MATLAB Quirks: struct with no fields are not empty

As far as struct() is concerned, I’m more inclined to using Struct of Array (SoA) over Array of Structs (AoS), unless all the use cases screams for SoA. Performance and memory overhead are the obvious reasons, but the true motivation for me to use SoA is that I’m thinking in terms of table-oriented programming (which I’ll discuss in later posts. See table() objects.): each field of a struct is a column in a table (heterogeneous array).

Since a table() is considered empty (by isempty()) if it has EITHER 0 rows INCLUSIVE OR 0 columns (no fields) and the default constructor creates a 0 \times 0 table, I thought struct() would do the same. NOT TRUE!

First of all, the default constructor of struct() gives ONE struct with NO FIELDS (so it’s supposed to correspond to a 1 \times 0 table). What’s even harder to remember is that struct2table(struct()) gives a 0 \times 0 table.

The second thing I missed is that a struct() with NO fields is NOT empty. You can have 3 structs with NO fields! So isempty(struct()) is always false!

I usually run into this problem when I want to seed the execution with an empty struct() and have the loop expand the fields if the file has contents in it, and I’ll check if the seeded struct was untouched to see if I can read data from the file. Next time I will remember to call struct([]) instead of struct(). What a trap!

At the end of the day, while struct is powerful, but I rarely find AoS necessary to do what I wanted once table() is out. AoS has pretty much the same restrictions as in table() that you cannot put different types in the same field across the AoS, but table allows you to index with variables (struct’s field) or rows (struct array index) without changing the data structure (AoS <-> SoA). So unless it’s a performance critical piece of the code, I’ll stick with tables() for most of my struct() needs.

 

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