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Viewing the internals of MATLAB Matrices

31st January, 2012 Leave a comment

A cool undocumented trick I just learnt from The MathWorks’ Bob Gilmore. If you type

format debug

Then printing any vector reveals information about its internal representation. For example:

x = magic(3)

x =


Structure address = 6bc1ab0 
m = 3
n = 3
pr = d8dccf0 
pi = 0
     8     1     6
     3     5     7
     4     9     2

The structure address is the address in memory where the matrix is stored, m and n are the number of rows and columns respectively of the matrix, and pr and pi are pointers to the addresses of the matrices storing the real and imaginary components of the matrix.

One interesting thing to look at is the representation of scalar numbers.

 y = 1

y =


Structure address = 6bc31e0 
m = 1
n = 1
pr = d790b90 
pi = 0
     1

Yep: they are stored in exactly the same way as matrices: in the same way the “everything in R is a vector”, everything in MATLAB is a matrix. To finish up, here are some more examples for you to explore:

% higher dimensional arrays
rand(2, 3, 4)
% cell arrays (unfortunately not that revealing)
{1, magic(3)}
% sparse matrices (very interesting)
sparse(ones(3))
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