Efficiently Use a Preallocated Array

I’m hoping to make my code more efficient my preallocating my arrays.

I am using matlab to monitor real-time data that I am grabbing from a pipe every 5 seconds or so. It comes into matlab like this:

A =

    [7.3468e+005]    'M001'    [0.5000]
    [7.3468e+005]    'M016'    [0.0861]
    [7.3468e+005]    'M002'    [0.2693]
    [7.3468e+005]    'M009'    [0.7381]
    [7.3468e+005]    'M004'    [    10]

Where A is an (x,3) cell array, with time, the measurement name, and measurement value as the columns.

I take this data and sort it by measurement name into a structure using dynamic field name:

[A_length ~] = size(A);

for xx = 1:1:A_length

data.(A{xx,2}).time = [data.(A{xx,2}).time; A{xx,1}];

data.(A{xx,2}).data = [data.(A{xx,2}).data; A{xx,3}];

end

I also check the length and if it’s > 2500 I truncate to 2300 so that we don’t truncate every time through the loop.

I am thinking of preallocating to see if that’s faster but I’m not sure the best way to do it. Here’s my initial idea:

[A_length ~] = size(A);

for xx = 1:1:A_length

if ~isfield(data,A{xx,2})

data.(A{xx,2}).time = NaN(2500,1);

data.(A{xx,2}).data = NaN(2500,1);

else

data.(A{xx,2}).time = [data.(A{xx,2}).time([2:2499]);A{xx,1}];

data.(A{xx,2}).data = [data.(A{xx,2}).data([2:2499]);A{xx,3}];

end

end

Is there a better way to shift the array (eg to add the data to the end of the arrays)?