How to use a precompiled inference algorithm when you want to sequentially update the posteriors? (Migrated from community.research.microsoft.com)

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• Friday, June 03, 2011 6:47 PM

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  John Guiver replied on 05-03-2011 10:37 AM

  Hi Shengbo

  You will need to implement the shared variable pattern explicitly rather than use the shared variable wrapper classes. The general pattern for shared variables is described in http://research.microsoft.com/en-us/um/cambridge/projects/infernet/docs/Sharing%20variables%20between%20models.aspx. To paraphrase:

  1. Run inference to convergence on one of the sub-models, say model/chunk A
  2. Extract the shared variable messages output from model/chunk A
  3. Initialise the next model, model/chunk B, say, by providing as input a product of all the output messages from all models/chunks except B (along with the prior)

  Note that you want the output message rather than the marginal from the compiled algorithm (at least if you are running Expectation Propagation); to generate code for this output message, you need to set an Output attribute on the shared variable in your original code (see http://research.microsoft.com/en-us/um/cambridge/projects/infernet/docs/Adding%20attributes%20to%20your%20model.aspx).

  There are several tricky aspects to this which govern how you should define your model. I think the first thing would be to understand how a simple scalar example works. I give code for a learning a Gaussian example below. Note that I am not using precompiled code in this example - its only purpose is to show you the shared variable pattern. The extension from scalar to array follows exactly the same pattern but is a bit more tricky syntactically; I can help you there in a follow-up post if necessary, depending on your needs.

  John

  using System;

  using System.Collections.Generic;

  using System.Linq;

  using System.Text;

  using MicrosoftResearch.Infer.Models;

  using MicrosoftResearch.Infer.Distributions;

  using MicrosoftResearch.Infer;

   

  namespace sharedvar_with_precompiled

  {

      class Program

      {

          static void Main(string[] args)

          {

              var numData = Variable.New<int>();

              Range n = new Range(numData);

              var input = Variable.New<Gaussian>();

              var a = Variable.Random<double, Gaussian>(input).Attrib(new Output());

              var d = Variable.Array<double>(n);

              d[n] = Variable.GaussianFromMeanAndPrecision(a, 1.0).ForEach(n);

   

              var engine = new InferenceEngine();

   

              // Prior

              var Prior = Gaussian.FromMeanAndPrecision(0.1, 0.2);

   

              // Data chunks

              double [][] data = new double[][]

              {

                  new double[] {1.0, 2.0, 3.0},

                  new double[] {1.5, 2.5},

                  new double[] {2.1, 2.2, 2.3}

              };

              int numChunks = data.Length;

   

              // Start with uniform output messages

              Gaussian[] outputs = new Gaussian[numChunks];

              for (int i = 0; i < numChunks; i++) outputs[i] = Gaussian.Uniform();

   

   

              int numPasses = 2;

             

              for (int p = 0; p < numPasses; p++)

              {

                  for (int i = 0; i < numChunks; i++)

                  {

                      // Multiply the prior with all output messages from other chunks

                      Gaussian inputValue = (Gaussian)Prior.Clone();

                      inputValue = Distribution.SetToProductWithAllExcept(inputValue, outputs, i);

   

                      input.ObservedValue = inputValue;

                      d.ObservedValue = data[i];

                      numData.ObservedValue = data[i].Length;

                      if (engine.Algorithm is ExpectationPropagation)

                          outputs[i] = engine.GetOutputMessage<Gaussian>(a);

                      else

                      {

                          outputs[i] = engine.Infer<Gaussian>(a);

                          outputs[i].SetToRatio(outputs[i], inputValue);

                      }

                  }

              }

              Console.WriteLine("Posterior = {0}", engine.Infer<Gaussian>(a));

          }

      }

  }

    

  • Marked As Answer by Microsoft ResearchOwner Friday, June 03, 2011 6:47 PM
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