Deleting specific values of fields inside a struct array

Question

Marc Laub 2020-5-2

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/522444-deleting-specific-values-of-fields-inside-a-struct-array

评论： Marc Laub 2020-5-5

采纳的回答： Ameer Hamza

在 MATLAB Online 中打开

Hello everyone,

I have a struct array with multiple fields. All fields are arrays with the same length.

a=struct();
a.field1=rand(20,1);
a.field2=rand(20,1);
a.field3=rand(20,1);

Is there away to delet a specific element from all field arrays at the same time?

So for example delete element 3 from field1, field2 and field3 under specific conditions, for example when field3 value==0.5.

I just know this way:

a.field1(a.field3==0.5)=[];
a.field2(a.field3==0.5)=[];
a.field3(a.field3==0.5)=[];

But is it possible to do this in on line instead of handling every field seperatly?

Best regrads

Marc

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Stephen23 2020-5-2

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"But is it possible to do this in on line instead of handling every field seperatly?"

You could use a loop or structfun (which just hides the loop inside).

Note that comparing floating point numbers for exact equality is a common cause of bugs, you should probably compare the absolute difference against a tolerance:

idx = abs(A-B)<tol

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Answer 1

Ameer Hamza 2020-5-2

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/522444-deleting-specific-values-of-fields-inside-a-struct-array#answer_429779

编辑：Ameer Hamza 2020-5-2

在 MATLAB Online 中打开

Something like this

a=struct();
a.field1=rand(20,1);
a.field2=rand(20,1);
a.field3=rand(20,1);
mask = a.field3==a.field3(5); % a.field3(5) is used as example
a = cell2struct(structfun(@(x) {x(~mask)}, a), fieldnames(a));

Result:

>> a
a = 
  struct with fields:
    field1: [19×1 double]
    field2: [19×1 double]
    field3: [19×1 double]

An alternative way is to use the for-loop. It will probably be faster than the above code because it does not need to create a temporary cell array and recreate the struct.

mask = a.field3==a.field3(5);
names = fieldnames(a);
for i=1:numel(names)
    a.(names{i})(mask) = [];
end

Note: If you are going to set the value for comparison with a.field3 arbitrarily, i.e., the comparison value is not already an element of the a.field3 itself. The comparison == for floating-point value can fail. In that case, you can define the mask based on tolerance.

mask = abs(a.field3 - 0.5) < 1e-6; % 1e-6 is the tolerance for the comparison with 0.5

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Ameer Hamza 2020-5-2

Good point. I assumed that the comparison value would be taken out of the vector itself, e.g., using min(), max(), etc. But if some other operations are performed on the value, then the == comparison can fail.

Marc Laub 2020-5-5