## Incompatibilities with MATLAB in Variable-Size Support for Code Generation

### Incompatibility with MATLAB for Scalar Expansion

Scalar expansion is a method of converting scalar data to match the dimensions of vector or matrix data. If one operand is a scalar and the other is not, scalar expansion applies the scalar to every element of the other operand.

During code generation, scalar expansion rules apply except when operating on two variable-size expressions. In this case, both operands must be the same size. The generated code does not perform scalar expansion even if one of the variable-size expressions turns out to be scalar at run time. Therefore, when run-time error checks are enabled, a run-time error can occur.

Consider this function:

```function y = scalar_exp_test_err1(u) %#codegen y = ones(3); switch u case 0 z = 0; case 1 z = 1; otherwise z = zeros(3); end y(:) = z; ```

When you generate code for this function, the code generator determines that `z` is variable size with an upper bound of `3`.

If you run the MEX function with `u` equal to 0 or 1, the generated code does not perform scalar expansion, even though `z` is scalar at run time. Therefore, when run-time error checks are enabled, a run-time error can occur.

```scalar_exp_test_err1_mex(0) Subscripted assignment dimension mismatch: [9] ~= [1]. Error in scalar_exp_test_err1 (line 11) y(:) = z;```

To avoid this issue, use indexing to force `z` to be a scalar value.

```function y = scalar_exp_test_err1(u) %#codegen y = ones(3); switch u case 0 z = 0; case 1 z = 1; otherwise z = zeros(3); end y(:) = z(1);```

### Incompatibility with MATLAB in Determining Size of Variable-Size N-D Arrays

For variable-size N-D arrays, the `size` function can return a different result in generated code than in MATLAB®. In generated code, `size(A)` returns a fixed-length output because it does not drop trailing singleton dimensions of variable-size N-D arrays. By contrast, `size(A)` in MATLAB returns a variable-length output because it drops trailing singleton dimensions.

For example, if the shape of array `A` is `:?x:?x:?` and `size(A,3)==1`, `size(A)` returns:

• Three-element vector in generated code

• Two-element vector in MATLAB code

#### Workarounds

If your application requires generated code to return the same size of variable-size N-D arrays as MATLAB code, consider one of these workarounds:

• Use the two-argument form of `size`.

For example, `size(A,n)` returns the same answer in generated code and MATLAB code.

• Rewrite `size(A)`:

```B = size(A); X = B(1:ndims(A));```

This version returns `X` with a variable-length output. However, you cannot pass a variable-size `X` to matrix constructors such as `zeros` that require a fixed-size argument.

### Incompatibility with MATLAB in Determining Size of Empty Arrays

The size of an empty array in generated code might be different from its size in MATLAB source code. The size might be `1x0` or `0x1` in generated code, but `0x0` in MATLAB. Therefore, you should not write code that relies on the specific size of empty matrices.

For example, consider the following code:

```function y = foo(n) %#codegen x = []; i = 0; while (i < 10) x = [5 x]; i = i + 1; end if n > 0 x = []; end y = size(x); end```

Concatenation requires its operands to match on the size of the dimension that is not being concatenated. In the preceding concatenation, the scalar value has size `1x1` and `x` has size `0x0`. To support this use case, the code generator determines the size for `x` as `[1 x :?]`. Because there is another assignment `x = []` after the concatenation, the size of `x` in the generated code is `1x0` instead of `0x0`.

This behavior persists while determining the size of empty character vectors which are denoted as `''`. For example, consider the following code:

```function out = string_size out = size(''); end```

Here, the value of `out` might be `1x0` or `0x1` in generated code, but `0x0` in MATLAB.

For incompatibilities with MATLAB in determining the size of an empty array that results from deleting elements of an array, see Size of Empty Array That Results from Deleting Elements of an Array.

#### Workaround

If your application checks whether a matrix is empty, use one of these workarounds:

• Rewrite your code to use the `isempty` function instead of the `size` function.

• Instead of using `x=[]` to create empty arrays, create empty arrays of a specific size using `zeros`. For example:

```function y = test_empty(n) %#codegen x = zeros(1,0); i=0; while (i < 10) x = [5 x]; i = i + 1; end if n > 0 x = zeros(1,0); end y=size(x); end```

### Incompatibility with MATLAB in Determining Class of Empty Arrays

The class of an empty array in generated code can be different from its class in MATLAB source code. Therefore, do not write code that relies on the class of empty matrices.

For example, consider the following code:

```function y = fun(n) x = []; if n > 1 x = ['a' x]; end y=class(x); end ```
`fun(0)` returns `double` in MATLAB, but `char` in the generated code. When the statement `n > 1` is false, MATLAB does not execute `x = ['a' x]`. The class of `x` is `double`, the class of the empty array. However, the code generator considers all execution paths. It determines that based on the statement `x = ['a' x]`, the class of `x` is `char`.

