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---+ [[GRM.NGramLibrary][OpenGrm NGram]] Forum

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---++ Infinity values / ill formed FST 
---+++ Main.RolandSchwarz - 03 Apr 2013 - 05:50
<noautolink>
Hi, 

I'm currently playing around with a test example and I noticed than after ngrammake if I call fstinfo (not ngraminfo) on the resulting language model fstinfo complains about the model being ill-formed. This is due to transitions (typically on epsilons) that have "Infinity" weight, which does not seem to be supported by openFST. Is that "working as intended"? The problem is later if I call fstshortestpath to get e.g. the n most likely sentences from the model the result contain not only "Infinity" weights but also "BadNumber" which might be a result of the infinite values.

Thanks,

Roland

</noautolink>


---+++ Main.BrianRoark - 03 Apr 2013 - 10:56
<noautolink>
Hi Roland,

yes, under certain circumstances, some states in the model end up with infinite backoff cost, i.e., zero probability of backoff.  In many cases this is, in fact, the correct weight to assign to backoff.  For example, with a very small vocabulary and many observations, you might have a bigram state that has observations for every symbol in the vocabulary, hence no probability mass should be given to backoff.  Still, this does cause some problems with OpenFst.  In the next version (due to be released in the next month or so) we will by default have a minimum backoff probability of some very small epsilon (i.e., very large negative log probability).  As a workaround in the meantime, I would suggest using fstprint to print the model to text, then use sed or perl or whatever to replace Infinity with some very large cost -- I think SRILM uses 99 in such cases, which would work fine. 

hope that helps,

brian
</noautolink>

---+++ Main.BrianRoark - 03 Apr 2013 - 10:58
<noautolink>
oh, I forgot to say that, once you have replaced Infinity with 99, use fstcompile to recompile the model into an FST.

brian
</noautolink>

---+++ Main.RolandSchwarz - 04 Apr 2013 - 05:12
<noautolink>
Hey, thanks a lot for the quick reply, I'll give that a go!

Roland
</noautolink>

---+++ Main.RolandSchwarz - 04 Apr 2013 - 05:32
<noautolink>
If I may add another quick question, when running fstshortestpath on the ngram count language model (i.e. after ngramcount but before ngrammake) I was expecting to get the most frequent n-gram, but instead the algorithm never seems to terminate. Any idea why that is? I though that shortestpath over the tropical sr should always terminate anyway.

Thanks,

Roland

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---++ Build failure on Fedora 17 
---+++ Main.JerryJames - 18 Dec 2012 - 10:42
<noautolink>
Hi.  I maintain several voice-recognition-related packages, including openfst, for the Fedora Linux distribution.  I am working on an OpenGrm NGram package.  My first attempt at building version 1.0.3 (with GCC 4.7.2 and glibc 2.15) failed:

<verbatim>
In file included from ngramrandgen.cc:32:0:
./../include/ngram/ngram-randgen.h:55:48: error: there are no arguments to 'getpid' that depend on a template parameter, so a declaration of 'getpid' must be available [-fpermissive]
./../include/ngram/ngram-randgen.h:55:48: note: (if you use '-fpermissive', G++ will accept your code, but allowing the use of an undeclared name is deprecated)
ngramrandgen.cc:39:1: error: 'getpid' was not declared in this scope
ngramrandgen.cc:39:1: error: 'getpid' was not declared in this scope
</verbatim>

It appears that an explicit =#include <unistd.h>= is needed in ngram-randgen.h.  That header was probably pulled in through some other header in previous versions of either gcc or glibc.
</noautolink>


---+++ Main.BrianRoark - 19 Dec 2012 - 17:22
<noautolink>
ok, that header file will be included in the next version.  Thanks for the heads up.

brian
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---++ Expected result when using a lattice with ngram perplexity? 
---+++ Main.JosefNovak - 28 Nov 2012 - 06:40
<noautolink>
I was wondering what the expected result is when feeding a lattice, rather than a string/sentence, to the ngramperplexity utility?  Is this supported?  It seems to report the perplexity of an arbitrary path through the lattice.
</noautolink>


