent itemset, type "M" in the first parameter. Additionally, we can give the following commands, to specify the output style:

q: no output to standard output (including messages w.r.t. input data)
i: do not output itemset to the output file (only rules are written)
f: output the frequency on the end of each itemset found,
Q: output the frequency on the head of each itemset found,
A: output positive/negative frequency, and (frequency)/(absolute
frequency); positive/negative frequency is the sum of weights of
the occurrence which have positive/negative weights, respectively.
Absolute frequency is the sum of absolute value of the weights of
the occurrences.
s: output confidence and item frequency by absolute values; in default,
they are written by ratio, but by this command they are written by
#transactions/sum of the transaction weights
I: output ID's of transactions including each itemset; ID of a
transaction is given by the number of line in which the transaction
is written. The ID starts from 0.
R: output redundant items for rule mining
usually, one might think that the redundant items whose removals
do not affect the frequency and the confidence are not necessary
to be output, thus LCM removes those items (but not all) from the
itemsets, in the case of rule mining. This command disables this
function, thus the itemsets to be output will be the same as the
usual itemset mining.
V: show the progress of computation
t: transpose the database so that item i will be transaction i, thus
if item i is included in j-th transaction, then item j will be
included in i-th transaction.

The output format is explained below. The following options are to restrict the itemset to be found. We can type them between the first parameter and the second parameter. Of course the option can be nothing.

-l,-u [num]: enumerate itemsets with size at least/most [num]
-U [num]: enumerate itemsets with frequency at most [num]
-w [filename]: read weights of transactions from the file (ver. 4 and 5)
the frequency of itemset will be the sum of weights of the transactions
which include the itemset.
-c [filename]: read constraints graph between items (ver. 5.2)
-C [filename]: read un-constraint graph between items (ver. 5.2)

Following options are only for ver. 5.x.

-m,-M [filename]:read/write item ordering from/to file [filename]
-K [num]: output the frequency of [num]-th largest frequent(closed/maximal)
itemsets (only for ver.5). Notice that it outputs only the frequency,
thus no itemset will be output
-S [num]: stop after outputting [num] solutions
-, [char]: give the separator of the numbers in the output
the numbers in the output file are separated by the given
character [char].
-Q [filename]: replace the output numbers
according to the permutation table written in the file of
[filename], replace the numbers in the output. The numbers in the
file can be separated by any non-numeric character such as newline
character.
-f,-F [ratio]: output itemsets with frequency [ratio] times smaller/larger
than that in the case of independence; let p be the product of
(frequency of i)/(#transactions) for all item i in the itemset.
-i [num]: find association rule for item [num]; output all rules of the form
{1,3,5} => [num]. The criteria to output a rule can be given by other
options.
-a,-A [ratio]: find association rules of confidence at least [ratio]; an
association rule of {1,3,5} => 2 will be output if the frequency of
{1,3,5,2} is no less/no more than the frequency of {1,3,5} times [ratio].
-r,-R [ratio]: find association rules of relational confidence at least
[ratio]; an association rule of {1,3,5} => 2 will be output if the
frequency of {1,3,5,2} is no less/no more than the frequency of {1,3,5}
times (frequency of {2}/(#transactions in the database)) times [ratio].
-p,-P [num]: output pattern itemset only if (frequency) / (absolute
frequency) is no less/no greater than [num]. Absolute frequency is the
sum of absolute weights of the occurrences of the pattern itemset.
-n,-N [num]: output pattern itemset only if its negative frequency is
no less/no greater than [num]. Negative frequency is the sum of
weights of the occurrences which have negative weights
-o,-O [num]: output pattern itemset only if its positive frequency is
no less/no greater than [num]. Positive frequency is the sum of
weights of the occurrences which have positive weights

-l, -u, and -U specify the upper/lower bound for itemset/frequency. itemsets exceed the upper/lower bound will be not output. When we give -S option and a number X, the program will terminate if it finds X solutions, even if there are more solutions.

If we specify "-w" and filename, then LCM reads the file as a weight database of transactions. Each i-th line of the file is regarded as the weight of i-th transaction. With transaction weights, LCM finds the itemsets such that the sum of weights of the transaction including the itemset is no less than the support. The weights can be real numbers. For version 5, we can give negative weights.

