Elegant way to extract part of a structure as an array

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Andrew 2011-5-10

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https://ww2.mathworks.cn/matlabcentral/answers/7155-elegant-way-to-extract-part-of-a-structure-as-an-array

评论： Samuel 2023-12-6

采纳的回答： Matt Fig

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Hi,

I would like to be able to extract part of a structure as an array. For example, suppose I create a simple structure:

s(1).x=1;
s(1).y=2;
s(2).x=3;
s(2).y=4;
s(3).x=5;
s(3).y=6;

Now suppose I want to extract the x values for the first two parts of this structure (that is, (1).x and (2).x). So I try:

>> s([1 2]).x
ans =
       1
ans =
       3

I get the x values for (1) and (2), but as two separate outputs. So if I make an assignment like the following:

>> vals=s([1 2]).x
vals =
       1

It only captures the first of the two outputs. I can get around this by putting the result of s([1 2]).x in a cell array, using curly braces:

>> vals={s([1 2]).x}
vals = 
      [1]    [3]

But I actually don't want these values in a cell array; I would like them an array, with each value in a row. I can do this by the following:

>> vals=cell2mat({s([1 2]).x}')
vals =
       1
       3

Now I have what I want. But, my question is, is there an easier, more elegant way to do this? My conversion of the output from array to cell array and then back to array seems very convoluted.

Thanks in advance.

Andrew DeYoung

Carnegie Mellon University

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Anna Mary Mc Cann 2021-6-18

If x happened to be a string variable, like a filename, the following works well: char({s(:).x})

Samuel 2023-12-6

An easier way to do this would be simply typing:

vals = [s([1 2]).x]

vals = [s([1 2]).x]'

to get a row/column vector for the answers to s(1).x and s(2).x

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采纳的回答

Matt Fig 2011-5-10

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vals = [s(1:2).x]

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Jolanda Müller 2023-9-25

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This is great, but what would be an equivalent of this if I have a more nested struct?

e.g.

s(1).x.a = 1
s(1).x.b = 10
s(1).y = 'one'
s(2).x.a = 2
s(2).x.b = 20
s(2).y = 'two'

When trying to get the array

[s.x.a]

it throws an error saying "Intermediate dot '.' indexing produced a comma-separated list with 2 values, but it must produce a single value when followed by subsequent indexing operations."

Is there an elegant way to do an equivalent to [s.x.a] that does not throw an error? The output I would like to see in this example would be [1 2].