error in array of struct matlab

Question

I want to train data on various labels using svm and want svm model as array of struct. I am doing like this but getting the error:

Subscripted assignment between dissimilar structures.

Please help me out

model = repmat(struct(),size);
for i=1:size
     model(i) = svmtrain(train_data,labels(:,i),'Options', options);    
end

What is `size`? Further, `size()` is a function. If you name a variable `size`, then you won't be able to call the `size()` function anymore. Usually, this is a bad idea - I'd consider changing this variable name to something like `sizeOfWhatever`, `whateverSize`, .... — hbaderts, Apr 06 '16 at 10:19
size is actually a variable which decides the length of array. I renamed this with some other variable but still I am getting same error. — garg, Apr 06 '16 at 11:26

score 0 · Accepted Answer · edited May 23 '17 at 12:24

A structure array in MATLAB can only contain structures with identical fields. By first creating an array of empty structures¹ and then trying to fill it with SVMStruct structures, you try to create a mixed array with some empty structures, and some SVMStruct structures. This is not possible, thus MATLAB complains about "dissimilar structures" (not-equal structures: empty vs. SVMStruct).

To allocate an array of structs, you will have to specify all fields during initialization and fill them all with initial values - which is rather inconvenient in this case. A very simple alternative is to drop this initialization, and run your loop the other way around^2,3:

for ii=sizeOfLabels:-1:1
     model(ii) = svmtrain(train_data,labels(:,ii),'Options', options);    
end

That way, for ii=sizeOfLabels, e.g. ii=100, MATLAB will call model(100)=..., while model doesn't exist yet. It will then allocate all space needed for 100 SVMStructs and fill the first 99 instances with empty values. That way you pre-allocate the memory, without having to worry about initializing the values.

¹Note: if e.g. size=5, calling repmat(struct(),size) will create a 5-by-5 matrix of empty structs. To create a 1-by-5 array of structs, call repmat(struct(),1,size).

²Don't use size as a variable name, as this is a function. If you do that, you can't use the size function anymore.

³i and j denote the imaginary unit in MATLAB. Using them as a variable slows the code down and is error-prone. Use e.g. k or ii for loops instead.

error in array of struct matlab

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