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Creating 1D array from frequency data/histogram?

Asked by Rose Lakatos on 15 Jun 2018
Latest activity Answered by Steven Lord
on 27 Jun 2018

I’m trying to run an algorithm that requires a 1 dimensional data to work. My data is grey values from ct images describing the frequency of 8bit pixel values from 0-255. Is it possible to create an 1D array [0,1,2,3,4,5...255] with corresponding frequency of that pixel value?

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2 Answers

Answer by Gayatri Menon on 27 Jun 2018

Hi,

Could you check out "histogram" command. Please refer to the below documentation to know more on this comment:

https://www.mathworks.com/help/matlab/ref/matlab.graphics.chart.primitive.histogram.html

Hope this helps.

Gayatri

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Answer by Steven Lord
on 27 Jun 2018

If you want the resulting array to contain exactly n(1) copies of x(1), n(2) copies of x(2), etc. use the repelem function. [Depending on how you want to use the result, you may also want to "shuffle" it using randperm.]

x = 0:5;
freq = [2 3 4 5 6 7];
A = repelem(x, freq);

If instead of frequencies you have probabilities for what each element of the resulting array should be, use discretize.

p = freq./sum(freq);
cumulativeprob = [0 cumsum(p)];
B = discretize(rand(1, 1000), cumulativeprob, x);

Let's check that the proportions of elements in B come close to the probabilities in p.

c = histcounts(B, 'BinEdges', [x x(end)], 'Normalization', 'probability');
[c; p]

The values in c look reasonably close to the probabilities in p for just 1000 samples.

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