In the following code, the code generator recognizes that the structure fields s.a and s.b are constants.

function y = mystruct()
s.a = 3;
s.b = 5;
y = zeros(s.a,s.b);

If any structure field is assigned inside a control construct, the code generator does not recognize the constant fields. This limitation also applies to arrays with constant elements. Consider the following code:

function y = mystruct(x)
s.a = 3;
if x > 1
    s.b = 4;
else
    s.b = 5;
end
y = zeros(s.a,s.b);

The code generator does not recognize that s.a and s.b are constant. If variable-sizing is enabled, y is treated as a variable-size array. If variable-sizing is disabled, the code generator reports an error.

In the following code, the code generator recognizes that a(1) is constant.

function y = myarray()
a = zeros(1,3);
a(1) = 20;
y = coder.const(a(1));

In the following code, because a(1) is assigned using non-scalar indexing, the code generator does not recognize that a(1) is constant.

function y = myarray()
a = zeros(1,3);
a(1:2) = 20;
y = coder.const(a(1));

For an output structure that has both constant and nonconstant fields, the code generator does not recognize the constant fields. This limitation also applies to arrays that have constant and nonconstant elements. Consider the following code:

function y = mystruct_out(x)
s = create_structure(x);
y = coder.const(s.a);

function s = create_structure(x)
s.a = 10;
s.b = x;

Because create_structure returns a structure s that has one constant field and one nonconstant field, the code generator does not recognize that s.a is constant. The coder.const call fails because s.a is not constant.