MATLAB Programming

6 Data Import and Export
• For small files that you want to read into memory once and access
frequently
• For data that you want to share between applications
• When you want to work with data in a file as if it were a MATLAB array
When the Advantage Is Less Significant. The following types of files do
not fully utilize the benefits of memory-mapping:
• Formatted binary files like HDF or TIFF that require customized readers
are not good for memory-mapping. For one thing, describing the data
contained in these files can be a very complex task. Also, you cannot access
data directly from the mapped segment, but must instead create arrays
to hold the data.
• Text or ASCII files require that you convert the text in the mapped region
toanappropriatetypeforthedatatobemeaningful. Thistakesup
additional address space.
• Files that are larger than several hundred megabytes in size consume a
significant amount of the virtual address space needed by MATLAB to
processyourprogram. MappingfilesofthissizemayresultinMATLAB
reporting out-of-memory errors more often. This is more likely if MATLAB
has been running for some time, or if the memory used by MATLAB
becomes fragmented.
ThememmapfileClass
MATLAB implements memory-mapping using an object-oriented class called
memmapfile. Thememmapfile classhasthepropertiesandmethodsyouneed
tomaptoafile,readandwritethefileviathemap,andremovethemapfrom
memory when you are done.
Properties of the memmapfile Class
There are six properties defined for the memmapfile class. These are shown in
the table below. These properties control which file is being mapped, where in
thefilethemappingistobeginandend, how the contents of the file are to
be formatted, and whether or not the file is writable. One property of the file
contains the file data itself.
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