# PrefixSpan

PrefixSpan is a sequential pattern mining algorithm described in Pei et al., Mining Sequential Patterns by Pattern-Growth: The PrefixSpan Approach. We refer the reader to the referenced paper for formalizing the sequential pattern mining problem.

spark.ml's PrefixSpan implementation takes the following parameters:

minSupport: the minimum support required to be considered a frequent sequential pattern. 

maxPatternLength: the maximum length of a frequent sequential pattern. Any frequent pattern exceeding this length will not be included in the results. 

maxLocalProjDBSize: the maximum number of items allowed in a prefix-projected database before local iterative processing of the projected database begins. This parameter should be tuned with respect to the size of your executors. 

sequenceCol: the name of the sequence column in dataset (default "sequence"), rows with nulls in this column are ignored.

```
import org.apache.spark.ml.fpm.PrefixSpan

val smallTestData = Seq(
  Seq(Seq(1, 2), Seq(3)),
  Seq(Seq(1), Seq(3, 2), Seq(1, 2)),
  Seq(Seq(1, 2), Seq(5)),
  Seq(Seq(6)))

val df = smallTestData.toDF("sequence")
val result = new PrefixSpan()
  .setMinSupport(0.5)
  .setMaxPatternLength(5)
  .setMaxLocalProjDBSize(32000000)
  .findFrequentSequentialPatterns(df)
  .show()
  
  /*
  Output:
+----------+----+
|  sequence|freq|
+----------+----+
|     [[3]]|   2|
|     [[2]]|   3|
|     [[1]]|   3|
|  [[1, 2]]|   3|
|[[1], [3]]|   2|
+----------+----+
  
  */
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://george-jen.gitbook.io/data-science-and-apache-spark/prefixspan.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.