"-c" and "-C" options (only for ver 5): "-c" option is for giving item constraints. An item constraint is given between two items which can not be included in frequent itemsets together with. In the other words, if an item constraint is given for items 1 and 2, then {1,2} can not be included in any frequent itemset. The set of item constraints forms an undirected graph. Thus, LCM inputs item constraints by a graph. The filename following to "-c" is regarded as a graph file. The format of the graph file is written below. Conversely, "-C" option is for giving "un-constraint" item pairs. In precise, when we give "-C" option, the item constraint between items 1 and 2 means that {1,2} can be included in frequent itemsets. That is, if no item constraint is given for items 1 and 2, the pair {1,2} can not be included in any frequent itemset. The un-constraints of "-C" option are also given by graph, of the same format to "-c" option.

If we specify "-w" and filename, then LCM reads the file as a weight database of transactions. Each i-th line of the file is regarded as the weight of i-th transaction. With transaction weights, LCM finds the itemsets such that the sum of weights of the transaction including the itemset is no less than the support. The weights can be real numbers. For version 5, we can give minus weights.

When we give -K option, LCM computes the frequency of the [num]-th largest frequent(closed/maximal) itemsets, and output it to the standard output (print it to the command line). By giving the output frequency to LCM, we can enumerate top-[num] frequent itemsets (closed, or maximal).

Examples)

- to find all frequent itemsets in "test.dat" for support 4, sizes of
no less than 3, output frequencies of each itemsets found, do not
output to file, show progress, and stop if 1,000,000 solutions will
be found;

% lcm FfV -l 3 -S 1000000 test.dat 4

- to find closed itemsets in "test.dat" with frequency at least 6, sizes
from 5 to 10, output itemsets to "out.dat";

% lcm C -l 5 -u 10 test.dat 6 out.dat

- to find maximal frequent itemsets in "test.dat" with weight file
"weight.dat" with frequency at least 8, output to "out.dat" with
transaction IDs, and no output for standard output;

% lcm MqI -w weight.dat test.dat 8 out.dat

- to specify item constraint file g.grh, for "test.dat" with frequency

at least 3, and output out.dat;

% lcm F -c g.grh test.dat 8 out.dat

When -m,-M [filename] options are given, LCM reads/writes item permutation from/to the file of [filename]. LCM internally permute the items for the efficient computation, and this option is to enforce the permutation to a specified one, or to know the permutation. The file format is just a list of indices, for example, if 0,1,2,3 will be 2,1,0,3 then the file is

2
1
0
3
[EOF]

For the items whose permutation is not written in the file, LCM determine its internal indices.

-f and -F options are for finding itemsets with high relational frequency. For each item i, let us consider (frequency of {i}) / (#transactions in D) as the appearance probability that item i is included in a transaction. If the appearance probability of each item is independent to each other, the frequency of an itemset S is expected to be the product of these appearance probabilities of its items times (#transactions in D). We here define the relational frequency of S by
(frequency of S) / (expected frequency of S). When we give -f [num] option, LCM outputs frequent itemsets with relational frequency no less than [num]. Similarly, frequent itemsets with relational frequency no greater than [num] is output when -F [num] option is given.

When -n/-N [num] options are given, LCM outputs only itemsets having negative frequency at least/most [num]. The negative frequency of itemset S is the sum of weights of the transactions that include S and have negative weights. Similarly, the positive weights of S is defined. When -o/-O options are given, LCM outputs only itemsets having positive weights.

-p/-P [ratio] options are to evaluate the absolute frequency ratio. The absolute frequency of itemset S is the sum of absolute weight of the transactions including S, and the absolute frequency ratio is given by the frequency of S divided by the absolute frequency of S. When -p/-P options are given, LCM outputs only itemset having absolute frequency ratio at least/most [ratio].

The above three kinds of options can be used to find emerging itemsets. For two databases D1 and D2 , the emerging itemsets are itemsets having quite different frequencies in D1 and D2, i.e., having a large frequency in one database and a small frequency in the other. There are several models to define emerging itemsets, such as (a) the difference of the frequencies between D1 and D2 are at least the given threshold s, (b) the frequency in D1 over the frequency in D2 is at least s, and so on. If we give a positive weight to each transaction of A, and a negative weight to each transaction of B, emerging itemsets defined as (a) have large frequencies, and those as (b) have large or small absolute frequency ratio. -n/-N, -o/-N options can be used to prune the itemset whose frequency in either D1 or D2 is too small or too large.

The options -m/-M [filename] is used to control the order of the output itemsets. Before computing the frequent itemsets, LCM gives an ordering to items according to the decreasing order of their frequency, and find the frequent itemsets in their lexicographically order. -M option outputs this ordering to the file. The format of the file is just a sequence of items in the order. Instead of using frequency ordering, the -m option inputs the item ordering from the file. The input file format is the same to the format of -M option, but it accepts any non-numeric separator. Using these options, you can control the order of finding the frequent itemsets, which can be used to compare the set of frequent itemsets in different databases.

