Avoiding catastrophic cancellation

Catastrophic cancellation occurs when subtracting two nearly equal numbers, leading to a significant loss of precision due to limited precision of floating-point numbers in a computer. Below are some examples and their better versions (reference):

log(a)−log(b) = log(a/b)

Computing cos(x)−1 for small values of x --> cos(x) ≈ 1−(x^2)/2 

Sharing files between Windows and Ubuntu virtual machine

I have a Windows 10 PC with Ubuntu 22.04 installed as a VirtualBox virtual machine. There are other ways to share files between Windows and Ubuntu, but the following is the most general way I know:

1. On Windows, share a folder (e.g. "temp") with your own windows user name.
2. Find the IP address of VirtualBox ethernet adapter:




3. On Ubuntu make sure you can ping that IP address.
4. Open a new Files window, at the bottom left, click on other locations. Then, at the bottom enter smb://<VirtualbBox ethernet adapter IP address>/<Windows folder name>



5. After clicking Connect button, you should see the temp folder:

Installing Eclipse CDT and build-essentials to offline Ubuntu

On my offline Ubuntu I use Eclipse C++ (CDT) and build-essentials to build C++ projects. To install Eclipse C++ (CDT) and build-essentials to offline Ubuntu:

1. Download Linux version of Eclipse C++, copy to offline Ubuntu and extract. You can directly run eclipse without any further installation, but you need to finish the following steps to build a C++ project.
2. On your Windows PC that is connected to the internet, install a virtualizer like VirtualBox  and install Ubuntu Desktop 22.04 as virtual machine.
3. Enable Windows - Ubuntu file sharing.
4. Use the following shell script to download build_essentials on your online PCs Ubuntu virtual machine and its dependencies. You can copy these downloads to your offline Ubuntu and install them following the steps written as comments down below:

Handling left over carriage return

Lines in text files created in Windows end with '\r\n'. If you read that text file in Linux with C++ getline(), your line will have a '\r' at the end because in Linux, getline() only gets rid of '\n'. If you have code that reads a certain number of characters and converts it to floating point using std::stod(), you might get std::invalid_argument exception when trying to read multiple values. You can use the following to take care of this problem:

Simulink signal

After generating code from a Simulink model, if you want to read the value of a parameter inside the model which is not an output port, you can use signals. In Matlab R2022b, signal definition is different from R2019a. The following R2022b method (assuming you have Embedded Coder) is only valid if you generate C code, I have not been able to find a way to generate signal code for C++:

1. Open Simulink model
2. Open Configuration Parameter window, go to Code Generation
3. System target file: ert.tlc (Embedded Coder)
4. Language: C
5. In Simulink APPS menu, click on Embedded Coder, A C CODE tabs appears to the right of APPS menu
6. Find the signal line you want the generate code for and double click on it, give it a name, e.g. "mySignal"
7. Single click on signal line, a pop up will appear, click on the left-most item, "Add selected signals to code mappings"
8. Under C CODE tab, click on Code Interface and then click Default Code Mappings
9. A Code Mappings panel will appear at the bottom of model
10. Click on right most tab, Signals/States
11. Under Signals, find mySignal
12. Click to the right of mySignal (Store Class column), change Auto to Model Default.
13. Generate code by pressing ctrl + b
14. If your model name was MyModel, the generated code will have a file called MyModel_ert_rtw/MyModel.h containing the data structure MyModel_B which has mySignal as a field that you can access from your own C/C++ code. 
15. If your code is C++, dont forget to use include in C wrapper: extern "C" {#include "MyModel_ert_rtw/MyModel.h"}

Sanity checks

If your software component is taking data from other components or sensors, that data should go through at least one sanity check. The world outside of your component is full of surprising errors, most of whom you won't be able to guess beforehand. A correctly working external component might get buggy after an update, don't assume that newer versions don't break existing functionality. Sanity checks prevent the simple ones from crashing your software, protecting you from embarrassment to even loss of life.

Compile s-function with C++17

To compile a Simulink s-function file with C++17 (assuming you have Visual Studio 2019 or higher) , use:

        mex COMPFLAGS='$COMPFLAGS /std:c++17' file.cpp

Note that this does NOT work: mex CXXFLAGS='$CXXFLAGS /std:c++17' file.cpp