Getting Information About a Matrix :: Data Structures (Programming)

Programming

Getting Information About a Matrix

This section explains how to get the following information about an existing matrix:

Dimensions of the Matrix

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

Function
Description

length
Return the length of the longest dimension. (The length of a matrix or array with any zero dimension is zero.)

ndims
Return the number of dimensions.

numel
Return the number of elements.

size
Return the length of each dimension.

Function	Description
`length`	Return the length of the longest dimension. (The length of a matrix or array with any zero dimension is zero.)
`ndims`	Return the number of dimensions.
`numel`	Return the number of elements.
`size`	Return the length of each dimension.

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

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
```

Logical Indexing Data Types Used in the Matrix