The remaining options are for the association rule mining. An association rule is a pair of an itemset S and an item x with the form "S => x". The confidence of the rule is the ratio of transactions including x, in the set of transactions including S. If the confidence is high, we can say that a transaction including S may include x with high probability. The remaining options are for enumerating all such association rules with higher or lower confidence

-a [ratio] and -A [ratio] options are for specifying the threshold value for the confidence. When -a option and [ratio] are given, LCM finds all association rules with confidence at least [ratio]. Conversely, association rules with confidence at most [ratio] are found when we give -A option.

-r [ratio] and -R [ratio] options are also for specifying the threshold, but the threshold values changes according to the frequency of each item. Precisely, when we give -R and [ratio], an association rule S => x is output if its confidence is no greater than
[ratio] * (frequency of {x}/#transactions),
thus it is output only if the confidence is small compared to the frequency of x in the database. Conversely, when we give -r and [ratio], an association rule S => x is output if (1 - its confidence) is no greater than [ratio] * (1- frequency of {x}).

When -i [num] option is given, LCM finds only the rules of the form
S => [num] ([num] is an item).

Examples)
- To find all association rule "A => b" such that frequency of A is no less
than 200, and the confidence is no less than 0.9. The input file is
"test.dat" and the output file is "out".

% lcm C -a 0.9 test.dat 200 out

- To find all association rule "A => b" such that frequency of A is no less
than 200, and the confidence is no greater than
0.1 * (frequency of {x}/#transactions). The item b is specified as item 5.
The input file is "test.dat" and the output file is "out".

% lcm C -R 0.1 -i 5 test.dat 200 out

Input File Format

Each line (row) of the input file is a transaction. The items included in a transaction are listed in a line. The items must be numbers begin from 1. They can not be minus. The item numbers do not have to be continuous, but notice that the program takes memory linear in the maximum item number. The separator of numbers can be any non-numeric letter, such as "," " " ":" "a", etc.

Example) ( "[EOF]" is the end of file )
0 1 2
1
2 3 4
4,1 2 3
2,1
[EOF]

The file format of item constraints (or complement) graphs is as follows. The ith row (line) of the file corresponds to node i-1. The first line corresponds to the node 0, and the 10th line corresponds to node 9. The node larger than i-1 and adjacent to i-1 is listed in the i-th line. The nodes are written by numbers. Separator to the number can be ",", but the graph load routine accepts any letter for the separator but not a number, +, and -. If the edge set is {(0,1),(0,2),(1,1),(1,3),(2,3)}, the file will be
===========
1,2
1 3
3

[EOF]
=========
where "[EOF]" is a symbol for the end of file.

Use General Names for Variables and Other Formats

We can transform variable names in general strings to numbers so that we can input the data to the program, by some script files.

- sortout < input-file > output-file
Sort the file in the lexicographical order. It is used to sort the output file.

- transnum.pl table-file [separator] < input-file > output-file
Read file from standard input, and give a unique number to each name written by general strings (such as ABC, ttt), and transform every string name to a number, and output it to standard output. The mapping from string names to numbers is output to table-file. The default character for the separator of the string names is " "(space). It can be changed by giving a character for the option [separator]. For example, A,B is a string name, if the separator is space, but if we set the separator to ",", it is regarded as two names A and B. This is executed by "transnum.pl table-file "," < input-file...".

- untransnum.pl table-file < input-file > output-file
According to the table-file output by transnum.pl, un-transform numbers to string names. The output of the program is composed of numbers, thus it is used if we want to transform to the original string names. It reads file from standard output, and output to the standard output.

- appendnum.pl < input-file > output-file
When we want to distinct the same words in different columns, use this script. This append column number to each words, so we can distinct them. Then, by using transnum.pl, we transform the strings to numbers.

- transpose.pl < input-file > output-file
Transpose the file. In the other words, consider the file as an adjacency matrix, and output the transposed matrix, or exchange the positions of items and transactions. For an input file, output the file in which the i-th line corresponds to item i, and includes the numbers j such that i is included in the j-th line of the input file.

- 01conv.pl [separator] < input-file > output-file
Transform the transaction database written in the style of 01 matrix to our input style. The file has to be a list of 0 and 1, separated by [separator]. If [separator] is omitted, the default separator " " is used.

