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?
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...
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
.