How can I count the number of times a value occurs

Hi there,

I have a 3D array (mask) with the numbers 0 background, 1, 2, 3 and 4. The numbers 1,2,3 and 4 correspond to particular regions.

I want to count how many regions in my 3D mask have the number 1,2,3, or 4.

Does anyone know how to do this?

Answer 1

[Rik]

You can use bwlabeln to label all your non-zero regions. then you can loop through each of the labeled regions to determine which number they have. With that, you essentially have the result you need.

If you don't have the image processing toolbox, let me know, because I have an alternative that doesn't require any toolbox.

Edit: added the example below.

%generate example data where max(L(:)) returns about 30
IM=randi(300000,100,100,100);
IM(IM>10)=0;

[L,n_regions]=bwlabeln(IM>0);
counts=zeros(1,max(IM(:)));
for n=1:n_regions
    ind=find(L==n,1);
    field_val=IM(ind);
    counts(field_val)=counts(field_val)+1;
end

[Gina Carts]

Hi Rik,

Thanks for your reply. I tried the bwlabeln function like this:

[L,n] = bwlabeln(mask);

When I printed n, I got the number 32. This is actually the correct number of regions I have (non-zero) but it doesn't give me how many regions of these 32 have number 1, how many number 2, how many number 3 etc.

How can I do that?

[Rik]

I added an example to my answer. Let me know if there is anything unclear.

[Gina Carts]

I think I see how it works. When I print the counts I got the following:

counts =

4 5 4 2 2 4 1 5 1 5

That means I have 4 regions with number 1, 5 regions with number 2 etc. Am I correct?

[Rik]

Yes, that is the idea. There might be more efficient ways to get to this vector, but this is what I could come up with.