Define Variable-Size Data for Code Generation

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.

Method	See
Assign the data from a variable-size matrix constructor such as: `ones` `zeros` `repmat`	Use a Matrix Constructor with Nonconstant Dimensions
Assign multiple, constant sizes to the same variable before using (reading) the variable.	Assign Multiple Sizes to the Same Variable
Define all instances of a variable to be variable-size.	Define Variable-Size Data Explicitly by Using coder.varsize

Use a Matrix Constructor with Nonconstant Dimensions

您可以定义一个适应矩阵通过使用一个constructor with nonconstant dimensions. For example:

functions = var_by_assign(u)%#codegeny = ones(3,u); s = numel(y);

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

functions = var_by_assign(u)%#codegenassert (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

functions = var_by_multiassign(u)%#codegenif(u > 0) y = ones(3,4,5);elsey = zeros(3,1);ends = numel(y);

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

The first dimension is fixed at size 3
The second dimension is variable-size with an upper bound of 4
The third dimension is variable-size with an upper bound of 5

代码生成器使用动态分配的时候,我t analyzes the dimensions ofydifferently:

The first dimension is fixed at size 3.
The second and third dimensions are unbounded.

Define Variable-Size Data Explicitly by Using coder.varsize

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

DefineBas a variable-size 2-dimensional array, where each dimension has an upper bound of 64.
```
coder.varsize('B', [64 64]);
```
DefineBas a variable-size array:
```
coder.varsize('B');
```
When you supply only the first argument,coder.varsizeassumes that all dimensions ofBcan vary and that the upper bound issize(B).

Specify Which Dimensions Vary

You can use the functioncoder.varsizeto specify which dimensions vary. For example, the following statement definesBas 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 orfalsehave fixed size. Dimensions that correspond to ones ortruevary in size.coder.varsizeusually treats dimensions of size 1 as fixed. SeeDefine Variable-Size Matrices with Singleton Dimensions.

Allow a Variable to Grow After Defining Fixed Dimensions

Functionvar_by_ifdefines matrixYwith fixed 2-by-2 dimensions before the first use (where the statementY = Y + ureads fromY). However,coder.varsizedefinesYas a variable-size matrix, allowing it to change size based on decision logic in theelseclause:

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

Withoutcoder.varsize, the code generator infersYto 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 whichsize(A,dim)= 1. Singleton dimensions are fixed in size when:

You specify a dimension with an upper bound of 1 incoder.varsizeexpressions.
For example, in this function,Ybehaves like a vector with one variable-size dimension:
```
functionY = dim_singleton(u)%#codegenY = [1 2]; coder.varsize('Y', [1 10]);if(u > 0) Y = [Y 3];elseY = [Y u];end
```
You initialize variable-size data with singleton dimensions by using matrix constructor expressions or matrix functions.
For example, in this function,XandYbehave like vectors where only their second dimensions are variable-size.
```
function[X,Y] = dim_singleton_vects(u)%#codegenY = ones(1,3); X = [1 4]; coder.varsize('Y','X');if(u > 0) Y = [Y u];elseX = [X u];end
```

You can override this behavior by usingcoder.varsizeto specify explicitly that singleton dimensions vary. For example:

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

In this example, the third argument ofcoder.varsizeis a vector of ones, indicating that each dimension ofYvaries 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:

functiony=struct_example()%#codegend = struct('values', zeros(1,0),'color', 0); data = repmat(d, [3 3]); coder.varsize('data(:).values');fori = 1:numel(data) data(i).color = rand-0.5; data(i).values = 1:i;endy = 0;fori = 1:numel(data)ifdata(i).color > 0 y = y + sum(data(i).values);endend

The expressioncoder.varsize('data(:).values')defines the fieldvaluesinside each element of matrixdatato be variable-size.

Here are other examples:

coder.varsize('data.A(:).B')
In this example,datais a scalar variable that contains matrixA. Each element of matrixAcontains a variable-size fieldB.
coder.varsize('data(:).A(:).B')
This expression defines fieldBinside each element of matrixAinside each element of matrixdatato be variable-size.