Deleting Elements in a Structure - Error Matrix Index is Out of Range

I have a struct with several fields

x.score = [98 75 22 75 29];
x.count = [1 2 3 4 5];

I want to delete all the elements in x (in all fields) that have a count < 4. So in this case I want X to be

x.score = [98 75 22]

x.count = [1 2 3]

Why doesn't this cmd work?

x([x.count<4]) = []

Matrix index is out of range for deletion.

[Stephen23]

"Why doesn't this cmd work?"

Because you are mixing up indexing into x (which is a scalar structure) with indexing into the fields of x (which are numeric vectors). If you want to remove elements from the fields of x then you need to index into those fields, and not by attempting to into the scalar structure x. Sometimes it is useful to index into a structure, but not if your goal is to index into its fields!

x.score = [98,75,22,75,29];
x.count = [1,2,3,4,5];
idx = x.count>=4;
x.score(idx) = [];
x.count(idx) = []

x = struct with fields:
    score: [98 75 22]
    count: [1 2 3]

Note that these square brackets are completely superfluous, get rid of them: [x.count<4] Even if x was non-scalar that would still be invalid syntax, so they indicate (as most situations where a user has superfluous square brackets) a confusion about what square brackets do and when to use them:

y(1).count = 1;
y(2).count = 2;
[y.count<4] % square brackets don't do anything useful here either

Error using <
Too many input arguments.

Answer 1

[Voss]

x.score = [98 75 22 75 29];
x.count = [1 2 3 4 5];

x.count<4 is a 1x5 logical array

x.count<4

ans = 1×5 logical array
   1   1   1   0   0

x(x.count<4) = [] would try to delete elements 1, 2, 3 of x (where x.count<4 is true), but you can't do that because x only has one element

size(x)

ans = 1×2

1 1

so you get the error.

Instead of deleting elements of x, you want to delete elements of each field of x, which can be done as follows:

f = fieldnames(x);
idx_to_delete = x.count<4;
for ii = 1:numel(f)
    x.(f{ii})(idx_to_delete) = [];
end
disp(x)

    score: [75 29]
    count: [4 5]

This gives a result different than what you specified the result should be. Perhaps the condition should be x.count>=4?

x.score = [98 75 22 75 29]; % re-create x
x.count = [1 2 3 4 5];

f = fieldnames(x);
idx_to_delete = x.count>=4;
for ii = 1:numel(f)
    x.(f{ii})(idx_to_delete) = [];
end
disp(x)

    score: [98 75 22]
    count: [1 2 3]

[Stephen23]

Note that these square brackets are misleading and completely superfluous: [x.count<4]

The output of any logical operator is already an array: https://www.mathworks.com/help/matlab/logical-operations.html

[Voss]

Thank you. Corrected.

[John]

Thanks. I was expecting a way to filter on data with a large number of fields without having to write a loop. Maybe using a table instead of struct is more appropriate. I have a large number of fields that I want to filter on multiple parameters to looks for patterns in a subset of the data.

[Voss]

You're welcome! Yes, a table may be more convenient, or just use a loop.