Deleting specific values of fields inside a struct array

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

[Stephen23]

"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

Answer 1

[Ameer Hamza]

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

[Marc Laub]

Yes something like this,

I used

a(1:somevalue)=struct();

before. So i could index the struct directly and delete the values from every field. But to allocate multiple values to the fields at one i had to use cell2mat and mat2cell. I wanted to bypass that by handling the fields itself as arrays. That leads me to the problem that i cant delete specific field elements without using cell2mat or cell2struct.

I thought there was a simpler way, without transforming between cells, mats and structs

Thank you for your help

[Ameer Hamza]

Check the second code in the updated answer.

[Image Analyst]

Sorry but that will only work for multiples of 2 or inverse multiples of 2 like 0.5. Why? Please refer to the FAQ: Why_is_0.3_-_0.2_-_0.1_or_similar_not_equal_to_zero?

Better show him how to use ismembertol(), because he'll soon come back to you saying it doesn't work again.

[Ameer Hamza]

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]

Its ok,

I modified it anyway to:

mask = a.field3==2;

since my trigger value isalways 2, so its fine.