double indexing
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Frank
am 12 Jan. 2012
Bearbeitet: DGM
am 20 Dez. 2024
Hello,
is it possible to do double or multiple indexing in matlab? E.g.
>> v = [1,2,5,7,9]
v =
1 2 5 7 9
>> v(1:4)
ans =
1 2 5 7
>> v(1:4)(2:3)
ans =
2 5
The first index create a new vector and the second index creates a new vector out of the newly created one. It is common in other languages, and it helps to avoid defining temp variables.
Cheers, Frank
2 Kommentare
Katharina Wellstein
am 20 Dez. 2024
Bearbeitet: Walter Roberson
am 20 Dez. 2024
A simple way to do this would be by using intersect, a function that finds commonalities between different arrays.
In this case it would be:
[value, position] = intersect(v(1:4), v(2:3))
For more complicated things like indexing into an array (or matrix) with different indices, the same thing can be used. E.g. if I want to find numbers that are higher than 5 and numbers that are even in x, I can do the following:
x = randi(randi(10,10,1);
bigIdx = find(x>5);
evenIdx = ~mod(x,2);
x(intersect(bigIdx,evenIdx);
Walter Roberson
am 20 Dez. 2024
The hypothetical v(1:4)(2:3) is intended to select elements 1 through 4 of v, and then select elements 2 to 3 of that.
Suppose we had instead asked for v(2:4)(2:3), then that would be elements 2 to 3 out of elements 2 to 4, so the end result would be v(3:4) .
[value, position] = intersect(v(2:4), v(2:3))
would give back v(2:3) instead, at best.
If we had
v = [1,5,5,7,9]
then [value, position] = intersect(v(1:4), v(2:3)) would be interesect([1 5 5 7], [5 5]) which is going to return [5] rather than [5 5] because the return values of intersect() are sorted and unique by default.
Overall, intersect() is just the wrong thing to use here.
Akzeptierte Antwort
Walter Roberson
am 12 Jan. 2012
The closest you can get is
subsref(V(1:4), struct('type', '()', 'subs', 2:3)))
You can follow {} referencing by () referencing, {}(), but you cannot use ()() or (){} or ().{} or ().(), and you cannot use function(){} or function()() or function().field
3 Kommentare
Petter
am 11 Jul. 2023
"You can follow {} referencing by () referencing, {}()"
This one line made my day so much easier, greatly appreciated. Thumbs up, gold star.
Walter Roberson
am 20 Dez. 2024
In the time since the above Answer was posted, Mathworks has since permitted function().field
Weitere Antworten (2)
mklcst mklcst
am 23 Jan. 2014
I think it could be very useful to have a short way to perform double indexing.
0 Kommentare
Jos (10584)
am 23 Jan. 2014
If the indices are stored in variables, this is trivial!
V = [1,2,5,7,9]
ii = 1:4
jj = [2 3]
out = V(ii(jj))
2 Kommentare
lee eugene
am 2 Aug. 2019
However, if the two index do not have the same starting indexing, there would be something wrong. For example, i would like to select index [2,4] from [1,2,5,7,9] at first, and then select index [1,2]. Then it would be [2,5,7] at first and [2,5] in the end.
DGM
am 20 Dez. 2024
Bearbeitet: DGM
am 20 Dez. 2024
That's what the result is supposed to be.
That said, there's still room for problems if the index vectors are programmatically generated. We'd have to safeguard against out-of-range indices. Consider one possibility:
% a vector
V = [1,2,5,7,9];
% potentially out-of-range indices
ii = [1:4 7];
jj = [2 3 5];
% one option might be to simply
% discard invalid indices
ii = intersect(ii,1:numel(V))
jj = intersect(jj,1:numel(ii))
% so it still works without indexing errors
out = V(ii(jj))
I'm sure there are other ways one might choose to handle it though. This might not be okay if you're not expecting to a different number of elements than what you requested. In that case, it might be better to just throw an error.
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