Product of probabilities by group

Question

Fernando 2012-12-8

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https://ww2.mathworks.cn/matlabcentral/answers/56038-product-of-probabilities-by-group

Hi,

I have matrix that contains in the first column a group identifier and in the second column a probability. I want to generate, for each group identifier, the product of the probabilities that correspond to that group.

For example

X=[1 0.5;1 0.3; 1 0.4; 2 0.7; 2 0.4];

What I want is to generate a matrix

Y=[1 (0.5*0.3*0.4);2 (0.7*0.4)];

and I can't do it "by hand" because the number of groups and the number of probabilities is huge.

Any ideas? Thanks.

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Answer 1

Matt Fig 2012-12-8

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编辑：Matt Fig 2012-12-8

在 MATLAB Online 中打开

Here is another method. This might work best if you have groups in increments of one because it uses the group number as the index of the return array.

Y = accumarray(X(:,1),X(:,2),[],@prod);

Y(I) is the product of the probabilities belonging to the Ith group. To understand this, look at:

X = [1    0.5000
  0.3000
  0.4000
  0.7000
  0.5000
  0.6000
  0.7000
  0.2000];  % No group 3 or 5.
Y = accumarray(X(:,1),X(:,2),[],@prod);

Since there is no group 3 or 5 in X, Y(3) and Y(5) are zero. If there are missing groups in your array, you can still get to the desired form like:

Z = [find(Y),Y(Y~=0)];

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Answer 2

the cyclist 2012-12-8

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Here's one way:

[uniqueX1,~,index] = unique(X(:,1));
numberUniqueX1 = numel(uniqueX1);
Y = nan(numberUniqueX1,1);
for nx = 1:numberUniqueX1
    Y(nx) = prod(X(index==nx,2));
end