[jira] [Created] (SPARK-14497) Use top instead of sortBy() to get top N frequent words as dict in ConutVectorizer

["Feng Wang (JIRA)" <ji...@apache.org>]

Feng Wang created SPARK-14497:
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             Summary: Use top instead of sortBy() to get top N frequent words as dict in ConutVectorizer
                 Key: SPARK-14497
                 URL: https://issues.apache.org/jira/browse/SPARK-14497
             Project: Spark
          Issue Type: Improvement
          Components: ML
            Reporter: Feng Wang


It's not necessary to sort the whole rdd to get top n frequent words.

        // Sort terms to select vocab
        wordCounts.sortBy(_._2, ascending = false).take(vocSize)
      
we could use top() instead since: 
top(n) - O(n)
sortBy - O(n*logn)

A minor side effect introduced by top() using default implicit Ordering in Tuple2: 

if the terms with same TF in dictionary would be sorted in descending order.
(a:1), (b:1),(c:1)  => dict: [c, b, a]






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