Define Variable-Size Data for Code Generation - MATLAB & Simulink (original) (raw)

For code generation, before using variables in operations or returning them as outputs, you must assign them a specific class, size, and complexity. Generally, after the initial assignment, you cannot reassign variable properties. Therefore, after assigning a fixed size to a variable or structure field, attempts to grow the variable or structure field might cause a compilation error. In these cases, you must explicitly define the data as variable-size by using one of these methods.

Use a Matrix Constructor with Nonconstant Dimensions

You can define a variable-size matrix by using a constructor with nonconstant dimensions. For example:

function s = var_by_assign(u) %#codegen y = ones(3,u); s = numel(y);

If you are not using dynamic memory allocation, you must also add anassert statement to provide upper bounds for the dimensions. For example:

function s = var_by_assign(u) %#codegen assert (u < 20); y = ones(3,u); s = numel(y);

Assign Multiple Sizes to the Same Variable

Before you use (read) a variable in your code, you can make it variable-size by assigning multiple, constant sizes to it. When the code generator uses static allocation on the stack, it infers the upper bounds from the largest size specified for each dimension. When you assign the same size to a given dimension across all assignments, the code generator assumes that the dimension is fixed at that size. The assignments can specify different shapes and sizes.

When the code generator uses dynamic memory allocation, it does not check for upper bounds. It assumes that the variable-size data is unbounded.

Inferring Upper Bounds from Multiple Definitions with Different Shapes

function s = var_by_multiassign(u) %#codegen if (u > 0) y = ones(3,4,5); else y = zeros(3,1); end s = numel(y);

When the code generator uses static allocation, it infers thaty is a matrix with three dimensions:

When the code generator uses dynamic allocation, it analyzes the dimensions ofy differently:

Grow an Array Using (end + 1) Indexing

In MATLAB® execution, you can grow any dimension of an array using end. For example, you can grow both dimensions of a two-dimensional matrix by assigning a value to the(end + 1) element of each dimension.

A =

17    24     1     8    15
23     5     7    14    16
 4     6    13    20    22
10    12    19    21     3
11    18    25     2     9

A =

17    24     1     8    15     0
23     5     7    14    16     0
 4     6    13    20    22     0
10    12    19    21     3     0
11    18    25     2     9     0
 0     0     0     0     0     1

To grow arrays using end in MATLAB code for code generation, you must adhere to these restrictions:

Define Variable-Size Data Explicitly by Using coder.varsize

To explicitly define variable-size data, use the function coder.varsize. Optionally, you can also specify which dimensions vary along with their upper bounds. For example:

If a MATLAB Function block output signal is variable-size, in the Property Inspector, you must specify that the signal is variable-size. You can specify the upper bounds or define the variable as unbounded (since R2023b). You do not have to usecoder.varsize with the corresponding output variable inside the MATLAB Function block. However, if you specify upper bounds withcoder.varsize, they must match the upper bounds in the Property Inspector.

Specify Which Dimensions Vary

You can use the function coder.varsize to specify which dimensions vary. For example, the following statement definesB as an array whose first dimension is fixed at 2, but whose second dimension can grow to a size of 16:

coder.varsize('B',[2, 16],[0 1])

.

The third argument specifies which dimensions vary. This argument must be a logical vector or a double vector containing only zeros and ones. Dimensions that correspond to zeros or false have fixed size. Dimensions that correspond to ones or true vary in size.coder.varsize usually treats dimensions of size 1 as fixed. See Define Variable-Size Matrices with Singleton Dimensions.

Allow a Variable to Grow After Defining Fixed Dimensions

Function var_by_if defines matrix Y with fixed 2-by-2 dimensions before the first use (where the statement Y = Y + u reads from Y). However,coder.varsize defines Y as a variable-size matrix, allowing it to change size based on decision logic in theelse clause:

function Y = var_by_if(u) %#codegen if (u > 0) Y = zeros(2,2); coder.varsize('Y'); if (u < 10) Y = Y + u; end else Y = zeros(5,5); end

Without coder.varsize, the code generator infersY to be a fixed-size, 2-by-2 matrix. It generates a size mismatch error.

Define Variable-Size Matrices with Singleton Dimensions

A singleton dimension is a dimension for which size(A,dim) = 1. Singleton dimensions are fixed in size when:

You can override this behavior by using coder.varsize to specify explicitly that singleton dimensions vary. For example:

function Y = dim_singleton_vary(u) %#codegen Y = [1 2]; coder.varsize('Y', [1 10], [1 1]); if (u > 0) Y = [Y Y+u]; else Y = [Y Y*u]; end

In this example, the third argument of coder.varsize is a vector of ones, indicating that each dimension of Y varies in size.

Define Variable-Size Structure Fields

To define structure fields as variable-size arrays, use a colon (:) as the index expression. The colon (:) indicates that all elements of the array are variable-size. For example:

function y=struct_example() %#codegen

d = struct('values', zeros(1,0), 'color', 0); data = repmat(d, [3 3]); coder.varsize('data(:).values');

for i = 1:numel(data) data(i).color = rand-0.5; data(i).values = 1:i; end

y = 0; for i = 1:numel(data) if data(i).color > 0 y = y + sum(data(i).values); end end

The expression coder.varsize('data(:).values') defines the field values inside each element of matrixdata to be variable-size.

Here are other examples:

See Also

coder.varsize

Topics