Batch Files for Simple use

For general string names, we have several batch files scripts for basic usages "exec_lcm", "exec_lcm_", "sep_lcm", or "sep_lcm_". For example, when a database with "general item names" is,

dog pig cat
cat mouse
cat mouse dog pig
cow horse
horse mouse dog
[EOF]

All these replace strings in the input database by numbers, execute LCM, and replace the numbers in the output file by the original strings. The usage of the scripts are

% exec_lcm [commands] input-filename support output-filename [options]

You have to give the commands in the first parameter, but the options have to locate at the end. The separator of the items is " " (blank, space). If you want to use other character as a separator, use "sep_lcm". The usage is

% sep_lcm separator [commands] input-filename support output-filename [options]

Almost same as "exec_lcm" but you have to specify separator at the fourth parameter. "exec_lcm_" and "sep_lcm_" are both for the aim to distinct the same items in the different columns. For example, it is used to the database such that different items are there in different columns, but some special symbols, such as "- is for missing data", are used commonly. An example is;

A true small
B - true -
C false middle
B - -
C - middle
A true -
[EOF]

In the output file, the items are followed by "." and numbers where the numbers are the column number. For example, "dog.0" means the item "dog" on the 0th(first) column.

The usage of them are the same as "exec_lcm" and "sep_lcm", respectively. All these scripts use files of the names "__tmp1__", "__tmp2__", and "__tmp3__". The files of these names will be deleted after the execution.

Example)

% exec_lcm F test2.dat 10 out.dat -w weight.dat -l 2

% sep_lcm_ "," C test3.dat 3 out.dat -U 5

Output Format

When the program is executed, the program prints out the #items, #transactions, and other features of the input database to standard error. After the termination of the enumeration, it outputs the total number of itemsets found (frequent/closed/maximal itemsets), and the numbers of itemsets of each size. For example, if there are 4 frequent itemsets of size 1, 2 frequent itemsets of size 3, and 1 frequent itemset of size 3, then the output to standard output will be,

9 <= total #frequent itemsets
1 <= #frequent itemsets of size 0 (empty set), it is always frequent
4 <= #frequent itemsets of size 1
3 <= #frequent itemsets of size 2
1 <= #frequent itemsets of size 3

If "q" is given in the first parameter, these do not appear in the standard output.

If output-filename was given, then the itemsets found are written to the output file. Each line of the output file is the list of items included in an itemset found, separated by " ". By giving "-," option we can change the separator. If "f" is given in the first parameter, the frequency follows each itemset, for example,

1 5 10 2 4 (22)

which means itemset {1,2,4,5,10} is included in 22 transactions. When "Q" option is given, the output will be

(22) 1 5 10 2 4

The output itemsets are not sorted. If you want to sort it, use the script "sortout.pl". The usage is just,

% sortout.pl < input-file > output-file

"input-file" is the name of file to which LCM outputs, and the sorted output will be written in the file of the name "output-file". The items of each itemset will be sorted in the increasing order of items, and all the itemsets (lines) will be also sorted, by the lexicographical order (considered as a string). (Actually, you can specify separator like sortout.pl ",").

An association rule S => x, for example {1,2,3} => 5 is output in the form

[xxx yyy] 5 <= 1 2 3

where xxx is the confidence of the rule, and yyy is
(frequency of y)/#transactions in D.

Performance

The performance of LCM is stable, for both computation time and memory use. The initialization and preprocess time of LCM is linear in the size of input database. The computation time for finding frequent itemsets depends on the number of itemsets found, and the minimum support. When the minimum support is large and the number of the itemsets found is small, the computation time for each itemset is relatively large. However, computation time per itemset decreases as the increase of the itemsets found, and roughly speaking when the size of output file is equal to the input database size, it will be constant, such as 1/1,000,000 sec (by PC with CPU of 2GHz.)

Memory usage of LCM is very stable. It is an advantage compared to other implementations. The memory usage of LCM is always linear in the size of the input database. Approximately LCM uses integers at most three times as much as the database size, which is the sum of the number of items of each transaction. The memory usage of the other implementations increases as the increase of the number of frequent itemsets, but that of LCM does not.

Solving Other Problems

- Enumerating Maximal Bipartite Cliques in a Bipartite Graph -

Enumerating all maximal bipartite cliques in a bipartite graph is equivalent to enumerating all closed itemsets. LCM can be used for this task. For a bipartite graph of two vertex sets A and B, construct the database such that each line is the list of vertices incident to a vertex in A. Then, by executing

% lcm CI input-filename 1 output-filename

you can enumerate maximal bipartite cliques.

- Enumerating Maximal Bipartite Cliques in a Graph -

By transforming the graph, we can enumerate bipartite cliques in graphs. For a given G=(V,E), we construct a graph G'=(V+V', E') where V' is a copy of V. If an edge (v,u) is in E, then, in E', vertex v of V and vertex u in V' are connected by an edge, and vertex u of V and vertex v in V' are connected by an edge. Then, G' is a bipartite graph, and a bipartite clique in G' is a bipartite clique in G, and vice versa. Thus, giving G' to LCM, we can enumerate all maximal bipartite cliques in general graphs.

