# Calculate probability of number appearing in a column

5 views (last 30 days)
Emma Kuttler on 21 Nov 2019
Answered: Walter Roberson on 23 Nov 2019
I have a 1000x296 matrix called Values. The rows represent trials of a simulation, and the rows represent the rank (1-296) of a particular element.
I want to create a matrix that returns the probability of a column having a particular value. I assume i will need a set of nested for loops to count the number of times each value appears in each column, and then divide by the number of rows (1000).
E.g.
The value in cell (1,1) will have the probability of a particular row of column 1 having value 1
The value in cell (1,15) will have the probability of a particular row of column 1 having value 15
The value in cell (200, 155) will have the probability of a particular row of column 200 having value 155.
Thanks!
Walter Roberson on 22 Nov 2019
Neither accumarray nor histcounts2 calculate probability distributions. In its simplest form, accumarray just counts.

Image Analyst on 21 Nov 2019
Just loop over the array getting the histogram in each column.
Something like (untested)
numBins = 25; % Whatever you want.
edges = linspace(0, max(Values(:), numBins);
[rows, columns] = size(Values);
allCounts = zeros(numBins, columns);
for col = 1 : columns
thisColumn = Values(:, col);
counts = histcounts(thisColumns, edges);
allCounts(:, col) = allCounts(:, col) + counts;
end
% Normalize
allCounts = allCounts / sum(allCounts(:));
bar3(allCounts); % Display

Walter Roberson on 23 Nov 2019
[rowidx, colidx] = ndgrid(1:size(Values,1), 1:size(Values,2));
probs = accumarray([colidx(:), Values(:)],1) ./ size(Values,1);;

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