Creating and Manipulating Models

size and ndims

You can access the dimensions and shape of an LTI array using:

• size to determine the lengths of each of the dimensions associated with an LTI array
• ndims to determine the total number of dimensions in an LTI array

When applied to an LTI array, size returns

[Ny Nu S1 S2 ... Sk]

where

• Ny is the number of outputs common to all models in the LTI array.
• Nu is the number of inputs common to all models in the LTI array.
• S1 S2 ... Sk are the lengths of the array dimensions of a k-dimensional array of models. Si is the number of models along the ith array dimension.

Note the following when using the size function:

• By convention, a single LTI model is treated as a 1-by-1 array of models.

For single LTI models, size returns only the I/O dimensions [Ny Nu].

• For LTI arrays, size always returns at least two array dimensions. For example, the size of a 2-by-1 LTI array in [Ny Nu 2 1]
• size ignores trailing singleton dimensions beyond the second array dimension. For example, size returns [Ny Nu 2 3] for a 2-by-3-by-1-by-1 LTI array of models with Ny outputs and Nu inputs.

The function ndims returns the total number of dimensions in an LTI array:

• 2, for single LTI models
• 2 + p, for LTI arrays, where p (greater than 2) is the number of array dimensions

Note that

ndims (sys) = length(size(sys))

To see how these work on the sample 2-by-3 LTI array m2d of two-output, one-input models, type

load LTIexamples
s = size(m2d)
s =
     2     1     2     3

Notice that size returns a vector whose entries correspond to the length of each of the four dimensions of m2d: two outputs and one input in a 2-by-3 array of models. Type

ndims(m2d)
ans =
     4

to see that there are indeed four dimensions attributed to this LTI array.

Dimensions, Size, and Shape of an LTI Array

reshape