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I have two probability distributions, a beta distribution and a lognormal distribution. What I want to do is draw a random value from within my beta distribution and based on where that value is in terms of a standard deviation range, then I also want to draw a random value from my lognormal distribution that also is within that standard deviation range.

For example: I draw a 0.2 from my beta distribution, which is above one standard deviation but below two standard deviations greater than the mean. Then I want to generate a random value from my lognormal distribution that is constrained between the range of my first standard deviation and second standard deviation (lets say, a value between 100 and 1000).

The standard deviation ranges are as follows: within one standard deviation, greater than one standard deviation but less than two standard deviations above the mean, greater than two standard deviations above the mean, less than one standard deviation but greater than two standard deviations below the mean, and less than two standard deviations below the mean.

What I've attempted with my code in MATLAB:

1) For each iteration, generate a random beta distribution value and a random lognormal distribution value. 2) If the beta value is within the specified standard deviation range and the lognormal value is also within that specified standard deviation range, then end the loop and print the values in an array

The issue that I am encountering is that the random beta value is generated perfectly fine, but for each lognormal value, I am left with the value as to which iteration the code has went through.

the array looks like:

0.1    1
0.12   2
0.05   3
0.07   4
.......

Here is my code:

close all;
clc;


d= xlsread('Poro perm data for Clarke Lake.xlsx');                                                                               
pdPor = d(:,2);
pdPerm = d(:,8);
porosity_permeability = []; % openspace for array
temp=[];
Perm = [1:300,1];
for i=1:300
    porRandom = betarnd(3.1800,44.87,[1 1]);
    mu=1.0130;
    sigma = 2.574;

    permRandom =lognrnd(mu,sigma);

    if porRandom <=(mean(pdPor) + std(pdPor)) && porRandom >= (mean(pdPor) - std(pdPor))                           

        if permRandom <=(mean(pdPerm) + std(pdPerm)) && permRandom >= (mean(pdPer) - std(pdPer)) 
            Perm(i) = permRandom; 
        end
    elseif porRandom < (mean(pdPor) - (std(pdPor))) && porRandom > (mean(pdPor) - ((std(pdPor))*2))                 

           if permRandom < (mean(pdPerm) - (std(pdPerm))) && permRandom > (mean(pdPerm) - ((std(pdPerm))*2)) 

              Perm(i) = permRandom; 
           end
    elseif porRandom < (mean(pdPor) - (((std(pdPor))*2)))                                              


           if permRandom < (mean(pdPerm) - (((std(pdPerm))*2)))  
              Perm(i) = PermRandom;
           end
    elseif porRandom > (mean(pdPor) + (std(pdPor))) && porRandom < (mean(pdPor) + ((std(pdPor))*2))         

            if permRandom > (mean(pdPerm) + (std(pdPerm))) && permRandom < (mean(pdPerm) + ((std(pdPerm))*2))
                Perm(i) = permRandom;
            end
    else porRandom > ((mean(pdPor)) + (((std(pdPor))*2)))                                        

            if permRandom > ((mean(pdPerm)) + (((std(pdPerm))*2)))   
                Perm(i) = permRandom;
            end
    end

    temp=[porRandom, Perm(i)]; % temporary space
    porosity_permeability=[porosity_permeability;temp]; 

end

Thank you for looking into my problem! I apologize if my explanation and terminology is lackluster. I am just learning coding and MATLAB.

reeno68
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1 Answers1

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Before the for loop, you defined Perm = [1:300,1];, so when all if-condition missed, you'll get the original value, that is the index of your iteration. It is possible because you cannot ensure your two random numbers drop in the same box, right?

If you expect to regenerate until the permRandom reach the same interval of porRandom, you should use another loop inside for-loop, like this:

...
porRandom=***;
while true
  permRandom=***
  if ****
    Perm(i) = permRandom;
    break
  end
end
***your print the good result

So only if you get the two in the same box will it break the loop and continue to print.

Something not about your question: your every operation inside if-end are the same. It is possible to merge into one.

RibomBalt
  • 190
  • 8