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C++ Lessons

C++ Quick Primer and Practice Exercises

Table of Contents

This document attempts to highlight the parts of the C++ language that are most helpful for FRC robotics. Although some of the examples are oriented towards robotics, these lessons do not require any of the FRC software to be installed.

Although there are simpler languages that could be used for robot programming, if you take the time to learn C++ it will be worth the effort. See this blogpost for an excellent argument for learning a lower-level language.

Compiling C++

The C++ language is a compiled language which means that a program (a compiler) processes the source code and converts it into an executable file. This allows most errors to be found before the program is actually run. It is possible to use either the Visual Studio IDE (next section) or an online compiler (Online CPP) to work on the practice exercises. Visual Studio allows you to work from your local harddisk which makes it easier to save you work and not lose anything and it works more similarly to the VSCode IDE we use for the robot code. The online compiler (Online CPP) lets you practice coding from any computer connected to the internet which is great if you use a family computer at home and you don’t want to install a (very) large IDE.

Integrated Development Environment (IDE)

An IDE is a system to write, compile, link, debug, and execute code. These tasks (which you probably do not understand yet) are all done from a common application (the IDE). FRC/WPILib uses Microsoft’s Visual Studio IDE. You should install the IDE on a laptop or desktop computer that you will use to learn C++. Follow the directions Visual Studio Community Install and choose the Workload "Desktop Development with C++".

Hello World

The ubiquitous first program of any programming language is the Hello World program. So ubiquitous in fact that Visual Studio creates the code to output "Hello World" by default when you create a Console Application project.

  1. Launch Visual Studio and select "Create a new project"

  2. Choose a "Console App"

  3. Give it the name HelloWorld, optionally choose a different location, and check the "Place Solution and Project in the same directory" checkbox

Hello World Project
Figure 1. Create a Hello World Console Project

The HelloWorld project is created and the HelloWorld.cpp is opened in an editor window. All of the lines that begin with double slashes (//) are comments. The contents of the file with the comments removed is:

#include <iostream>

int main()
{
    std::cout << "Hello World!\n";
}

Choose Debug → Start Debugging to run the program. You should see the following output:

Hello World!

C:\Users\mylogin\source\repos\HelloWorld\x64\Debug\HelloWorld.exe (process 3356) exited with code 0.
To automatically close the console when debugging stops, enable Tools->Options->Debugging->Automatically close the console when debugging stops.
Press any key to close this window . . .

If all of the above goes correctly then you now have a working C++ compiler and can start writing code! If you cannot get this to work, ask for help because this should just work.

C++ Syntax, Functions, Data Types

There are several good websites for learning C++ online. I will point you in the direction of LearnCPP for a lot of these lessons. The team has a couple of copies of the book "C for Dummies", by Stephen Davis_ that you can use as a secondary resource. If you prefer to learn from a book and want your own copy then I suggest you get _"Programming: Principles and Practice Using C", by Bjarne Stroustrup. It is a really good book but it is a bit expensive.

Each chapter in LearnCPP has several sections and you need to click on the green NEXT LESSON banner near the bottom of each section. There are also quizes for most sections and entire chapter summaries and quizes.

Exercise 1

Write a program that asks the user to enter a number, and then enter a second number. The program should tell the user what the result of adding and subtracting the two numbers is.

The output of the program should match the following (assuming inputs of 6 and 4):

Enter an integer: 6
Enter another integer: 4
6 + 4 is 10.
6 - 4 is 2.
GOALS:

  1. Create a program that takes input from the user.

  2. Send output to the console.
QUESTIONS:

  1. What happens if you enter floating point numbers instead of integers (i.e. 6.5 and 4.2)?

  2. What happens if you don’t enter a number (just hit enter)?

  3. What is a void function?

  4. What is a declaration versus a definition?

Exercise 2

Write a program that has a function that takes two numbers as parameters and returns the ratio of two numbers. It’s function declaration should be:

double ratio( double numerator, double denominator );

Take two numbers as user input and print out the ratio of the numbers.

Enter a number: 6.0
Enter another number: 4
6.00000 / 4 is 1.50000.
GOALS:

  1. Create function that returns a value.

  2. Understand integer types and floating point types.
QUESTIONS:

  1. What happens if you change the ratio() function to take integer parameters and input integers from the user?

  2. What happens if you enter zero for the denominator?

Exercise 3

Add an if statement to Exercise 2 to check if the denominator is zero to avoid divide by zero errors. Print an error if the denominator is zero.

Note
 In Chapter 8.2 they show if .. else single line statements not inside blocks ({}). Always use blocks with your if statements as it helps prevent errors.
GOALS:

  1. Use an if statement to execute code conditionally.
QUESTIONS:

  1. You should never check a floating point number for equality with another number. To properly check if the denominator is zero you should check if the absolute value of the denominator is smaller than a very small number. How would you use the fabs() function to do this? Search for std::fabs.

Exercise 4

Write a program to convert RPMs to radians per second with a function declaration of double RPMtoRadPerSec(double rpm). Use a for loop to give the following output:

500.0000 RPM is 52.3598 rad/s
1000.0000 RPM is 104.71976 rad/s
1500.0000 RPM is 157.07963 rad/s
2000.0000 RPM is 209.43951 rad/s

Object Orientation

The real power of C++ is that it is an object oriented language. The techniques of Object Oriented Programming (OOP) help organize a program by spliting tasks into logical pieces. OOP turns out to be a very powerful way of organizing complex software.

Exercise 5

Write a Point2d class that has two constructors. One takes no arguments and defaults to the (0,0) point. The second takes two doubles as arguments (x, y). Write a member function print() that displays the points coordinates. The class should have two private member variables m_x and m_y of type double.

The following main() function should run and give the following output:

int main()
{
    Point2d first{};            // Contruct an 'empty' Point2d at the origin
    Point2d second{ 6.0, 5.0 }; // Construct a Point2d with two doubles

    // Point2d third{ 2.0 };    // error!  no constructor for (double)

    first.print();
    second.print();

    return 0;
}
Point2d(0.000, 0.000)
Point2d(6.000, 5.000)

Types, Templates, and Template Classes

Standard Library

C++ comes with many built-in classes that implement commonly needed components. For example, it contains several container classes like std::vector that allow for dynamically sized collections of objects (see Containers Library).

The most commonly used components in robot code are:

  • std::string — Very useful for containing strings. Strings can be added together etc.

  • std::vector — Used to store an unknown number of items (e.g. doubles) in an ordered way.

  • std::optional — Used to provide a way to signal that a variable doesn’t contain a valid value. Used extensively in the Phoenix 6 library and some in WPILib.