Resolve Error: Fixed Size on the Left Side but Variable Size on the Right

Issue

For code generation, a variable is either fixed size or variable size. If the code generator can determine that the size of a variable does not change, it fixes the size of the variable in the generated code. Otherwise, it allows the variable to change size. If you assign a variable-size value to a variable that the code generator previously defined as fixed size, the code generator produces one of these errors:

Unable to make this assignment because dimension dimension is fixed size on the left side but variable size on the right

Unable to make this assignment because dimension dimension in element 'element' is fixed size on the left side but variable size on the right

Unable to make this assignment because dimension dimension in field 'field' is fixed size on the left side but variable size on the right

Possible Solutions

To resolve this error, try one of these solutions.

Force the Variables to Be the Same Size by Using Explicit Indexing

If you do not need the variable on the right side to be variable size, you can force the variable to be fixed size by using explicit indexing. For example, suppose you have a function that defines variables A and B. A is a variable-size scalar and B is a fixed-size scalar. Code generation fails because the function tries to assign the variable-size variable A to the fixed-size variable B.

function out = varsizeError2(n) %#codegen
coder.varsize("A",[1 1],[true true]);
A = ones(n,n);
B = 3;
numel(B);
if (n == 3)
    B = A;
end
out = B;
end

To generate code for this function, use explicit indexing to force A to be a fixed-size scalar.

function out = varsizeExample2(n) %#codegen
coder.varsize("A",[1 1],[true true]);
A = ones(n,n);
B = 3;
numel(B);
if (n == 3)
    B = A(1);
end
out = B;
end

Allow the Variable on the Left Side to Vary by Using coder.varsize

Use coder.varsize (MATLAB Coder) to instruct the code generator to treat the variable on the left side of the assignment as variable size. For example, consider this function:

function out = varsizeError1(n) %#codegen
A = ones(n,n);
B = magic(3);
numel(B);
if (n == 3)
    B = A;
end
out = B;
end

The code generator recognizes array A as variable size because the size of A depends on the input value. The code generator identifies B as fixed size because the size of this array is constant when the code uses B by passing it to the numel function. Code generation fails for the assignment B = A because A is variable size and B is fixed size. To resolve this error, use coder.varsize to instruct the code generator to allow the size of B to vary.

function out = varsizeExample1(n) %#codegen
A = ones(n,n);
B = magic(3);
coder.varsize("B");
numel(B);
if (n == 3)
    B = A;
end
out = B;
end

Prevent Implicit Expansion

When performing binary operations on arrays of different but compatible sizes, the code generator implicitly expands the arrays so that they are the same size. If you assign the implicitly expanded output of a binary operation or function to a fixed-size variable of different size, the code generator might produce an error. For example, consider this function:

function out = varsizeError3(n) %#codegen
A = ones(n,1);
B = ones(n,n);
if n == 1
    A = A+B;
end
out = A;
end

The second dimension of array A is fixed size. To perform the operation A+B, the code generator implicitly expands the second dimension of A to match the second dimension of B. The result of A+B is an array that is variable size in both dimensions. Code generation fails because the code generator cannot assign an variable with a second dimension that is variable size to an variable with a second dimension that is fixed size. In this example, the operation A+B only executes if both arrays are 1-by-1. Therefore, you can disable implicit expansion for this function by using coder.noImplicitExpansionInFunction (MATLAB Coder), or for this operation by using coder.sameSizeBinaryOp (MATLAB Coder). For example:

function out = varsizeExample3(n) %#codegen
coder.noImplicitExpansionInFunction
A = ones(n,1);
B = ones(n,n);
if n == 1
    A = A+B;
end
out = A;
end