## Specifying Inputs in Code Generation from MATLAB

### Defining Input Size and Type

When you use Signal Processing Toolbox™ functions for code generation, you must define the size and type of the function inputs. One way to do this is with the `-args` compilation option. The size and type of inputs must be defined because C is a statically typed language. To illustrate the need to define input size and type, consider the simplest call to `xcorr` requiring two input arguments. The following demonstrates the differences in the use of `xcorr` in MATLAB® and in Code Generation from MATLAB.

Cross correlate two white noise vectors in MATLAB:

```x = randn(512,1); %real valued white noise y = randn(512,1); %real valued white noise [C,lags] = xcorr(x,y); x_circ = randn(256,1)+1j*randn(256,1); %circular white noise y_circ = randn(256,1)+1j*randn(256,1); %circular white noise [C1,lags1] = xcorr(x_circ,y_circ);```

`xcorr` does not require the size and type of the input arguments. `xcorr` obtains this information at runtime. Contrast this behavior with a MEX-file created with `codegen` (MATLAB Coder). Create the file `myxcorr.m` in a folder where you have read and write permission. Ensure that this folder is in the MATLAB search path. Copy and paste the following two lines of code into `myxcorr.m` and save the file. The compiler tag `%#codegen` must be included in the file.

```function [C,Lags]=myxcorr(x,y) %#codegen [C,Lags]=xcorr(x,y);```

Enter the following command at the MATLAB command prompt:

`codegen myxcorr -args {zeros(512,1),zeros(512,1)} -o myxcorr `

Run the MEX-file:

```x = randn(512,1); %real valued white noise y = randn(512,1); %real valued white noise [C,Lags] = myxcorr(x,y);```

Define two new inputs `x1` and `y1` by transposing `x` and `y`.

```x1 = x'; %x1 is 1x512 y1 = y'; %y1 is 1x512```

Attempt to rerun the MEX-file with the transposed inputs.

```[C,Lags] = myxcorr(x1,y1); %Errors ```

The preceding program errors with the message ```??? MATLAB expression 'x' is not of the correct size: expected [512x1] found [1x512]```.

The error results because the inputs are specified to be `512x1` real-valued column vectors at compilation. For complex-valued inputs, you must specify that the input is complex valued. For example:

```codegen myxcorr -o ComplexXcorr ... -args {complex(zeros(512,1)),complex(zeros(512,1))}```

Run the MEX-file at the MATLAB command prompt with complex-valued inputs of the correct size:

```x_circ = randn(512,1)+1j*randn(512,1); %circular white noise y_circ = randn(512,1)+1j*randn(512,1); %circular white noise [C,Lags] = ComplexXcorr(x_circ,y_circ);```

Attempting to run `ComplexXcorr` with real valued inputs results in the error: ```??? MATLAB expression 'x' is not of the correct complexness```.

### Inputs must be Constants

For a number of supported Signal Processing Toolbox functions, the inputs or a subset of the inputs must be specified as constants at compilation time. Use `coder.Type` with the `-args` compilation option, or enter the constants directly in the source code.

Specifying inputs as constants at compilation time results in significant advantages in the speed and efficiency of the generated code. For example, storing filter coefficients or window function values as vectors in the C source code improves performance by avoiding costly computation at runtime. Because a primary purpose of Code Generation from MATLAB is to generate optimized C code for desktop and embedded systems, emphasis is placed on providing the user with computational savings at runtime whenever possible.

To illustrate the constant input requirement with `ellip`, create the file `myLowpassFilter.m` in a folder where you have read and write permission. Ensure that this folder is in the MATLAB search path. Copy and paste the following lines of code into `myLowpassFilter.m` and save the file.

```function output = myLowpassFilter(input,N,Wn) %#codegen [B,A] = ellip(N,Wn,'low'); output = filter(B,A,input);```

If you have the MATLAB Coder™ software, enter the following command at the MATLAB command prompt:

```codegen myLowpassFilter -o myLowpassFilter ... -args {zeros(512,1),coder.newtype('constant',5),coder.newtype('constant',0.1)} -report```

Once the program compiles successfully, the following message appears in the command window: ```Code generation successful: View report```.

Click on `View report`. Click on the `C code` tab on the top left and open the target source file `myLowpassFilter.c`. The source code includes the numerator and denominator filter coefficients.

Run the MEX-file without entering the constants:

`output = myLowpassFilter(randn(512,1));`

If you attempt to run the MEX-file by inputting the constants, you receive the error ```??? Error using ==> myLowpassFilter 1 input required for entry-point 'myLowpassFilter'```.

You may also enter the constants in the MATLAB source code directly. Edit the `myLowPassFilter.m` file and replace the MATLAB code with the lines:

```function output = myLowpassFilter(input) %#codegen [B,A] = ellip(5,0.1,'low'); output = filter(B,A,input);```

Enter the following command at the MATLAB command prompt:

`codegen myLowpassFilter -args {zeros(512,1)} -o myLowpassFilter`

Run the MEX-file by entering the following at the MATLAB command prompt:

`output = myLowpassFilter(randn(512,1));`