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MATLAB C/C++ API - Update big variable

Posted on 2013-06-12
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Hi there,

I have just realized that if I have a variable in a MAT-file (big array) and I want to update this variable by appending a new row (suppose a 2D array) using the MATLAB C/C++ API then it is necessary to load the variable into memory, create a copy with the new row and finally overwrite the existing array with the new one.

Because I am working with big/huge arrays I would like to know how to update a variable without loading the whole thing into memory.

Thank you.
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Question by:ScreenFox
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Expert Comment

by:sarabande
ID: 39241292
you probably have to use the cat function. see

http://radio.feld.cvut.cz/matlab/toolbox/mathlib/cppmathug/workin15.html

Sara
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Expert Comment

by:yuk99
ID: 39267174
Not sure if you can use it with C/C++ API, but MATLAB has the MATFILE object that allows you to associate at with variables in mat-file, so you can read and update them without loading the whole thing into memory.

Here is an example:

>> x = rand(1000);
>> save x.mat x
>> matObj = matfile('x.mat','Writable',true)
matObj =
  matlab.io.MatFile

  Properties:
      Properties.Source: 'x.mat'
    Properties.Writable: true                                          
                      x: [1000x1000 double]                            

  Methods

>> whos x matObj %compare the Bytes
  Name           Size                Bytes  Class                Attributes

  matObj         1x1                   112  matlab.io.MatFile              
  x           1000x1000            8000000  double                        

>> matObj.x = [matObj.x; 1:1000]; %adding a row
>> matObj
matObj =
  matlab.io.MatFile

  Properties:
      Properties.Source: 'x.mat'
    Properties.Writable: true                                          
                      x: [1001x1000 double]                            

  Methods

>> clear matObj %the mat file will still exist
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ScreenFox earned 0 total points
ID: 39280712
Thanks yuk99, but I don't know either.

Anyway, not sure if Matlab actually loads the x matrix when used on a right hand side expression like matObj.x = [matObj.x; 1:1000] or it tries to map to memory the relevant part of the file.
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Expert Comment

by:yuk99
ID: 39282140
Good question and I don't know the exact answer. It requires some tests.
You can try profiling memory usage as described here:
http://undocumentedmatlab.com/blog/profiling-matlab-memory-usage/
I'll try it later when I'll have more time.
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Author Closing Comment

by:ScreenFox
ID: 39329156
Could not find a proper solution.
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