[GitHub] [madlib] hpandeycodeit opened a new pull request #432: MADLIB-1351 : Added stopping criteria on perplexity to LDA

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hpandeycodeit opened a new pull request #432: MADLIB-1351 : Added stopping criteria on perplexity to LDA
URL: https://github.com/apache/madlib/pull/432
 
 
   LDA:
   Added stopping criteria on perplexity to LDA. 
   
   [MADLIB-1351](MADLIB-1351)
   
   Currently, in LDA there are no stopping criteria. It runs for all the provided iterations. 
   This PR calculated the Perplexity on each iteration and when the difference between the last two Perplexity values is less than the perplexity_tol, it stops the iteration. 
   
   These are the two new parameters added to the function: 
   
   ```
   evaluate_every      Integer,
   perplexity_tol      Double Precision
   ```
   
   And there is a change to the Model output table as well. It will have these two extra columns 
   
   ```
   perplexity  DOUBLE PRECISION[]
   perplexity_iters INTEGER[]
   ```
   
   Where 
   perplexity is an Array of perplexity values as per the 'evaluate_every' parameter.
   perplexity_iters is an Array indicating the iterations for which perplexity is calculated
   

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