how do i isolate the elements above both main triangles of a square matrix?

is it better to make the other elements zeroes or to find some kind of returning motive in the indexes?

[dpb]

Depends on the definition of "better"...

Indeterminate as posed.

[Image Analyst]

And the definition of isolate.

Answer 1

[dpb]

l=logical(ones(size(x)));  % a logical array of size of given array (x)
mask=triu(l)&fliplr(triu(fliplr(l));  % mask for those wanted (assume keep diagonal)
ans=x(mask);                          % as vector
ans=x&mask);                          % keeping the values only in array

One can simplify above w/ fewer temporaries, etc., but shows one way to skin the cat explicitly.

Golf, anybody??? :)

Answer 2

[Image Analyst]

How about this:

m = magic(4)
upperTriangle = triu(m)
lowerTriangle = tril(m)

In the command window:

m =
    16     2     3    13
     5    11    10     8
     9     7     6    12
     4    14    15     1
upperTriangle =
    16     2     3    13
     0    11    10     8
     0     0     6    12
     0     0     0     1
lowerTriangle =
    16     0     0     0
     5    11     0     0
     9     7     6     0
     4    14    15     1

[Image Analyst]

David's "Answer" moved here:

that wasn't my question. imagine that you have a square matrix and you draw the 2 main diagonals. you get 4 triangles right? i need the elements of the upper one. sorry if i wasn't clear enough.

better= easier/shorter

[Image Analyst]

Do you want the diagonals also, like 16, 11, 6, or just the values inside, like 2,10,3. Please use magic(5) as an example and show us the output array or vector that you want as output.

>> magic(5)
ans =
    17    24     1     8    15
    23     5     7    14    16
     4     6    13    20    22
    10    12    19    21     3
    11    18    25     2     9

[Image Analyst]

Check out Mohammad's answer? Does that do what you want? If you don't want the elements exactly underneath the diagonals, then there is an input parameter you can pass in to move the diagonal location up or down.

Answer 3

[Roger Stafford]

Try this:

   % Create some arbitrary square matrix, M
   n = 12;
   M = magic(n);

   % Erase all but your "upper triangle"
   [I,J] = ndgrid(1:n);
   M((I>=J)|(I+J>=n+1)) = 0;