---+++ Main.BrianRoark - 28 Nov 2012 - 20:46
<noautolink>
Hi Josef,

ngramperplexity reports the perplexity of the path through the lattice that you get by taking the first arc out of each state that you reach. (Note that this is what you want for strings encoded as single-path automata.)  Not sure what the preferred functionality should be for general lattices.  Could make sense to show a warning or an error there; but at this point the onus is on the user to ensure that what is being scored is the same as what you get from farcompilestrings - unweighted, single-path automata.  If you have an idea of what preferred functionality would be for non-string lattices, email me.

brian
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---++ is there a way to use NGramApply in c++ 
---+++ Main.MarkusFreitag - 19 Oct 2012 - 12:55
<noautolink>
Hi,

I do not want to print my fst and execute NGramApply in bash before reading the new fst again in c++.

Is there a method to use the method NGramApply directly in c++ ?

Thanks
</noautolink>


---+++ Main.BrianRoark - 21 Oct 2012 - 21:32
<noautolink>
Hi Markus,

there is no single method; rather there are several ways to perform composition with the model, depending on how you want to interpret the backoff arcs.  The most straightforward way to do this in your own code is to look at src/bin/ngramapply.cc and use the composition method for the particular kind of backoff arc, e.g., ngram.FailLMCompose() when interpreting the backoff as a failure transition.  In other words, write your own ngramapply method based on inspection of the ngramapply code.

Hope that helps,

brian
</noautolink>

---+++ Main.MarkusFreitag - 22 Oct 2012 - 09:58
<noautolink>
Hi,

thanks, I think yes that should work.
I am using FailureArcs and my LM fst is created, so I do not need to build a lm fst out of strings or an ARPA lm.

I first just need to read the fst lm from my disk:

#include <ngram/ngram.h>

fst::StdMutableFst *fstforNGram;
fstforNGram->Read($MYNGRAMFST);
ngram::NGramModel ngram(fstforNGram);
// that seems not to work, as: undefined reference to `ngram::NGramModel::InitModel()'

If I read the lm , I could then just add:

ngram.FailLMCompose(*lattice, &cfst, kSpecialLabel);

and the composed fst should be ready, right?

Thanks for helping
</noautolink>

---+++ Main.BrianRoark - 23 Oct 2012 - 09:05
<noautolink>
Correct, that is the method for composing with failure arcs.
</noautolink>

---+++ Main.MarkusFreitag - 23 Oct 2012 - 09:30
<noautolink>
yes, but I have a problem to read the fst lm in c++:

fst::StdMutableFst *fstforNGram;

fstforNGram->Read($MYNGRAMFST);

to that point it works.

ngram::NGramModel ngram(fstforNGram);

 that seems not to work, as: undefined reference to `ngram::NGramModel::InitModel()'

Thanks
</noautolink>
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---++ Fractional Kneser Ney 
---+++ Main.JosefNovak - 09 Oct 2012 - 04:31
<noautolink>
Hi,  I have been using OpenGrm with my Grapheme-to-Phoneme conversion tools for a while now and recently added some functionality to output weighted alignment lattices in .far format.  

It is my understanding that these weighted lattices can only currently be utilized with Witten-Bell smoothing; is this correct?  

Is there any plan to support fractional counts with Kneser-Ney smoothing, for instance along the lines of,

"Correlated Bigram LSA for Unsupervised Language
Model Adaptation", Tam and Schultz. 

or would I be best advised to implement this myself?  
</noautolink>


---+++ Main.BrianRoark - 09 Oct 2012 - 09:32
<noautolink>
Hi Josef,

Witten-Bell generalizes straightforwardly to fractional counts, as you point out.  No immediate plans for new versions of other smoothing methods along those lines, so if that's something that you need urgently, you would need to implement it. 

brian
</noautolink>

---+++ Main.JosefNovak - 09 Oct 2012 - 18:21
<noautolink>
Understood, and thanks very much for your response!
</noautolink>
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---++  FATAL: NegLogDiff
---+++ Main.LukeCarmichael - 25 Sep 2012 - 17:21
<noautolink>
Hello,
I run this sequence of commands with the following output.

