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.

Answer 2

There's a nice discussion about this in Loren on the Art of MATLAB blog.

If I understand you correctly, here's a ways to initialize the struct you want:

a(100).x = 100; 

With this method, we can see that elements are filled in with empty arrays.