How do I Preallocate a Global Array of Structures in Matlab

Question

In Matlab, I'm trying to preallocate a global array of structures. 1. Prepending the keyword global gets an error. 2. I'm getting an error trying to preallocate the object

Eg - Subscripted assignment dimension mismatch.

Error in IronCondor (line 41) OptionsChain(MaxOptions+1) = s; % make sure compatibility

MaxOptions = 3000;
OptionsChain = struct('symbol', {}, 'expiration', {}, 'strike_price', {}, 'bid', {}, 'ask', {}, 'last', {}, 'volume', {}, 'last_time', {});

if ~isempty(OptionsChain) && isstruct(OptionsChain)
   OptionsChain(MaxOptions+1) = s; % make sure compatibility
end

Answer 1

Do this instead:

MaxOptions = 3000;
OptionsChain = struct('symbol', {}, 'expiration', {}, 'strike_price', {}, 'bid', {}, 'ask', {}, 'last', {}, 'volume', {}, 'last_time', {});
OptionsChain = repmat(OptionsChain, MaxOptions, 1);

Your code doesn't work because OptionsChain is originally a structure of size 1. Doing OptionsChain(MaxOptions + 1) means that you are trying to put a structure at location 3001, where that's out of bounds.

Therefore, you can do what I did above by using repmat to duplicate OptionsChain 3000 times so that you get 3000 elements of that structure, or in a less elegant way, you can do it in a for loop:

OptionsChain = struct('symbol', {}, 'expiration', {}, 'strike_price', {},     'bid', {}, 'ask', {}, 'last', {}, 'volume', {}, 'last_time', {});
s = OptionsChain;
for idx = 1 : MaxOptions-1
    OptionsChain(end+1) = s;
end

The end+1 allows you to tack on something at the end of the array, and we only need to do it for MaxOptions-1 times, as we already have one instance of it created.