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## Nonconstant Index into `varargin` or `varargout` in a `for`-Loop

### Issue

Your MATLAB® code contains a `for`-loop that indexes into `varargin` or `varargout`. When you generate code, you see this error message:

```Non-constant expression or empty matrix. This expression must be constant because its value determines the size or class of some expression.```

### Cause

At code generation time, the code generator must be able to determine the value of an index into `varargin` or `varagout`. When `varargin` or `varagout` are indexed in a `for`-loop, the code generator determines the index value for each loop iteration by unrolling the loop. Loop unrolling makes a copy of the loop body for each loop iteration. In each iteration, the code generator determines the value of the index from the loop counter.

The code generator is unable to determine the value of an index into `varargin` or `varagout` when:

• The number of copies of the loop body exceeds the limit for loop unrolling.

• Heuristics fail to identify that loop unrolling is warranted for a particular `for`-loop. For example, consider the following function:

```function [x,y,z] = fcn(a,b,c) %#codegen [x,y,z] = subfcn(a,b,c); function varargout = subfcn(varargin) j = 0; for i = 1:nargin j = j+1; varargout{j} = varargin{j}; end```

The heuristics do not detect the relationship between the index `j` and the loop counter `i`. Therefore, the code generator does not unroll the `for`-loop.

### Solution

Use one of these solutions:

#### Force Loop Unrolling

Force loop unrolling by using `coder.unroll`. For example:

```function [x,y,z] = fcn(a,b,c) %#codegen [x,y,z] = subfcn(a,b,c); function varargout = subfcn(varargin) j = 0; coder.unroll(); for i = 1:nargin j = j + 1; varargout{j} = varargin{j}; end ```

#### Rewrite the Code

Rewrite the code so that the code generator can detect the relationship between the index and the loop counter. For example:

```function [x,y,z] = fcn(a,b,c) %#codegen [x,y,z] = subfcn(a,b,c); function varargout = subfcn(varargin) for i = 1:nargin varargout{i} = varargin{i}; end ```

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