- Enumerating Maximal Directed Bipartite Cliques in a directed Graph -

For a directed graph G=(V,A), a directed bipartite clique is vertex sets B and C such that for any pair of a vertex b in B and c in C, there is an arc from b to c. In a similar way to the above, we construct a directed graph G'=(V+V', A') where V' is a copy of V. If an arc (v,u) is in A, then, in A', there is an edge connecting vertex v of V and vertex u in V' in A'. Then, G' is a bipartite graph, and a bipartite clique in G' is a directed bipartite clique in G, and vice versa. Thus, giving G' to LCM, we can enumerate all maximal bipartite cliques in general graphs.

- Finding Pairs of Transactions Having Large Intersection -

Suppose that we want to see which pair of transactions having many common items, i.e., for a threshold t, we want to find all pairs of transactions A and B such that at least t items are included in both A and B. For the sake, execute lcm with -l 2 and -u 2, for example,

lcm F -l 2 -u 2 test.dat 20

then we can find all pairs of transactions having at least 20 common items.

- Mining "itemset" sets.
Consider a data base each whose record is a set of itemset (transaction database). a set S of itemsets is included in a record R if and only if each itemset of S is included in some itemset in R. Note that an itemset of R may include more than one itemsets of S. Here the problem is to enumerate all sets of itemsets included in no less than theta records of the given database. When we give a restriction of the size of itemsets in a pattern S to be enumerated, say at most some small k, we can tract the problem by using LCM. Transform each record of the database to an itemset by listing all the itemsets included in at least one itemsets in the record, and regarding each itemset listed as an item.

For example, an itemset {A,B,C} is considered as an item "{A,B,C}". A frequent itemset in the database obtained corresponds to a frequent set of itemsets. They correspond to each other one-to-one. Moreover, by enumerating closed itemsets, we can ignore unnecessary patterns automatically. If a frequent itemset includes a item {A,B,C}, corresponding to an itemset {A,B,C} in the original problem, then without changing the frequency we can add any item corresponding to a subset of {A,B,C} to the itemset. A closed itemset always includes such subsets, thus a closed itemset is given by a set of itemsets such that any two itemsets does not satisfy the inclusion relation. Thus, we can discard unnecessary patterns automatically.

Introductions to Frequent Itemset Mining

Let D be a database such that each record is transaction data. Here transaction data is a set of items. Thus, the database is a set of subsets of items. Such a database is called a "transaction database".

For example, let us see the following database of transactions:

transaction A: 1,2,5,6,7
transaction B: 2,3,4,5
transaction C: 1,2,7,8,9
transaction D: 1,7,9
transaction E: 2,7,9
transaction E: 2,7,9
transaction F: 2

Transaction A is a set of items 1, 2, 5, 6, and 7, and the others are so on. Now we consider a problem of finding sets of items which are included in many transactions, since such sets give some interesting structures of the database. Since "many transactions" is not well-defined, we introduce a number called "minimum support" or simply "support", and consider that itemsets included in at least "minimum support" transactions are included in many transactions. We call such itemsets "frequent itemsets", and the number of transactions including an itemset "frequency" of the itemset. For the above transaction database, by setting minimum support to 3, frequent itemsets are

{}, {1}, {2}, {7}, {9}, {1,7}, {1,9}, {2,7}, {2,9}, {7,9}, and {2,7,9}.

Note that the first itemset is the empty set, and it is considered as a frequent itemset. If the support is large, #frequent itemsets is small. The number of frequent itemsets increases as the decrease of the support.

A popular usage of the frequent itemset mining is to find an interesting knowledge from database. In this sense, large support usually gives trivial frequent itemset, which are not interesting. In the other hand. when we set minimum support to a small number, the number of frequent itemsets will be so huge. Dealing with a large huge of itemsets is itself a hard task. Thus, next we consider how to decrease the number of itemsets to be found, without missing interesting knowledge.

One approach to this task is to find only maximal frequent itemsets, which are included in no other frequent itemset. For example, the maximal frequent itemsets in the above database are

{1,7}, {1,9}, and {2,7,9}.

The idea is that any frequent itemset is included in at least one maximal frequent itemset, at least we can get the itemset from maximal frequent itemsets. However, we can not know how much frequent they are. For example, {2,7} can be obtained from {2,7,9}, but we can not know whether {2,7} is more frequent than {2,7,9} or not. In this sense we miss how much interesting each frequent itemset is.