<verbatim>
home$ ngramcounts a.far > a.cnts
home$  ngrammake --v=4 --method=katz a.cnts > katz.mod
INFO: FstImpl::ReadHeader: source: a.cnts, fst_type: vector, arc_type: standard, version: 2, flags: 3
Count bin   Katz Discounts (1-grams/2-grams/3-grams)
Count = 1   -nan/0.253709/-0.343723
Count = 2   -nan/1.26571/1.19095
Count = 3   -nan/0.467797/-0.532465
Count = 4   -nan/1.29438/1.18879
Count = 5   -nan/0.0740557/-0.489831
Count > 5   1/1/1
FATAL: NegLogDiff: undefined -10.2649 -10.2651
</verbatim>
Other methods work fine.

How can I diagnose this problem?

Thanks,
Luke

</noautolink>


---+++ Main.BrianRoark - 26 Sep 2012 - 13:12
<noautolink>
Hi Luke, 

this is basically a floating point precision issue, the system is trying to subtract two approximately equal numbers (while calculating backoff weights).   The new version of the library coming out in a month or so has much improved floating point precision, which will help.  In the meantime, you can get this to work by modifying a constant value in src/include/ngram/ngram-model.h which will allow these two numbers to be judged to be approximately equal.  Look for: 
static const double kNormEps = 0.000001;
near the top of that file.  Change to 0.0001, then recompile.

This sort of problem usually comes up when you train a model with a relatively small vocabulary (like a phone or POS-tag model) and a relatively large corpus.  The n-gram counts end up not following Good-Turing assumptions about what the distribution should look like (hence the odd discount values).  In those cases, you're probably better off with Witten-Bell smoothing with the --witten_bell_k=15 or something like that.  Or even trying an unsmoothed model.

And stay tuned for the next release, which deals more gracefully with some of these small vocabulary scenarios.

Brian
</noautolink>
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---++ FATAL: NGramModel: bad ngram model topology 
---+++ Main.BenoitFavre - 10 Sep 2012 - 09:18
<noautolink>
I generated an ngram model from a .arpa file with the following command:

ngramread --ARPA lm.arpa > lm.model

ngramread does not complain, but ngraminfo and trying to load the model from C++ code generate the following error:

FATAL: NGramModel: bad ngram model topology

How can I troubleshoot the problem?
</noautolink>


---+++ Main.BenoitFavre - 10 Sep 2012 - 09:27
<noautolink>
Adding verbosity results in more mystery...

ngraminfo --v=2 lm.model 
INFO: FstImpl::ReadHeader: source: lm.model, fst_type: vector, arc_type: standard, version: 2, flags: 3
INFO: Incomplete # of ascending n-grams: 1377525
FATAL: NGramModel: bad ngram model topology
</noautolink>

---+++ Main.BrianRoark - 11 Sep 2012 - 10:33
<noautolink>
Hi,

that error is coming from a sanity check that verifies that every state in the language model (other than the start and unigram states) is reached by exactly one 'ascending' arc, that goes from a lower order to a higher order state.  ARPA format models can diverge from this, by, for example, having 'holes' (e.g., bigrams pruned but trigrams with that bigram as a suffix retained).  But ngramread should plug all of those.  maybe duplication?  I'll email you about this.
</noautolink>

---+++ Main.BrianRoark - 18 Sep 2012 - 10:50
<noautolink>
Benoit found a case where certain 'holes' from a pruned ARPA model were not being filled appropriately in the conversion.  The sanity check routines on loading the model ensured that this anomaly was caught (causing the errors he mentioned), and we were able to find the cases where this was occurring and update the code.  The updated conversion functions will be in the forthcoming version update release of the library, within the next month or two.  In the meantime, if anyone encounters this problem, let me know and I can provide a workaround.
</noautolink>
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-- Main.CyrilAllauzen - 09 Aug 2012
Topic revision: r23 - 2013-04-04 - RolandSchwarz
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