Efficiently filtering a MATLAB struct array?

Question

Suppose I have a struct array in MATLAB:

a= struct('a1',{1,2,3},'a2',{4,5,6})

How can I efficiently (i.e. with vectorized code) filter the elements of the struct so that they satisfy some boolean property?

For instance:

How would I create a new struct array b whose elements are the subset of a where both a1 is a multiple of 3 and a2 is a multiple of 3? The expected result is a struct array of size 1 with the element struct('a1', 3, 'a2', 6).
How would I create a new struct array b whose elements are the subset of a where a1 is odd or a2 is a multiple of 3? The expected result is the following struct array of size 2: struct('a1', {1,3}, 'a2', {4,6}).

Steve · Accepted Answer · 2012-06-28 18:20:02Z

You can solve this problem with the mod(...) function and proper use of brackets and referencing. Consider

Mod(x,3) will return zero if your number is a multiple of 3. mod(x,2) will return 1 if x is odd.
You can get all your a1 or a2 values in a vector by typing [a.a1]. Just typing a.a1 gives a mess.
You can filter our from your a structure by writing a = a([1 3]); or by writing a = a(logical([1 0 1])) to get the same result.
You can use the & for logical and and | for logical or (see here).

Altogether, the following code solves your problem:

%% Part 1:
a= struct('a1',{1,2,3},'a2',{4,5,6});
logForA1isMod3 = (mod([a.a1], 3) == 0);
logForA2isMod3 = (mod([a.a2], 3) == 0);

a = a(logForA1isMod3  & logForA2isMod3);

%% Part 2:
a= struct('a1',{1,2,3},'a2',{4,5,6});
logForA1isOdd = (mod([a.a1], 2) == 1);

a = a(logForA1isOdd | logForA2isMod3);

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