All pixels except one pixel in matlab

Question

Say that we have a 3x3 matrix in matlab. If we type x(:), this will select all the elements in the matrix, right?

How can we select all the elecments except element x(2,2)? What should we type in this case?

Thanks.

Possible duplicate? http://stackoverflow.com/q/11054728/2065121 — Roger Rowland, May 23 '13 at 09:47
@EitanT: I just created a new tag [tag:matrix-indexing], which should apply to lots of questions.. I cant believe this hasn't been done before :) The existing tags refer to databases indexing — Amro, May 23 '13 at 12:47
@Amro I've done my share of tagging now (by the way, I've also noticed the misuse of the [tag:delete] tag, which really refers to deleting files, not elements from an array). — Eitan T, May 23 '13 at 13:20
@EitanT: appreciate it, thanks. I wish stack exchange would introduce new 10K tools for mass retagging :) — Amro, May 23 '13 at 13:27
@Amro Nice idea, you should post it on Meta. Some tool that accepts rules, for example: "_retag all [tag:index]-tagged questions that are also [tag:matlab] questions with [tag:matlab-indexing]_". — Eitan T, May 23 '13 at 14:01
I wonder if moderators have access to such tools... Another area lacking is synonyms suggestion; speaking of which could you take a look at this one: http://stackoverflow.com/tags/matlab-compiler/synonyms , its so hard to get attention to these suggestions — Amro, May 23 '13 at 14:18

devrobf · Answer 1 · 2013-05-23T10:56:42.377

1

I would do it like this: first, create a logical array of trues, which would select all elements if used as an index mask:

mask = true(size(x) );

Now set element 2,2 to be false, therefore deselecting it:

mask(2,2) = false;

Now use this mask to select elements from x:

myValues = x(mask);

EDIT: Removed second, incorrect answer.

edited May 23 '13 at 10:56

answered May 23 '13 at 09:48

devrobf

Thanks for your reply. When I tried your *simpler* solution, I got this error: `Subscripted assignment dimension mismatch.`. Why is that? – Simplicity May 23 '13 at 09:58
You can't remove an element that would result in a "non-rectangular matrix", so to speak. The method based on logical indexing is more expressive, anyways, and does not involve mutating a matrix. – Joshua Barr May 23 '13 at 10:04
@JoshuaBarr You're correct, apologies. As I said I saw it on another answer, I hadn't followed it through properly. – devrobf May 23 '13 at 10:55
@Med-SWEng My mistake (see Joshua's comment), go with the first solution in this case! – devrobf May 23 '13 at 10:56
@jazzbassrob The second answer is also fine, with a minor adjustments. Convert the original matrix into a column vector first and then use or linear indices (or `sub2ind`) to remove elements. – Eitan T May 23 '13 at 12:01

score 0 · Answer 2 · answered May 23 '13 at 11:36

You could use:

 A(setdiff(1:numel(A),ceil(numel(A)/2)))

For example, for the input as:

>> A = randi(100,3)

A =

    49    71    68
    45    76    66
    65    28    17

The output is:

>> A(setdiff(1:numel(A),ceil(numel(A)/2)))

ans =

    49    45    65    71    28    68    66    17

2 Answers2