problem in finding Mean value

Question

Vennila Gangatharan 2013-1-6

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https://ww2.mathworks.cn/matlabcentral/answers/58056-problem-in-finding-mean-value

编辑： Walter Roberson 2017-6-26

Hi,

I want to find mean value of a mammogram only by choosing the values greater than zero. I used for loops with condition but it gives 255 as answer constantly. I don't know what's the mistake i have done.

Code:

[x,y] = size(I);
s = 0;
for i = 1:x
      for j = 1:y
          if I(i,j) > 0 
               s = s + I(i,j);
          end
      end
end
disp('s = ');
disp(s);

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Answer 1

Walter Roberson 2013-1-6

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Change to

s = s + double(I(i,j));

Question: when you are calculating the mean, are you going to be dividing by the number of values in I, or by the number of non-negative values?

Also are you sure that I will be two-dimensional and not 3 dimensional?

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Vennila Gangatharan 2013-1-6

在 MATLAB Online 中打开

Thank you walter. It works. I am going to divide the sum by no. of non-negative values.

Corrected Code:

[x,y] = size(I);

s = 0;

cnt = 0;

for i = 1:x

    for j = 1:y
        if I(i,j) > 0 
             s = s + double(I(i,j));
             cnt = cnt + 1;
        end
    end

end

m = s/cnt;

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Answer 2

Image Analyst 2013-1-6

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Why are you doing two loops, which will make it slow? Why not just do:

nonZeroPixels = yourImage > 0;
meanValue = mean(yourImage(nonZeroPixels));

The way above is vectorized, faster, and very MATLABish.

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Answer 3

Jan 2013-1-6

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https://ww2.mathworks.cn/matlabcentral/answers/58056-problem-in-finding-mean-value#answer_70303

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Alternative method without loops:

s = mean(double(I(I > 0)));

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Walter Roberson 2013-1-6

mean() works even without the double()

Jan 2013-1-6

Yes, Walter, because the underlying SUM uses the DOUBLE format as default, when 'native' is not specified. I would prefer to let functions reply the same type as the input as default, therefore I tend to cast more often than needed.

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