Create README.md

Author: Cheng-Lin-Li

A-Priori_SON/README.md

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+## This is an implementation of Savasere, Omiecinski, and Navathe (SON) algorithm in Spark 2.1.1 with Python 2.7
+Finding Frequent Itemsets: SON Algorithm by A-Priori algorithm in stage 1
+
+## Algorithm: Savasere, Omiecinski, and Navathe (SON) Algorithm, A-Priori algorithm
+
+## Task:
+The task is to implement SON algorithm in Apache Spark using Python. 
+Given a set of baskets, SON algorithm divides them into chunks/partitions and then proceed in two stages. 
+First, local frequent itemsets are collected, which form candidates; 
+next, it makes second pass through data to determine which candidates are globally frequent.
+
+#### Usage: bin/spark-submit A-Priori_SON.py <baskets.txt> <.3> <output.txt>
+
+  1. baskets.txt is a text file which contains a basket (a list of comma-separated item numbers) per line. 
+  For example
+  1,2,3 
+  1,2,5 
+  1,3,4 
+  2,3,4 
+  1,2,3,4 
+  2,3,5 
+  1,2,4 
+  1,2
+  1,2,3 
+  1,2,3,4,5
+  
+  2. <.3> = minimum support ratio (that is, for an itemset to be frequent, it should appear in at least 30% of the baskets).
+   
+  3. output.txt is the output result file.
+  
+
+#### Input: Take a utility matrix (mat.dat) as the input
+
+#### Output: Output root-mean-square deviation (RMSE) into standard output or a file after each iteration
+After each iteration, output RMSE with 4 floating points.
+The "%.4f" % RMSE is adapted to format the RMSE value, and save into file as follows. 
+
+1.0019 
+
+0.9794 
+
+0.8464 
+
+...
+