Asked by Vennila
on 6 Jan 2013

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);

Answer by Walter Roberson
on 6 Jan 2013

Accepted answer

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?

Vennila
on 6 Jan 2013

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;

Answer by Image Analyst
on 6 Jan 2013

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.

Answer by Jan Simon
on 6 Jan 2013

Alternative method without loops:

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

Walter Roberson
on 6 Jan 2013

mean() works even without the double()

Jan Simon
on 6 Jan 2013

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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