If statements with matricies

I am trying to display the matrix positions (Row and column) where the entries are greater than 10 in a given matrix.

I figured a good way to do this is to use the find(x>10) command and if the values are within a range I specify (In this case if find(x>10) >2) then I would subtract two (if the matrix is 2x2) from those two values only. (This would find the row number)

So say find(x>10) yielded the following

ans=
    2
    3
    4

Then that would mean that in position 2,3 and 4 there are values greater than 10 and I would need to subtract 2 from the specific entries in ans. I would do something similar for the columns.

Is this a good way to go about this? Any suggestions?

Answer 1

[Matt Fig]

You might want the two output version of FIND. The one output version does not return the row number, but the linear index.

x = magic(6);  % Sample data
[I,J] = find(x>10);  % I is rows, J is columns.

Now if your goal is only to subtract 2 from the values of x which are greater than 10, and whos row and column position are greater than two (as I read your somewhat confusing post), do this:

idx = I>2 & J>2;
idx = sub2ind(size(x),I(idx),J(idx));
x(idx) = x(idx)-2

Note, if you want to display the positions, you could just use:

x>10 % Shows in command window
spy(x>10)  % Shows a graph.

[Matt]

I apologize for the confusing post, I was writing it in a hurry.

But I see! The problem is, my professor wants us to do it without the [I,J] command. I recall someone asking about it in class and I did not know what they were talking about until I read your post. How would I go about this without that command?

[Paulo Silva]

For matrices, [I,J] = ind2sub(size(A),find(A>5)) returns the same values as [I,J] = find(A>5)

[Matt Fig]

Well, you do see the SUB2IND command, right? If you look at the help for this function, you will see its cousin IND2SUB.

The only other suggestion would be to do it in a double FOR loop:

for ii = 3:size(x,1)

for jj = 3:size(x,2)

% Do your stuff here.

end

end

[Matt]

Cool! Thanks a lot!

[Jan]

@Matt: Does this mean, that you accept the answer? Then enable the "Accepted" flag, please.