Problem with generate exterior border of regions

Dear colleagues,

I need to calculate the mean intensity of exterior border of regions. This regions are identified in a labels matrix called L. The number of regions is num=47.

For this purpose, I put in Matlab the following sentences:

pborde=strel('disk',3);  
for k=1:num
pborde2(k)=imdilate((L==k),pborde);
Bext(k)=xor ((L==k),pborde2(k));
Bext(k)=double(Bext(k));

but Matlab give me the following error "Subscripted assignment dimension mismatch"

I have tested the same sentence but with k=47

pborde247=imdilate((L==47),pborde);
Bext47=xor ((L==47),pborde247);
Bext47=double(Bext47);

And It calculates correct. What is the problem? What sentences is wrong?

After solution this problem I calculate the mean intensity with the following order:

p2ext(k)=regionprops(Bext(k),yb,'MeanIntensity')

where yb is the original signal

Thank you vey much

[Walter Roberson]

dbstop if error

and then rerun your program. When the error is encountered, examine the steps one by one.