Generate Code for Variable-Size Arrays

Construct Matrix with Nonconstant Dimensions

You can construct a matrix with one or more variable-size dimensions by using a matrix-creation function such as zeros or ones and specifying at least one nonconstant dimension. For example, consider this function:

function M = constructVarsize(x) %#codegen
M = ones(3,x);
end

If you generate code for this function, the code generator defines M with a first dimension that has the fixed-size of 3 and a second dimension that is variable size and unbounded. If you hover over M in the code generation report, the tooltip displays the size of M in the generated code.

Grow Array by Using end+1 Indexing

When you grow an array or cell array dimension by using end+1 indexing, the code generator defines that dimension as variable size. For example, consider this function:

function M = growVarsize(x) %#codegen
M = 0;
for i = 1:x
    M(end+1) = i;
end

If you generate code for this function, the code generator defines M with a first dimension that has the fixed-size of 1 and a second dimension that is variable size and unbounded. If you hover over M in the code generation report, the tooltip displays the size of M in the generated code.

For more information about code generation when the MATLAB code uses end+1 to grow an array, see Code Generation for Arrays That Grow Via end+1 Indexing.

Assign Multiple Sizes to the Same Variable

Before you use a variable in your code, such as by passing it to or returning from a function, you can assign it values of different shapes. The code generator specifies that array dimensions that vary in size across assignments are variable size.

For example, consider this function:

function M = multisizeVarsize(x) %#codegen
if x>1
    M = ones(10,3,10);
else
    M = ones(10,5);
end

If you generate code for this function, the code generator determines that the maximum size of M across both code branches is 10-by-5-by-10. In the generated code, M has a first dimension with a fixed size of 10, a second dimension that is variable size with an upper bound of 5, and a third dimension that is variable size with an upper bound of 10. If you hover over M in the code generation report, the tooltip displays the size of M in the generated code.

Use coder.varsize

If you change the size of a variable after you pass it to or return it from a function, the code generator can be unable to determine that one or more of the dimensions of the variable are variable size. In such cases, you can use the coder.varsize directive to force the code generator to recognize that the variable changes size.

For example, this function changes the size of array M on the else code branch after it passes variable to the + function on the if branch. This function uses the coder.varsize directive to force the code generator to allow M to vary in size.

function M = functionVarsize(in) %#codegen
coder.varsize("M")
if (in > 0)
    M = zeros(2,2);
    M = M+in;
else
    M = ones(5,5);
end
end

If you generate code for this function, the code generator determines that the maximum size of M across both code branches is 5-by-5. In the generated code, both dimensions of M are variable size with an upper bound of 5. If you hover over M in the code generation report, the tooltip displays the size of M in the generated code.