Using closed itemsets, we can avoid this problem. An itemset is closed if it is included in no other itemset of the same frequency. For example, in the above database, the closed itemsets are

{}, {2}, {2,5}, {7,9}, {1,7,9}, {2,7,9}, {1,2,7,9}, {2,3,4,5},
{1,2,7,8,9}, and {1,2,5,6,7,9}.

Closed itemset is a maximal frequent itemset if the support is equal to its frequency. Thus, the set of closed itemsets is the collection of maximal frequent itemsets for all possible supports. Any non-closed itemset is dominated by a closed itemset, thus we lose nothing about the frequency.

Algorithms and Implementation Issue

The basic idea of the algorithm is depth first search. Let D be a transaction database, t be the minimum support, 1,...,n be items, T1,...,Tm be the transactions in D. D(I) denotes the set of transactions including I. We denote the largest item of an itemset I by tail(I). LCM first computes the frequency of each itemset composed of one item. If an itemset {i} is frequent, then enumerate frequent itemsets obtained by adding one item to {i}. In this way, recursively, LCM enumerates all frequent itemsets. To avoid duplications, LCM adds items j to {i} only if j>i. This algorithm is written as follows:

FrequentItemsetMining (D:database, I:itemset )
Output I
For each item j > tail(I),
if (I \cup j) is frequent, then FrequentItemsetMining (D, I\cup{j})

By calling FrequentItemsetMining (D, emptyset), we can enumerate all frequent itemsets.

However, a straightforward implementation of this algorithm is very slow, since computing frequency of (I\cup{j}) takes long time. To be fast, we use conditional database and occurrence deliver as follows.

==== Conditional database of itemset I ===
Conditional database of itemset I, denoted by D(I), is given by the database obtained by

1. D(I) := all transactions including I
2. remove all unnecessary items from each transaction of D(I)
3. merge the identical transactions into one.
(do for all such identical transactions)

Here unnecessary item is an item satisfying
(a) included in less than t transactions of D(I),
(b) included in all transactions in D(I), or
(c) less than the largest item in I.
Then, the frequency of itemset I plus J is the same, in D and D(I), thus LCM uses D(I) instead of D in the recursive call with respect to I. This technique is quite common in implementations of frequent itemset mining.

Constructing D(I) possibly takes much memory, when the recursion is deep. Thus, LCM uses a slight modification.

1. D(I) := IDs of all transactions including I
2. If there are transactions T1,...,Tk become the same after deleting
unnecessary items, make a new transaction equal to it, and replace the
IDs of T1,...,Tk by the ID of new transaction

After terminating the recursive call with respect to I, we delete the transactions generated in the process. By this, we can bound the memory usage for the transactions by twice the database size. From this, LCM allocates the memory in the initialization, and never do again in the main routine. Thus the memory usage is very stable, and is very fast since computation time for memory allocation is not small in the frequent itemset mining implementations.

=== occurrence deliver ===
Let T be a transaction database (subset family), a collection of subsets of E = {1,...,n}, and T(S) be the set of transactions (subsets) in T including a subset S.
We suppose that each transaction is sorted. The occurrence deliver computes T(i) for all i>j, for a given j, in time linear in the sum of their sizes. First, for each i for which we compute T(i), we give an empty bucket. Then, for each transaction t in T, we scan it in decreasing order of items until we meet the item less than j, and for each item i>j, insert t to the bucket of i. After scanning all the transactions, the bucket of i is T(i). When we have T(S) and want to compute T(S\cup {i}) for all i>tail(S), we can do in the same way by setting T:=T(S), and j:=tail(S). For the check whether the current itemset is a maximal frequent itemset of not, we also use the occurrence deliver. If there is no item i such that S\cup {i} is frequent, then the frequent itemset S is maximal frequent itemset.

=== closed itemset mining ===
To enumerate closed itemsets, we use the ppc extension (prefix preserving closure extension). For an itemset P, we define P(i) by the items of P which are smaller than i. The closure C(P) of P is the closed itemset including P and having the same frequency to P. When P is a closed itemset, we define its core index, denoted by core_i(P), by the minimum item i such that C(P(i)) = P. Then, a closed itemset P' is said to be a ppc extension of P if and only if

(i) P' = C(P\cup {i}) for some i>core_i(P). (ii) P(i) = P'(i).

Note that P' has smaller frequency than P. It is proved that any closed itemset is a ppc extension of the other unique closed itemset. Thus, the binary relation ppc extension induces a rooted tree whose root is C(emptyset). Thus, starting from the smallest closed itemset (which is usually empty set) and find ppc extension recursively for all i, we can perform depth first search on the rooted tree. The computation of closure of P can be done by taking the intersection of transactions including P, thus it can be done by occurrence deliver, in the linear time of T(P).

