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From "Nicolas Hernandez (JIRA)" <...@uima.apache.org>
Subject [jira] [Commented] (UIMA-2106) Handling tokens not present in the language model (and also with no suffix present in the model) causes a null pointer exception in the tagger process
Date Thu, 31 Mar 2011 18:24:05 GMT

    [ https://issues.apache.org/jira/browse/UIMA-2106?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13014118#comment-13014118

Nicolas Hernandez commented on UIMA-2106:

As soon as I found how to assign me the task I can submit a patch. There are two lines to
change in org.apache.uima.examples.tagger.Viterbi.java
available_pos = word_probs.get("("); 
available_pos.put("null", Double.MIN_VALUE);

possible_pos_next =  word_probs.get("(");
possible_pos_next.put("null", Double.MIN_VALUE);

> Handling tokens not present in the language model (and also with no suffix present in
the model) causes a null pointer exception in the tagger process
> ------------------------------------------------------------------------------------------------------------------------------------------------------
>                 Key: UIMA-2106
>                 URL: https://issues.apache.org/jira/browse/UIMA-2106
>             Project: UIMA
>          Issue Type: Bug
>          Components: Sandbox-Tagger
>    Affects Versions: 2.3
>         Environment: OS
> Linux version 2.6.32-30-generic (buildd@vernadsky) (gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5)
) #59-Ubuntu SMP Tue Mar 1 21:30:21 UTC 2011
> java version "1.6.0_17"
> Java(TM) SE Runtime Environment (build 1.6.0_17-b04)
> Java HotSpot(TM) Server VM (build 14.3-b01, mixed mode)
>            Reporter: Nicolas Hernandez
>            Priority: Minor
>             Fix For: 2.3
>   Original Estimate: 5m
>  Remaining Estimate: 5m
> The HMMTagger Analysis Engine class uses the org.apache.uima.examples.tagger.Viterbi.java
implementation to determine the pos tag list of a given sentence.
> In practice this implementation is partially dependant on the part of speech tagging
(likewise the remaining HMMTagger classes actually).
> For exemple it makes strong assumptions on the kind of tokens it can take as input. It
assumes no restriction about the token covertext values.
> It results in using some covertext probabilities for initialization or default value
when the tagger processes an unknown token...
> As a consequence if the coveredText used for setting the default value is not present
in the training model an error occurs. Roughly speaking, the process looks first for probability
associated to the current token coverText, if the coverText is not present in the model, it
looks in the model for the probability of its longest suffix, and finally if it does not found
a match, the process assigns to the unknown coverText the probability of the arbitrary coverText
: "("  
> The problem is that if the probability of this coverText is not available in the model,
the probability of the unknown token is not defined and a null pointer exception occurs latter
when the variable is called.
> Why the probability of the "(" text would not be available in the model ? In a large
training corpus if we consider all the tokens, there is little chance not to find at least
one occurrence of "(". 
> Nevertheless if we use the HMM training  AE to build a model for predicting noun gender
and number, or verb tense and person, or "being a part of" named entity... these tokens won
t have the "(" coverText... and consequently an error will occurs when the tagging will be

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