# False Output from find-function?

2 views (last 30 days)
Rene on 30 Aug 2020
Heyho
I'm looking for maximum values of a table and am using
[rows, columns] = find(A==max(A));
to do so in the attached matrix.
It gets me this:
1 1
2 2
3 3
4 4
5 5
6 6
7 7
8 8
9 9
10 10
11 11
12 12
13 13
14 14
15 15
16 16
19 17
17 18
18 18
19 19
20 20
The matrix is 20x20 and the output has 21 values. :S
Also in my opinion the line...
19 17
...isn't true, cause I can't see a maximum there? This shouldn't be there at first place, should it?
Before and after this everything looks fine to me.
Why is this and how can I prevent that from happening?

max(A) is a vector, not a number:
max(A)
ans =
Columns 1 through 15
1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 0.9999 1.0000 1.0000 1.0000 1.0000
Columns 16 through 20
1.0000 0.7950 1.0000 1.0000 0.9998
The value at A(19,17) is the 0.795, which is not a maximum of the matrix but rather a maximum of the column.
if you want to get the maximum of the whole matrix you need this:
max(A(:))
ans =
1.0000
And when you compare, remember that float value comparison is everything but reliable, so you need to have a given tolerance.
In your code it would be something like this:
[rows, columns] = find( abs(A-max(A(:)))<1e-4 );
ans =
1 1
2 2
3 3
4 4
5 5
6 6
7 7
8 8
9 9
10 10
11 11
12 12
13 13
14 14
15 15
16 16
17 18
18 18
19 19
This will probably work for you
A = [1 0.5 0.7;0.4 0.8 0.4;0.2 0.3 0.7];
Amax = max(A); % Get maximum from each column
DiffFromMax = A-Amax; % Zero values are maximum locations
tol = -1e-4; % Note that the tolerance is negative
[rows, columns] = find( DiffFromMax>tol )

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