==== sweep pointer method ====
Actually, we do not have to compute the intersection completely. We choose one transaction T among them, and check whether there is an item in T\setminus P which is included in all the other transactions. We trace T in the increasing order of items, and when we meet an item i in T\setminus P, we trace the other transactions from the head whether they include i or not. Notice that to check the next item j in T\setminus P, we can start the trance of the other transactions from the positions which we terminated the trace for checking i. This does not increase the computation time much since the time complexities are the same, and usually we check only few items and conclude that the itemset is not a child. Thus, we can do ppc check very quickly.

Moreover, when the frequency of the itemset is small, we can use bit operations so that we can operate 32 or 64 bits at once. Suppose that the frequency frq(I) of an itemset I is less than 32. We give ID's 0,1,2,4,...,2^{frq(I)} to Occ(I), which is the set of occurrences of I. Let the bit-mask b(S) of a subset S of Occ(I) be the sum of ID's of the occurrences in S. For each item j included in at least one occurrence of I, we compute b(Occ(I\cup {j})). When we compute the closure C(I\cup {j}) of I\cup {j}, an item e not in I\cup {j} is included in C(I\cup {j}) if and only if b(Occ(I\cup {e})) \cap b(Occ(I\cup {j})) = b(Occ(I\cup {j})). Here \cap means the and operation for bit string, and two numbers are regarded as bit-strings. This operation can be done in very few steps, thus we can accelerate the closure computation. If I\cup {j} is a closed itemset, we can use the bit-masks for the ppc extensions of I\cup {j}. Thus we do not have to re-compute the bit-masks in the recursive calls. In practice, the bit-mask method accelerates the speed of the algorithm 2 or 3 times when the data set is sparse and the minimum support is small, say 10.

==== recursive pruning for closed itemset ===
For the efficient computation of closed itemsets, we have one more technique. Suppose that for a closed itemset I and an item j, C(I\cup {j}) is not a ppc extension of I, because C(I\cup {j}) includes an item e<j, not in I. Then, we can see for any ppc extension I\cup {j'}, j'<j, C(I\cup {'j}\cup {j}) includes item e, thereby C(I\cup {'j}\cup {j}) is not a ppc extension of I\cup {j'}. It implies that we can never get a ppc extension by adding j, in the recursion. This reduces the number of candidates of ppc extensions, so reduce the computation time.
In practice, when the difference between the number of frequent closed itemsets and the number of the frequent itemsets is large, it speeds the algorithm about 2 or 3 times.

The details of the algorithm is described as follows.

- scan the file and for each item i, count #transactions, sum of transaction size, frq(i), where frq(i) = #transactions including i
- allocate pointer array for T[][] (2d array for transactions) , Occ[][] (2d array of integers)
- allocate memory for w[] (transaction weights), "buf" for T[][] and occ_buf for Occ[][] (size of buf is ||D||*2, and size of occ_buf = ||D||, where D is the removal of infrequent items (items i, frq(i)<th had removed) ). Size of Occ[i] is equal to frq(i)
- load the file to buffer. If neither gap nor window constraint is given, skip items i with frq(i) < th,
set T[t] to the pointer to the beginning of T[i] in "buf", put "end mark" (large int) to the last of each T[]
- sort each transactions in increasing order
- load the weight file, or set the transaction weights to 1
- find the same transactions by radix sort (with using Occ[][])
- unify the same transactions into one, with adding the transaction weights
- set occ to the array of {0,1,...,#transactions-1}
- allocate int array "jump" of size #items
- call LCM_seq ( occ, #transactions, current end of buffer )

2. LCM (occ, ii, t_new, buf)
- call Delivery(occ, ii), to compute for each item i<ii, sum of transaction weights (occ_w[i]), and positive transactions weights (occ_pw[i]) in transactions in occ including i.
( Delivery set "jump" to items where occ_pw[i] or occ_w[i] is not equal to 0)
- in the case of closed/maximal itemset mining, instead of call Delivery(occ, ii), call Delivery(occ, "end mark")

- in the case of closed/maximal itemset mining, return if there is an item j > ii, j is not in the current itemset such that occ_w[j] = sum of transaction weights in occ, and occ_pw[j] = sum of positive transaction weights in occ
(it is the check of "prefix preserving closure extension")

- for each i in jump with occ_pw[i] < th, set occ_w[i] and occ_pw[i] to 0 and remove i from jump
- if the sum of transaction weights in occ >= th, (and if there is no j>ii, j is not in the current itemset with occ_w[j]>=th ,in the case of maximal itemset mining) output itemset, or rules

