Strange behaviour: struct with cell values

Hi there,

I'm writing a bigger piece of code and I am using structs as input variables. Now I changed (or tried to change) one input parameter in this struct from an array to cell. A little bit of code first:

order = 1;                  %Dumm
param = {1,2,zeros(2,2)};

opt1.order = order;
opt1.param = param;

opt2 = struct('order',order,'param',param);

As we can see from that bit of sample code,:opt1 is a 1x1 struct (which is what I want) and opt2 is a 1x3 struct (which I do ABSOLUTELY not want).

Why do the two lines of code behave differently?

Is there a way, that opt2 does not get expanded to a 1x3 struct?

Thanks so much!

Answer 1

[Stephen23]

"Strange behaviour: struct with cell values"

The STRUCT() documentation explains what happens with cell array inputs: "... if value is a scalar cell array, then s is a scalar structure... If value is a nonscalar cell array, then s is a structure array with the same dimensions as value."

So it is very simple: if you want the output to be a scalar structure, then the input must be a scalar cell array.

You can achieve this very easily by nesting your (non-scalar) cell array in a scalar cell array:

order = 1;                  %Dumm
param = {1,2,zeros(2,2)};
opt2 = struct('order',order,'param',{param})

opt2 = struct with fields:
    order: 1
    param: {[1]  [2]  [2×2 double]}

[Simon Baeuerle]

Oh ok - I missunderstood that line of the documentation. And I guess

opt1.param = param 

automatically assumes the input to be a scalar cell array?!

[Steven Lord]

The command opt1.param = param doesn't call the struct function. It performs indexed assignment. Indexed assignment into a struct array doesn't have the same behavior as calling the struct function.

Why does struct behave this way? That predates the start of my time at MathWorks, and I haven't done the archaeology to try to determine the reason for that behavior.

[Simon Baeuerle]

Ok perfect. I'll just write a wrapper-function to ensure user-independent, predictable input in the struct and then I'm fine. Perfect! Thank you!