Question

I’ve often found myself doing something like this:

unprocessedData = fetchData();  % returns a vector of structs or objects processedData = [];             % will be full of structs or objects  for dataIdx = 1 : length(unprocessedData)      processedDatum = process(unprocessedData(dataIdx));     processedData = [processedData; processedDatum]; end 

Which, whilst functional, isn’t optimal – the processedData vector is growing inside the loop. Even mlint warns me that I should consider preallocating for speed.

Were data a vector of int8, I could do this:

% preallocate processed data array to prevent growth in loop processedData = zeros(length(unprocessedData), 1, 'int8'); 

and modify the loop to fill vector slots rather than concatenate.

is there a way to preallocate a vector so that it can subsequently hold structs or objects?

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Update: inspired by Azim’s answer, I’ve simply reversed the loop order. Processing the last element first forces preallocation of the entire vector in the first hit, as the debugger confirms:

unprocessedData = fetchData();  % note that processedData isn't declared outside the loop - this breaks  % it if it'll later hold non-numeric data. Instead we exploit matlab's  % odd scope rules which mean that processedData will outlive the loop % inside which it is first referenced:   for dataIdx = length(unprocessedData) : -1 : 1      processedData(dataIdx) = process(unprocessedData(dataIdx)); end 

This requires that any objects returned by process() have a valid zero-args constructor since MATLAB initialises processedData on the first write to it with real objects.

mlint still complains about possible array growth, but I think that’s because it can’t recognise the reversed loop iteration…

Answer 1

Since you know the fields of the structure processedData and you know its length, one way would be the following:

unprocessedData = fetchData(); processedData = struct('field1', [], ...                        'field2', []) % create the processed data struct processedData(length(unprocessedData)) = processedData(1); % create an array with the required length for dataIdx = 1:length(unprocessedData)     processedData(dataIdx) = process(unprocessedData(dataIdx)); end 

This assumes that the process function returns a struct with the same fields as processedData.