- if jump_t = 0, clear occ_w, occ_pw and jump, and return

- find the same transactions by radix sort, with ignoring the items not included in "jump", and items larger than ii
- create a new transaction for the same transactions detected above into one, with adding the transaction weights.
(the buffer and ID of created transactions start from buf, and t_new
update buf/t_new to the end of buffer/ID )
--- in the case of closed itemset mining, take intersection of the suffixes of transactions to be merged
--- in the case of maximal itemset mining, compute the union of the suffixes of transactions to be merged, and compute the weight of each item (item weight of item i is the sum of weights of transactions to be merged including i).
- replace the "merged" transactions from occ, and insert the newly created transactions to occ.

- call Delivery(occ, ii) to compute Occ[i] for each item i with occ_pw[i]>=th

- for each item i in jump in increasing order,
call LCM (Occ[i], t_new, buf ) (note that t_new and buf are updated)
clear Occ[i], occ_w[i] to 0, occ_pw[i] to 0
end for
end of LCM

=== Delivery is done as follows

Delivery (occ, ii)
- for each transaction t in occ
- for each item i in t with i<ii
( for computing occ_pw and occ_w)
if occ_pw[i] = 0 and occ_w[i] = 0, insert i to "jump"
add the weight of t to occ_w[i] and add to occ_pw[i] if the weight is positive
( for computing Occ[i])
if occ_pw[i] >= th, then insert t to Occ[i]
end for
end for
end of Delivery

This data storage method is called "forward star", in graph data structures.

1. allocate int array of size #transactions + (sum of frq(i) with frq(i)>=th) for "buf"
2. allocate pointer (to int) array of size #transactions for T[][]
3. buf' := pointer to the start position of buf, t=0

4. while ( not end of the file )
4.1 set T[t] to the pointer to buf'
4.2 read integer to memory starting from buf', until newline
4.3 put "end mark" to the tail of the integers
4.4 buf' = the position next to "end mark", t = t+1
end while

Acknowledgments

We thank to Ken Satoh of National Institute of Informatics Japan, Hiroki Arimura of Hokkaido University for their contribution for the research, to Tatsuya Asai, Yuzo Uchida, Masashi Kiyomi for their contribution for computational experiments and coding. We would also like to present our sincere thanks to Bart Goethal, one of the organizers of FIMI (Frequent Itemset Mining Implementation) for his organizing the workshop to which the first version of LCM was submitted, which was the start of our research.

A part of the research of LCM is supported by joint-research fund of National Institute of Informatics Japan, and Grant-in-aid from the Ministry of Education, Science, Sport and Culture of Japan (Monbu-Kagaku-Sho).

We also thank to Ilpo Lyytinen of University of Helsinki, Sancho, R. Arun Kumar, Sethu Institue of Technology, Pulloor, Tamilnadu, Koji Tsuda of Advanced Industrial Science and Technology, JAPAN, Yukinobu Hamuro of Kwansei gakuin University, JAPAN, Hiroyuki Morita of Osaka Prefecture University, JAPAN, Yasuyuki Shirai of Mitsubishi Research Institute, Nicolas Pasquier of Universite' de Nice Sophia-Antipolis, Giuseppe Rizzo, Payas Gupta for their bug reports.

References

Bart Goethal, "FIMI repository", http://fimi.cs.helsinki.fi/ (various frequent itemset implementation, benchmark databases, computational experiments comparing all implementations, and papers explaining the implementations, very useful site)

Takeaki Uno, Masashi Kiyomi, Hiroki Arimura, LCM ver.3: Collaboration of Array, Bitmap and Prefix Tree for Frequent Itemset Mining, Open Source Data Mining Workshop on Frequent Pattern Mining Implementations 2005, Aug/2005

Takeaki Uno, Masashi Kiyomi, Hiroaki Arimura, "LCM ver.2: Efficient Mining Algorithms for Frequent/Closed/Maximal Itemsets," in Proceedings of IEEE ICDM'04 Workshop FIMI'04, 1/Nov/2004, http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-126/

Takeaki Uno and Tatsuya Asai, Hiroaki Arimura and Yuzo Uchida, "LCM: An Efficient Algorithm for Enumerating Frequent Closed Item Sets," Workshop on Frequent Itemset Mining Implementations (FIMI'03), http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-90/

Takeaki Uno,Tatsuya Asai,Yuzo Uchida, Hiroki Arimura, "An Efficient Algorithm for Enumerating Closed Patterns in Transaction Databases," Lecture Notes in Artificial Intelligence 3245 (Proceedings of Discovery Science 2004), 4/Oct/2004