Matrix linear indexing: how to add more than once to the same index
1 vue (au cours des 30 derniers jours)
Afficher commentaires plus anciens
Pablo García Auñón
le 25 Août 2016
Commenté : Pablo García Auñón
le 26 Août 2016
Here is my question. Let's imagine we have this matrix:
A = [1 2 ; 3 4];
So if I use linear indexing to add 1 to, for instance, positions 1 and 3:
A([1 3]) = A([1 3]) + 1
A =
2 3
3 4
Now, let's imagine that, having the initial A matrix, I have an index vector in which some indices are repeated. For example, I need to add twice 1 to the first position, and once to the index 3:
A([1 3 1])=A([1 3 1])+1
A =
2 3
3 4
And surprisingly I get A(1,1)=2, instead of A(1,1)=3. How can I make it work?
The first idea was to put it inside a loop, but the matrix is normally big and this operation runs many times, and it becomes very slow. Any ideas?
Thank you very much in advance!
2 commentaires
per isakson
le 25 Août 2016
"And surprisingly I get A(1,1)=2, instead of A(1,1)=3."   Matlab obviously doesn't work the way you think it should. I cannot find any support for this behavior in the documentation. Neither that Matlab should work this way, nor should not. In cases like this Matlab rules ;-)
>> B = 1
B =
1
>> B([1,1,1]) = B([1,1,1])+1
B =
2
Guillaume
le 25 Août 2016
The behaviour is not surprising at all and totally expected.
With
A = [10 20; 30 40];
A([1 3 1]) is the content of indices 1, 3 and 1, so: [10 30 10]. Therefore A([1 3 1]) + 1 is the vector [11 21 11].
You then ask matlab to assign that vector to A([1 3 1]), so A ends up being
[11 21; 30 40]
Réponse acceptée
Guillaume
le 25 Août 2016
Another option, possibly faster than a loop or unique + histc:
A = [10 20;30 40];
idx = [1 3 1];
A(:) = A(:) + accumarray(idx(:), 1, [numel(A), 1]);
2 commentaires
dpb
le 26 Août 2016
Yeah, but I thought it "cute"... :)
I always have a heckuva' time coming up w/ "the right stuff" with accumarray...
Plus de réponses (2)
Azzi Abdelmalek
le 25 Août 2016
A = [1 2 ; 3 4]
idx=[1 3 1]
for k=1:numel(idx)
A(idx(k))=A(idx(k))+1
end
0 commentaires
Voir également
Catégories
En savoir plus sur Creating and Concatenating Matrices dans Help Center et File Exchange
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!