#### Workaround

Instead of using `x=[]` to create an empty array, create an empty array of a specific class. For example, use `blanks(0)` to create an empty array of characters.

```function y = fun(n) x = blanks(0); if n > 1 x = ['a' x]; end y=class(x); end```

### Incompatibility with MATLAB in Matrix-Matrix Indexing

In matrix-matrix indexing, you use one matrix (the index matrix) to index into another matrix (the data matrix). In MATLAB, the general rule for matrix-matrix indexing is that the dimensions of the result are the same as the dimensions of the index matrix. For example, if `A` and `B` are matrices, `size(A(B))` equals `size(B)`. However, when `A` and `B` are vectors, MATLAB applies a different rule. When performing vector-vector indexing, the orientation of the result is the same as the orientation of the data matrix. For example, if `A` is 1-by-5 and `B` is 3-by-1, then `A(B)` is 1-by-3.

The code generator attempts to apply the same matrix-matrix indexing rules as MATLAB. If `A` and `B` are variable-size matrices at code generation time, the code generator follows the general MATLAB indexing rule and assumes that `size(A(B))` equals `size(B)`. At run time, if `A` and `B` are vectors with different orientations, then this assumption is incorrect. Therefore, when run-time error checks are enabled, an error can occur.

To avoid this run-time error, try one of these solutions:

• If `A` or `B` is a fixed-size matrix at run time, define this matrix as fixed-size at code generation time.

• If `A` and `B` are both vectors at run time, make sure that their orientations match.

• If your code intentionally accepts matrices as well as vectors of different orientations at run time, include an explicit check for vector-vector indexing and force vectors into the same orientation. For example, use the `isvector` function to determine whether both `A` and `B` are vectors and, if so, use the `colon` operator to force both vectors to be column vectors.

```... if isvector(A) && isvector(B) Acol = A(:); Bcol = B(:); out = Acol(Bcol); else out = A(B); end ...```

### Incompatibility with MATLAB in Vector-Vector Indexing

In vector-vector indexing, you use one vector (the index vector) to index into another vector (the data vector). In MATLAB, the rule for vector-vector indexing is that the orientation of the result vector is the same as the orientation of the data vector. For example, if `A` is 1-by-5 and `B` is 3-by-1, then `A(B)` is 1-by-3. However, this rule does not apply if `A` is a scalar. If `A` is scalar, then the orientation of `A(B)` is the same as the orientation of the index vector `B`.

The code generator attempts to apply the same vector-vector indexing rules as MATLAB. If `A` is a variable-size vector at code generation time, the code generator assumes that the orientation of `A(B)` is the same as the orientation of `A`. However, this assumption is false and a run time error occurs if both of these conditions are true:

• At code generation time, the orientation of `A` does not match that of `B`.

• At run time, `A` is a scalar and `B` is a vector.

To avoid this run-time error, try one of these solutions:

• If `A` is a scalar at run time, define `A` as a scalar at code generation time.

• If `A` and `B` are defined as vectors at code generation time, make sure that their orientations are the same.

• If `A` and `B` are variable-size vectors with different orientations at code generation time, make sure that `A` is not a scalar at run time.

• If `A` and `B` are variable-size vectors with different orientations at code generation time, make sure that `B` is not a vector at run time.

### Incompatibility with MATLAB in Logical Indexing

In logical indexing, you use an array of logical values (the index array) to index into another array (the data array). In MATLAB execution, the shape of the resulting array depends on whether the index and data arrays are matrices, vectors, or scalars. When you generate C/C++ code for MATLAB code that performs logical indexing using variable-size arrays, the code generator can make assumptions about the shape of the indexing result that are false for some run-time inputs, causing a run-time error.

If the index and data arrays are fixed-size at run time, you can avoid this run-time error by defining both of these arrays as fixed-size at code generation time.

If either or both arrays must be defined as variable-size at code generation time, this table shows some possible solutions for the run-time errors that can occur for certain inputs. In this table, `A` is the data array and `B` is the logical index array.

