Nerd Food: Neurons for Computer Geeks - Part III: Coding Interlude

If you are anything like me, the first two parts of this series have already bored you silly with theory (Part I, Part II) and you are now hankering for some code - any code - to take away the pain. So part III is here to do exactly that. However, let me prefix that grandiose statement by saying this is not the best code you will ever see. Rather, its just a quick hack to introduce a few of the technologies we will make use of for the remainder of these series, namely:

• CMake and Ninja: this is how we will build our code.
• Wt: provides a quick way to knock-up a web frontend for C++ code.
• Boost: in particular Boost Units and later on Boost OdeInt. Provides us with the foundations for our numeric work.

What I mean by a "quick hack" is: there is no validation, no unit tests, no "sound architecture" and none of the things you'd expect from production code. But it should serve as an introduction to modeling in C++.

All the code is available in GitHub under neurite. Lets have a quick look at the project structure.

CMake

We just took a slimmed down version of the Dogen build system to build this code. We could have gotten away with a much simpler CMake setup, but I intend to use it for the remainder of this series so that's why its a bit more complex than what you'd expect. It is made up of the following files:

• Top-level CMakeLists.txt: ensures all of the dependencies can be found and configured for building, sets up the version number and debug/release builds.
• build/cmake: any Find* scripts that are not supplied with the CMake distribution. We Google for these and copied them here.
• projects/CMakeLists.txt: sets up all of the compiler and linker flags we need to build the project. Uses pretty aggressive flags such as -Wall and -Werror.
• projects/ohms_law/src/CMakeLists.txt: our actual project, the bit that matters for this article.

ohms_law Project

The project is made up of two classes, in files calculator.[hc]pp and view.[hc]pp. The names are fairly arbitrary but they try to separate View from Model: the user interface is in view and the "number crunching" is in calculator.

The View

Lets have a quick look at view. In the header file we simply define a Wt application with a few widgets:

class view : public Wt::WApplication {
public:
  view(const Wt::WEnvironment& env);

private:
  Wt::WLineEdit* current_;
  Wt::WLineEdit* resistance_;
  Wt::WText* result_;
};

It is implemented in an equally trivial manner. We just setup the widgets and hook them together. Finally, we create a trivial event handler that performs the "computations" when the button is clicked.

view::view(const Wt::WEnvironment& env) : Wt::WApplication(env) {
  setTitle("Ohm's Law Calculator");

  root()->addWidget(new Wt::WText("Current: "));
  current_ = new Wt::WLineEdit(root());
  current_->setValidator(new Wt::WDoubleValidator());
  current_->setFocus();

  root()->addWidget(new Wt::WText("Resistance: "));
  resistance_ = new Wt::WLineEdit(root());
  resistance_->setValidator(new Wt::WDoubleValidator());

  Wt::WPushButton* button = new Wt::WPushButton("Calculate!", root());
  button->setMargin(5, Wt::Left);
  root()->addWidget(new Wt::WBreak());
  result_ = new Wt::WText(root());

  button->clicked().connect([&](Wt::WMouseEvent&) {
      const auto current(boost::lexical_cast<double>(current_->text()));
      const auto resistance(boost::lexical_cast<double>(resistance_->text()));

      calculator c;
      const auto voltage(c.voltage(resistance, current));
      const auto s(boost::lexical_cast<std::string>(voltage));
      result_->setText("Voltage: " + s);
    });
}

The Model

The model is equally as simple as the view. It is made up of a single class, calculator, whose job is to compute the voltage using Ohm's Law. It does this by making use of Boost Units. This is obviously not necessary, but we wanted to take the opportunity to explore this library as part of this series of articles.

double calculator::
voltage(const double resistance, const double current) const {
  boost::units::quantity<boost::units::si::resistance>
    R(resistance * boost::units::si::ohms);
  boost::units::quantity<boost::units::si::current>
    I(current * boost::units::si::amperes);
  auto V(R * I);
  return V.value();
}

Compiling and Running

If you are on a debian-based distribution, you can do the following steps to get the code up and running. First install the dependencies:

$ sudo apt-get install libboost-all-dev witty-dev ninja-build cmake clang-3.5

Then obtain the source code from GitHub:

$ git clone https://github.com/mcraveiro/neurite.git

Now you can build it:

cd neurite
mkdir output
cd output
cmake ../ -G Ninja
ninja -j5

If all went according to plan, you should be able to run it:

$ stage/bin/neurite_ohms_law --docroot . --http-address 0.0.0.0 --http-port 8080

Now using a web browser such as chrome, connect to http://127.0.0.1:8080 and you should see a "shiny" Ohm's Law calculator! Sorry, just had to be done to take away the boredom a little bit. Lets proceed with the more serious matters at hand, with the promise that the real code will come later on.

Created: 2015-09-04 Fri 17:16

Emacs 24.5.1 (Org mode 8.2.10)

Validate