MATLAB Answers


Load Part of a Matrix or Cell array

Asked by Richard
on 10 Jun 2011

Hey All,

I was wondering if the latest version of Matlab R2011a has any built in functionality, or if there's anything recent on the file exchange, that can load in only part of a vector, matrix or cell array stored in a ".mat" file.

For example if I have a file "output.mat" which contains a single variable "large_matrix" which is a (100x100) matrix is there a way I can load say the 10th element, large_matrix(10), without loading the entire matrix into memory?

The above is to just illustrate my question I know Matlab could easily load a 100x100 matrix into memory.

The most Matlab seems to offer is selectively loading variables e.g.

loaded_file = load('output','other_variable');
other_variable = loaded_file.other_variable;

But I'm hoping I'm wrong.

Any advice would be greatly appreciated, Thanks



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3 Answers

Answer by Bjoern Hendrik Fock on 19 Jun 2011


Jan Simon
on 19 Jun 2011

+1. A nice summary of the frequency this feature is requested here.
Modern MAT-Files store the values of array in a compressed format. Accessing the values demands for a decompression of complete blocks. Therefore I assume, that it does not matter the efficiency if you cut out the wanted part inside the MAT reading function or in Matlab.

Walter Roberson
on 19 Jun 2011

Jan, it could matter for memory use purposes. Suppose you wanted a small number of elements from a very large matrix. The decompression routine could read values and throw them away until it got to the correct place -- and if the values happened to be consecutive in the file, it wouldn't even have to hunt around and decompress other blocks.

There are, of course, efficiency issues such as if the user's request did not index the elements in monotonically increasing order and the list of requested elements was too large to sort in the available memory. None the less, there are cases where it would make sense for memory use and file efficiency to read selectively.

Answer by Titus Edelhofer
on 10 Jun 2011


unfortunately the answer is no (for R2011a). What I have done is to write a simple function that splits the variable into pieces when saving to a file, and read the pieces afterwards while processing the data...



Answer by Bjoern Hendrik Fock on 6 Sep 2011

The feature seems to be added in Matlab R2011b by a new function called matfile. I will try this out the next days. MathWorks, this looks like an important improvement. Thank you for listening to your customers.



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