How to initialize an array of structs in MATLAB?

Question

How can I preallocate an array of structs in MATLAB? I want to preallocate "a" in this example so that it does not resize several times.

a = []
for i = 1:100
  a(i).x = i;
end

Answer 1

Using repmat is by far the most efficient way to preallocate structs :

N = 10000;    
b = repmat(struct('x',1), N, 1 );

This is ~10x faster using Matlab 2011a than preallocating via indexing, as in

N      = 10000;
b(N).x = 1

The indexing method is only marginally faster than not preallocating.

No preallocation:            0.075524    
Preallocate Using indexing:  0.063774
Preallocate with repmat:     0.005234

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Code below in case you want to verify.

        clear;
        N = 10000;

    %1) GROWING A STRUCT
        tic;
        for ii=1:N
            a(ii).x(1)=1;    
        end
        noPreAll = toc;        

    %2)PREALLOCATING A STRUCT
        tic;
        b = repmat( struct( 'x', 1 ), N, 1 );
        for ii=1:N
            b(ii).x(1)=1;    
        end;  
        repmatBased=toc;        

    %3)Index to preallocate
        tic;
        c(N).x = 1;
        for ii=1:N
            c(ii).x(1)=1;    
        end;  
        preIndex=toc;

        disp(['No preallocation:        ' num2str(noPreAll)])            
        disp(['Preallocate Indexing:    ' num2str(preIndex)])
        disp(['Preallocate with repmat: ' num2str(repmatBased)])

Results in command window:

No preallocation:        0.075524    
Preallocate Indexing:    0.063774
Preallocate with repmat: 0.0052338
>> 

P.S. I'd be interested to know why this is true, if anyone can explain it.