MATLAB Programming

6 Data Import and Export
Note To successfully modify the file you are mapping to, you must have write
permission for that file. If you do not have write permission, you can still set
the Writable property to true, but attempting to write to the file generates
an error.
You can change the value of the Writable property at any time. To make the
memory map to records.dat read only, type
m.Writable = false;
Property names, like Writable, arenotcasesensitive.
Reading a Mapped File
The most commonly used property of the memmapfile classistheData
property. It is through this property of the memory-map object that MATLAB
provides all read and write access to the contents of the mapped file.
TheactualmappingofafiletotheMATLABaddressspacedoesnottake
place when you construct a memmapfile object. A memory map, based on the
information currently stored in the mapped object, is generated the first time
you reference or modify the Data property for that object.
Once you have mapped a file to memory, you can read the contents of that
file using the same MATLAB statements used to read variables from the
MATLAB workspace. By accessing the Data property of the memory map
object, the contents of the mapped fileappearasiftheywereanarrayin
the currently active workspace. You simply index into this array to read the
desired data from the file.
This section covers the following topics:
• “Improving Performance” on page 6-55
• “Example 1 — Reading a Single Data Type” on page 6-55
• “Example 2 — Formatting File Data as a Matrix” on page 6-56
• “Example 3 — Reading Multiple Data Types” on page 6-57
6-54