Definition at Code Generation TimeInput at Run TimePossible Solution
`A` is a fixed-size matrix and `B` is a variable-size matrix.`B` is a row vector.Force `B` to be a column vector at run time using the `colon` operator.
`A` is a variable-size matrix and `B` is a fixed-size matrix.`A` is a row vector.Force `A` to be a column vector at run time using the `colon` operator.
`A` is a variable-size matrix and `B` is a variable-size matrix.`A` is a row vector.Force `A` to be a column vector at run time using the `colon` operator.
`A` is a variable-size matrix and `B` is a variable-size matrix.`A` is a matrix and `B` is a row vector.Force `B` to be a column vector at run time using the `colon` operator.
`A` is a variable-size matrix and `B` is a vector. `A` and `B` are vectors, and the orientations of `A` and `B` are different.Force `A` and `B` to have the same orientation at run time.

### Incompatibility with MATLAB in Matrix Indexing Operations for Code Generation

The following limitation applies to matrix indexing operations for code generation:

• Initialization of the following style:

```for i = 1:10 M(i) = 5; end ```

In this case, the size of `M` changes as the loop is executed. Code generation does not support increasing the size of an array over time.

For code generation, preallocate `M`.

```M = zeros(1,10); for i = 1:10 M(i) = 5; end ```

The following limitation applies to matrix indexing operations for code generation when dynamic memory allocation is disabled:

• `M(i:j)` where `i` and `j` change in a loop

During code generation, memory is not dynamically allocated for the size of the expressions that change as the program executes. To implement this behavior, use `for-`loops as shown:

```... M = ones(10,10); for i=1:10 for j = i:10 M(i,j) = 2*M(i,j); end end ...```

Note

The matrix `M` must be defined before entering the loop.

### Incompatibility with MATLAB in Concatenating Variable-Size Matrices

For code generation, when you concatenate variable-size arrays, the dimensions that are not being concatenated must match exactly.

### Differences When Curly-Brace Indexing of Variable-Size Cell Array Inside Concatenation Returns No Elements

Suppose that:

• `c` is a variable-size cell array.

• You access the contents of `c` by using curly braces. For example, `c{2:4}`.

• You include the results in concatenation. For example, ```[a c{2:4} b]```.

• `c{I}` returns no elements. Either `c` is empty or the indexing inside the curly braces produces an empty result.

For these conditions, MATLAB omits `c{I}` from the concatenation. For example, `[a c{I} b]` becomes `[a b]`. The code generator treats `c{I}` as the empty array `[c{I}]`. The concatenation becomes `[...[c{i}]...]`. This concatenation then omits the array `[c{I}]`. So that the properties of `[c{I}]` are compatible with the concatenation `[...[c{i}]...]`, the code generator assigns the class, size, and complexity of `[c{I}]` according to these rules:

• The class and complexity are the same as the base type of the cell array.

• The size of the second dimension is always 0.

• For the rest of the dimensions, the size of `Ni` depends on whether the corresponding dimension in the base type is fixed or variable size.

• If the corresponding dimension in the base type is variable size, the dimension has size 0 in the result.

• If the corresponding dimension in the base type is fixed size, the dimension has that size in the result.

Suppose that `c` has a base type with class `int8` and size`:10x7x8x:?`. In the generated code, the class of `[c{I}]` is `int8`. The size of `[c{I}]` is `0x0x8x0`. The second dimension is 0. The first and last dimensions are 0 because those dimensions are variable size in the base type. The third dimension is 8 because the size of the third dimension of the base type is a fixed size 8.

Inside concatenation, if curly-brace indexing of a variable-size cell array returns no elements, the generated code can have the following differences from MATLAB:

• The class of `[...c{i}...]` in the generated code can differ from the class in MATLAB.

When `c{I}` returns no elements, MATLAB removes `c{I}` from the concatenation. Therefore, `c{I}` does not affect the class of the result. MATLAB determines the class of the result based on the classes of the remaining arrays, according to a precedence of classes. See Valid Combinations of Unlike Classes. In the generated code, the class of `[c{I}]` affects the class of the result of the overall concatenation `[...[c{I}]...]` because the code generator treats `c{I}` as `[c{I}]`. The previously described rules determine the class of `[c{I}]`.

• In the generated code, the size of `[c{I}]` can differ from the size in MATLAB.

In MATLAB, the concatenation `[c{I}]` is a 0x0 double. In the generated code, the previously described rules determine the size of `[c{I}]`.