Editing multiple elements of an array in one line

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Kyle Ruzic
Kyle Ruzic 2018-5-16
评论： Rik 2018-5-16
My problem is this, I have a huge array and I want to edit multiple elements of the same array that I know the location of. Here's how I would assume this should work based on how matlab handles arrays
h = zeros(10,10,10,2); % just a simple example array
b = [10 5]; % elements I want to place in new location
i = [1 2]; % "locations" I want to place them in
h = h(1,1,1,i) + b(i)
Now what I would expect is for h(1,1,1,1) = 10 and h(1,1,1,2) = 5. But instead what you get is h(1,1,1,1) = [10 5] and h(1,1,1,2) = [10 5]
Why is this? I know a solution is to simple edit them in a loop, however this isn't actually answering the problem.
Thanks

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Rik
Rik 2018-5-16
The error you get for your example will probably depend the release. On R2018a, the implicit expansion tries to expand b to fit the 4D selection from h. What I needed to do is make sure that b is 1x1x1x(numel(b)):
h = zeros(10,10,10,2); % just a simple example array
b = [10 5]; % elements I want to place in new location
i = [1 2]; % "locations" I want to place them in
h(1,1,1,i) = h(1,1,1,i) + permute(b(:),numel(size(h)):-1:1);
h(1,1,1,1)
h(1,1,1,2)
  2 个评论
Kyle Ruzic
Kyle Ruzic 2018-5-16
Sorry I edited the question as you were typing your answer to change h(1,1,1,i) to just h, which is what I was doing to produce the result I mentioned in the question.
I'm a bit confused by how this
permute(b(:),numel(size(h)):-1:1);
is changing the size of b.
Thanks for the quick response!
Rik
Rik 2018-5-16
If you read the doc for permute, you can see that it changes the order of the dimensions, so in this case, the 2x1 vector resulting from the b(:) operation, is changed. A 2x1 vector is the same as a 2x1x1x1 array, so permute uses the vector [4 3 2 1] (the output of numel(size(h)):-1:1)) to change this to 1x1x1x2, which fits the excerpt from h defined by the h(1,1,1,i) operation.

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