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File Size Increases Unexpectedly When Growing Array

This example shows how to prevent an array from growing when writing one million double-precision values to a file, by assigning initial values to the array.

Construct a object for writing.

fileName = 'matFileOfDoubles.mat';
matObj = matfile(fileName);
matObj.Properties.Writable = true;

Define parameters of the values to write. In this case, write one million values, fifty thousand at a time. The values should have a mean of 123.4, and a standard deviation of 56.7.

size = 1000000;
chunk = 50000; 
mean = 123.4;
std = 56.7;

Assign an initial value of zero to the last element in the array prior to populating it with data.,size) = 0;

View the size of the file.

  • On Windows® systems, use dir.

    system('dir matFileOfDoubles.mat');

  • On UNIX® systems, use ls -ls:

    system('ls -ls matFileOfDoubles.mat');

matFileOfDoubles.mat is less than 5000 bytes. Assigning an initial value to the last element of the array does not create a large file.

Write data to the array, one chunk at a time.

nout = 0;
while(nout < size)
    fprintf('Writing %d of %d\n',nout,size);
    chunkSize = min(chunk,size-nout);
    data = mean + std * randn(1,chunkSize);,(nout+1):(nout+chunkSize)) = data;
    nout = nout + chunkSize;

View the size of the file.

system('dir matFileOfDoubles.mat');

The file size is now larger now because the array is populated with data.

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