Normalizing double values for probability calculus

Question

I want to normalize weights in a list of particles. These weights belong to particle-objects. I try to normalize them by dividing them with the sum of the weights. All the weights are declared in doubles. When the program starts dividing at the start of the list, the value is correct, but soon after second or third division, I get wrong results.. which has the consequence that the sum of the weights after the operation is not 1, which it should be. Can anyone help me with this problem ? Maybe something to do with threading ? Thx in advance..

// normalizing weights
double weightsum = 0;
double check = 0;
List<ParticleRobot> temporalparticleSet = new List<ParticleRobot>();

for (int i = 0; i < particleSet.Count; i++)
{
    weightsum = weightsum + this.particleSet[i].Weight;
}

Program.Weightsum = weightsum;

Console.WriteLine("Sum of unnormalized particleweights is " + weightsum);

foreach (ParticleRobot p in this.particleSet)
{
    Program.Weight = p.Weight;
    p.Weight = Program.Weight / Program.Weightsum;
    Console.WriteLine("Updated Particleweight is now : " + p.Weight);
}

// checking that they sum up to 1
for (int i = 0; i < particleSet.Count; i++)
{
    check = check + this.particleSet[i].Weight;
}

Console.WriteLine("Check: Sum of particles-weights is = " + check);

Answer 1

First off, what is temporalparticleSet? Do you actually mean to be looping through that instead of this.particleSet? Other than that I don't see any issues with the code at all. I would modify:

Program.Weight = p.Weight;
p.Weight = Program.Weight / Program.Weightsum;

to

p.Weight = p.Weight / Program.Weight;

Also is p.Weight a double? Might be some rounding issue. Have you tried using a breakpoint and stepping through it?