How do I preallocate memory

I get the error message at the end of the first for loop (after code: OutData(ii).hucID = hucID;) that I should consider preallocating memory. How do I do that for the following code:

for ii = 1:length(hucPoly)   
    hucSub      =   polyshape(hucPoly(ii).X, hucPoly(ii).Y);
    hucID       =   hucPoly(ii).Name;
    OutData(ii).hucID   =   hucID;
    
    for i = 1:size(inFiles,1)
        inShapeOld  =   shaperead([dataFolder char(inFiles.OldShape(i))]);
        
        inPolyOld   =   polyshape([inShapeOld.X],[inShapeOld.Y], 'Simplify', false);
        
        inShapeNew  =   shaperead([dataFolder char(inFiles.NewShape(i))]);
        inPolyNew   =   polyshape([inShapeNew.X],[inShapeNew.Y], 'Simplify', false);
        
        inPolyNewsub    =   intersect(inPolyNew, hucSub,'KeepCollinearPoints',true);
        inPolyOldsub    =   intersect(inPolyOld, hucSub);
        [OutData(ii).intsecArea(i).Data, S] = polyIntersect(CoRegErrData, inPolyNewsub, inPolyOldsub);
        OutData(ii).intsecArea(i).ID    =   char(inFiles.NewShape(i));
        if ~isnan(S(1).X(1))
            shapewrite(S, [dataFolder 'outShapes_S\' hucID '_outInt_' char(inFiles.NewShape(i))])
        end
        T   =   table(OutData(ii).intsecArea(i).Data, 'VariableNames', {'outInt_acres1' });
        writetable(T, [dataFolder 'outTables_S\' hucID '_outInt_' char(inFiles.NewShape(i)) '.txt'])
    end
end

Answer 1

[Voss]

One way is to do this before the outer loop:

clear OutData
OutData(length(hucPoly)) = struct();

[Voss]

Another way is to do this before the outer loop:

OutData = struct('hucID',num2cell(zeros(1,length(hucPoly))),'intsecArea',[]);

Answer 2

[Rik]

A third way to do it: loop backwards.

for ii = numel(hucPoly):-1:1

This also replaces the call to the length function. I have yet to see a situation where people actually mean max(size(A)) instead of the number of elements.

But the main point is that for a struct it is much less important to pre-allocate properly.