Diagnose and Fix Variable-Size Data Errors

Diagnosing and Fixing Size Mismatch Errors

Check your code for these issues:

 Assigning Variable-Size Matrices to Fixed-Size Matrices

You cannot assign variable-size matrices to fixed-size matrices in generated code. Consider this example:

function Y = example_mismatch1(n) %#codegen
assert(n < 10);
B = ones(n,n);
A = magic(3);
A(1) = mean(A(:));
if (n == 3)
    A = B;
end
Y = A;

Compiling this function produces this error:

??? Dimension 1 is fixed on the left-hand side 
but varies on the right ...

There are several ways to fix this error:

• Allow matrix A to grow by adding the coder.varsize construct:

  function Y = example_mismatch1_fix1(n) %#codegen
  coder.varsize('A');
  assert(n < 10);
  B = ones(n,n);
  A = magic(3);
  A(1) = mean(A(:));
  if (n == 3)
      A = B;
  end
  Y = A;

• Explicitly restrict the size of matrix B to 3-by-3 by modifying the assert statement:

  function Y = example_mismatch1_fix2(n) %#codegen
  coder.varsize('A');
  assert(n == 3)
  B = ones(n,n);
  A = magic(3);
  A(1) = mean(A(:));
  if (n == 3)
      A = B;
  end
  Y = A;

• Use explicit indexing to make B the same size as A:

  function Y = example_mismatch1_fix3(n) %#codegen
  assert(n < 10);
  B = ones(n,n);
  A = magic(3);
  A(1) = mean(A(:));
  if (n == 3)
      A = B(1:3, 1:3);
  end
  Y = A;

 Empty Matrix Reshaped to Match Variable-Size Specification

If you assign an empty matrix [] to variable-size data, MATLAB^® might silently reshape the data in generated code to match a coder.varsize specification. For example:

function Y = test(u) %#codegen
Y = [];
coder.varsize('Y', [1 10]);
if u < 0
    Y = [Y u];
end

In this example, coder.varsize defines Y as a column vector of up to 10 elements, so its first dimension is fixed at size 1. The statement Y = [] designates the first dimension of Y as 0, creating a mismatch. The right hand side of the assignment is an empty matrix and the left hand side is a variable-size vector. In this case, MATLAB reshapes the empty matrix Y = [] in generated code to Y = zeros(1,0) so it matches the coder.varsize specification.

Diagnosing and Fixing Errors in Detecting Upper Bounds

Check your code for these issues:

 Using Nonconstant Dimensions in a Matrix Constructor

You can define variable-size data by assigning a variable to a matrix with nonconstant dimensions. For example:

function y = dims_vary(u) %#codegen
if (u > 0)
    y = ones(3,u);
else
    y = zeros(3,1);
end

However, compiling this function generates an error because you did not specify an upper bound for u.

There are several ways to fix the problem:

• Enable dynamic memory allocation and recompile. During code generation, MATLAB does not check for upper bounds when it uses dynamic memory allocation for variable-size data.

• If you do not want to use dynamic memory allocation, add an assert statement before the first use of u:

  function y = dims_vary_fix(u) %#codegen
  assert (u < 20);
  if (u > 0)
      y = ones(3,u);
  else
      y = zeros(3,1);
  end