MATLAB®

Getting Information About a Matrix

Dimensions of the Matrix

These functions return information about the shape and size of a matrix.

The following examples show some simple ways to use these functions. Both use the 3-by-5 matrix A shown here:

rand('state', 0);     % Initialize random number generator
A = rand(5) * 10;
A(4:5, :) = []
A =
    9.5013    7.6210    6.1543    4.0571    0.5789
    2.3114    4.5647    7.9194    9.3547    3.5287
    6.0684    0.1850    9.2181    9.1690    8.1317

Example Using numel

Using the numel function, find the average of all values in matrix A:

sum(A(:))/numel(A)
ans =
    5.8909

Example Using ndims, numel, and size

Using ndims and size, go through the matrix and find those values that are between 5 and 7, inclusive:

if ndims(A) ~= 2
   return
end

[rows cols] = size(A);
for m = 1:rows
   for n = 1:cols
      x = A(m, n);
      if x >= 5 && x <= 7
         disp(sprintf('A(%d, %d) = %5.2f', m, n, A(m,n)))
      end
   end
end

The code returns the following:

A(1, 3) =  6.15
A(3, 1) =  6.07

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Classes Used in the Matrix

These functions test elements of a matrix for a specific data type.

Example Using isnumeric and isreal

Pick out the real numeric elements from this vector:

A = [5+7i 8/7 4.23 39j pi 9-2i];

for m = 1:numel(A)
   if isnumeric(A(m)) && isreal(A(m))
      disp(A(m))
   end
end

The values returned are

    1.1429
    4.2300
    3.1416

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Data Structures Used in the Matrix

These functions test elements of a matrix for a specific data structure.

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Matrix Indexing

Resizing and Reshaping Matrices