docs update

Author: YY-OnCall

docs/ml-frequent-pattern-mining.md

@@ -27,27 +27,32 @@ explicitly, which are usually expensive to generate.
 After the second step, the frequent itemsets can be extracted from the FP-tree.
 In `spark.mllib`, we implemented a parallel version of FP-growth called PFP,
 as described in [Li et al., PFP: Parallel FP-growth for query recommendation](http://dx.doi.org/10.1145/1454008.1454027).
-PFP distributes the work of growing FP-trees based on the suffices of transactions,
-and hence more scalable than a single-machine implementation.
+PFP distributes the work of growing FP-trees based on the suffixes of transactions,
+and hence is more scalable than a single-machine implementation.
 We refer users to the papers for more details.
 
 `spark.ml`'s FP-growth implementation takes the following (hyper-)parameters:
 
 * `minSupport`: the minimum support for an itemset to be identified as frequent.
   For example, if an item appears 3 out of 5 transactions, it has a support of 3/5=0.6.
-* `minConfidence`: minimum confidence for generating Association Rule. The parameter will not affect the mining
-  for frequent itemsets,, but specify the minimum confidence for generating association rules from frequent itemsets.
+* `minConfidence`: minimum confidence for generating Association Rule. Confidence is an indication of how often an
+  association rule has been found to be true. For example, if in the transactions itemset `X` appears 4 times, `X`
+  and `Y` co-occur only 2 times, the confidence for the rule `X => Y` is then 2/4 = 0.5. The parameter will not
+  affect the mining for frequent itemsets, but specify the minimum confidence for generating association rules
+  from frequent itemsets.
 * `numPartitions`: the number of partitions used to distribute the work. By default the param is not set, and
-  partition number of the input dataset is used.
+  number of partitions of the input dataset is used.
 
 The `FPGrowthModel` provides:
 
 * `freqItemsets`: frequent itemsets in the format of DataFrame("items"[Array], "freq"[Long])
 * `associationRules`: association rules generated with confidence above `minConfidence`, in the format of 
   DataFrame("antecedent"[Array], "consequent"[Array], "confidence"[Double]).
-* `transform`: The transform method examines the input items in `itemsCol` against all the association rules and
-  summarize the consequents as prediction. The prediction column has the same data type as the
-  `itemsCol` and does not contain existing items in the `itemsCol`.
+* `transform`: For each transaction in itemsCol, the `transform` method will compare its items against the antecedents
+  of each association rule. If the record contains all the antecedents of a specific association rule, the rule
+  will be considered as applicable and its consequents will be added to the prediction result. The transform
+  method will summarize the consequents from all the applicable rules as prediction. The prediction column has
+  the same data type as `itemsCol` and does not contain existing items in the `itemsCol`.
 
 
 **Examples**

docs/mllib-frequent-pattern-mining.md

@@ -24,7 +24,7 @@ explicitly, which are usually expensive to generate.
 After the second step, the frequent itemsets can be extracted from the FP-tree.
 In `spark.mllib`, we implemented a parallel version of FP-growth called PFP,
 as described in [Li et al., PFP: Parallel FP-growth for query recommendation](http://dx.doi.org/10.1145/1454008.1454027).
-PFP distributes the work of growing FP-trees based on the suffices of transactions,
+PFP distributes the work of growing FP-trees based on the suffixes of transactions,
 and hence more scalable than a single-machine implementation.
 We refer users to the papers for more details.
 

mllib/src/main/scala/org/apache/spark/ml/fpm/FPGrowth.scala

@@ -227,9 +227,12 @@ class FPGrowthModel private[ml] (
 
   /**
    * The transform method first generates the association rules according to the frequent itemsets.
-   * Then for each association rule, it will examine the input items against antecedents and
-   * summarize the consequents as prediction. The prediction column has the same data type as the
-   * input column(Array[T]) and will not contain existing items in the input column. The null
+   * Then for each transaction in itemsCol, the transform method will compare its items against the
+   * antecedents of each association rule. If the record contains all the antecedents of a
+   * specific association rule, the rule will be considered as applicable and its consequents
+   * will be added to the prediction result. The transform method will summarize the consequents
+   * from all the applicable rules as prediction. The prediction column has the same data type as
+   * the input column(Array[T]) and will not contain existing items in the input column. The null
    * values in the itemsCol columns are treated as empty sets.
    * WARNING: internally it collects association rules to the driver and uses broadcast for
    * efficiency. This may bring pressure to driver memory for large set of association rules.