Compare nearby elements in array

Hello all,

i am currently working on a project and I need your help.

I have an array with around 500 x 500 elements. The elements are values from 0,1...255.

I need something to compare nearby elements in this array.

The comparsion should work like this:

for i=1:size(A)
    for j=1:size(A)
If  current_element>=Tresh & previous_element>=Tresh & next_element>=Tresh
    B(i) = A(i);
    elseif current_element>=Tresh & next_element>=Tresh & next_after_next_element>=Tresh
    B(i) = A(i);
    elseif second_to_last_element>=Tresh & last_element>=Tresh & current_element>=Tresh
    B(i) = A(i);
    elseif ... (the same with every columm)
    else B(i,j) = 0;
    end

for example:

A = [4 0 0 0 3; 6 0 0 9 2; 6 7 9 8 1; 6 0 0 6 4; 0 10 0 0 0];

the result should be:

B = [0 0 0 0 0; 6 0 0 9 0; 6 7 9 8 0; 6 0 0 6 0; 0 0 0 0 0];

this example already has a threshold which only indicates elements equal to or greater than 5

I tried different posibilites but could not find any solution for my problem. I read a lot about the "Game of Life"-problem but think this is not suitable for my problem.

What i have tried so far is following code:

A = [4 0 0 0 3; 6 0 0 9 2; 6 7 9 8 1; 6 0 0 6 4; 0 10 0 0 0];
B=double(A);
Thresh=5;
for i=1:size(A)
    for j=1:size(A)
    if      A(i)>=Thresh & A(i+1)>=Thresh 
            B(i) = A(i);
    elseif  A(j)>=Thresh & A(j+1)>=Thresh
            B(j) = A(j);
    else    B(i,j) = 0;
    end
    end
end

and the result is:

B = [0 0 0 0 0; 6 0 0 9 2; 6 7 9 8 1; 0 0 0 0 0 ;0 10 0 0 0]

As you can see, it almost works how I need it. Unfortunateley values above 9 are a problem because they are not replaced by 0 if possible.

What can I do to refer to the second to last, previous and next after next element?

I know that for example

elseif  A(i)>=Thresh & A(i+1)>=Thresh & A(i+2)>=Thresh

and

elseif  A(i)>=Thresh & A(i-1)>=Thresh & A(i-2)>=Thresh    

will not work.

I know that '-2' or similar is not working due to logical issues. But why '+2' as well?

Thanks in advance

Answer 1

[KALYAN ACHARJYA]

"I have an array with around 500 x 500 elements"

Please note that you have mentioned about 2-D array , Here I have considered the previous elements and next element based on column number (considering same row),

data=randi(255,[500,500]);
Tresh=?? % Define
for i=2:499
    for j=1:500
    if data(i,j)>=Tresh && data(i-1,j)>=Tresh &&  data(i+1,j)>=Tresh
    B(i,j)=A(i,j);   
    elseif 
        ......% Hope you can implement it now
    end
    end 
end 

[Enthusiasten]

Thanks Kalyan for your answer!

This is the solution I was looking for. To avoid those logical issues, I have to set new limits. This was a thing, I have not considerd.

for i=3:498
    for j=3:498

But is there anyway to consider the first two and last two elements without inserting and later deleting new rows and columns containg zeros?

I mean, I can do other loops like

for i=1
    for i=1
    if      data(i,j)>=Tresh && data(i+2,j)>=Tresh &&  data(i+1,j)>=Tresh
            B(i,j)=data(i,j); 
    elseif  data(i,j)>=Tresh && data(i,j+2)>=Tresh &&  data(i,j+1)>=Tresh
            B(i,j)=data(i,j);
    else    B(i,j)=0
    end
    end
end

and

for i=2
    for i=2
    if     data(i,j)>=Tresh && data(i-1,j)>=Tresh &&  data(i+1,j)>=Tresh
           B(i,j)=data(i,j);
    elseif data(i,j)>=Tresh && data(i,j-1)>=Tresh &&  data(i,j+1)>=Tresh    
           B(i,j)=data(i,j);
    else   B(i,j)=0
    end
    end
end

and the same for the last two rows and columns. But it is not ressource-saving, is it?

Thanks for your help!

[KALYAN ACHARJYA]

Yes, I agreed, if you can avoid multiple loops, it would be better.

[Enthusiasten]

Thanks for your help!

I have finally added two rows and columns at the beginning and the end.

After the loops, I have deleted them so i can evaluate every element during the loops.

Answer 2

[Steven Lord]

I think the islocalmax function, specifying both the dimension over which to operate and a 'ProminenceWindow', will do what you want.

[Enthusiasten]

Thanks Steven for your answer!

I tried this function but it was not, what i exactly wanted.

But it might be helpful for an other problem of mine.

Thanks!