episode content

episodes/02-cpp-hello-world.md

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+---
+title: "Lightning overview of C++"
+teaching: 5
+exercises: 10
+---
+
+:::::::::::::::::::::::::::::::::::::: questions 
+
+- How do I write and execute C++ code?
+
+::::::::::::::::::::::::::::::::::::::::::::::::
+
+::::::::::::::::::::::::::::::::::::: objectives
+
+- To write a *hello-world* C++ program
+
+::::::::::::::::::::::::::::::::::::::::::::::::
+
+## Setting up your working area
+
+If you completed the [Docker pre-exercises](https://cms-opendata-workshop.github.io/workshopwhepp-lesson-docker/) 
+you should already have worked through 
+[this episode](https://cms-opendata-workshop.github.io/workshopwhepp-lesson-docker/03-docker-for-cms-opendata/index.html), under **Download the docker images for ROOT and python tools and start container**, and you will have
+
+- a working directory `cms_open_data_root` on your local computer
+- a docker container with name `my_root` created with the working directory `cms_open_data_root` mounted into the `/code` directory of the container.
+
+Start your ROOT container with
+
+```
+docker start -i my_root
+```
+{: .language-bash}
+
+In the container, you will be in the `/code` directory and it shares the files with your local `cms_open_data_root` directory.
+
+
+## Your first C/C++ program (Optional Review!)
+
+Let's start with writing a simple `hello world` program in C. First we'll edit the
+*source* code with an editor of your choice. 
+
+Note that you will
+*edit* the file in a local terminal on your computer and then *run* the file 
+in the Docker environment. This is because we mounted the `cms_open_data_root` directory
+on your local disk such that it is visible inside the Docker container.
+
+Let's create a new file called `hello_world.cc` in the `cms_open_data_root` directory, using your preferred editor. 
+
+The first thing we need to do, is `include` some standard libraries. These libraries
+allow us to access the C and C++ commands to print to the screen (`stdout` and `stderr`) as
+well as other basic function.
+
+At the very beginning of your file, add these three lines
+
+```cpp
+#include <cstdlib>
+#include <cstdio>
+#include <iostream>
+```
+
+The first library, `cstdlib`, you will see in almost every C++ program as it has many of the very
+basic functions, including those to allocate and free up memory, or even just exit the program.
+
+The second library, `cstdio`, contains the basic C functions to print to screen, like `printf`.
+
+The third library, `iostream`, contains C++ functions to print to screen or write to files.
+
+Usually people will use one or the other of the C or C++ printing functions, but for pedagogical purposes,
+we show you both.
+
+Every C++ program must have a `main` function. So let's define it here. The scope of this function
+is defined by curly brackets `{ }`. So let's add
+
+```cpp
+int main() {
+
+
+    return 0;
+}
+```
+
+The `int` at the beginning tells us that this function will be returning an integer value. At the end of
+the `main` function we have `return 0`, which usually means the function has run successfully to completion.
+
+:::::::::::::::::: callout
+## Warning!
+
+Note that at the end of `return 0`, we have a semicolon `;`, which is how C/C++ programs terminate lines.
+If you're used to programming in python or any other language that does not use a similar terminator, this
+can be tough to remember. If you get errors when you compile, check the error statements for the lack
+of `;` in any of your lines!
+::::::::::::::::::
+
+For this function, we are not passing in any arguments so we just have the empty `( )` after the `main`.
+
+This function would compile, but it doesn't do anything. Let's print some text to screen. Before
+the `return 0;` line, let's add these three lines.
+
+```cpp
+    printf("Hello world! This uses the ANSI C 'printf' statement\n");
+
+    std::cout << "Hello world! This uses the C++ 'iostream' library to direct output to standard out." << std::endl;
+
+    std::cerr << "Hello world! This uses the C++ 'iostream' library to direct output to standard error." << std::endl;
+```
+
+The text itself, should explain what they are doing. If you want to learn more about standard error and standard
+output, you can read more on [Wikipedia](https://en.wikipedia.org/wiki/Standard_streams).
+
+OK! Your full `hello_world.cc` should look like this.
+
+::::::::::::::::::::::: spoiler
+## Full source code file for `hello_world.cc`
+
+```cpp
+#include <cstdlib>
+#include <cstdio>
+#include <iostream>
+
+
+int main() {
+
+    printf("Hello world! This uses the ANSI C 'printf' statement\n");
+
+    std::cout << "Hello world! This uses the C++ 'iostream' library to direct output to standard out." << std::endl;
+
+    std::cerr << "Hello world! This uses the C++ 'iostream' library to direct output to standard error." << std::endl;
+
+    return 0;
+
+}
+```
+:::::::::::::::::::::::::::
+
+This won't do anything yet though! We need to *compile* the code, which means turning this into
+[*machine code*](https://en.wikipedia.org/wiki/Machine_code). To do this, we'll use the GNU C++ compiler, `g++`.
+Once you have saved your file, go to the container shell, make sure (e.g. with `ls -l`) that you your file is in the current directory and type this in your shell.
+
+```bash
+g++ hello_world.cc -o hello_world
+
+```
+
+This compiles your code to an executable called `hello_world`. You can now run this by typing the following on
+the shell command line, after which you'll see the subsequent output.
+
+```bash
+./hello_world
+```
+
+```output
+Hello world! This uses the ANSI C 'printf' statement
+Hello world! This uses the C++ 'iostream' library to direct output to standard out.
+Hello world! This uses the C++ 'iostream' library to direct output to standard error.
+```
+
+When you are working with the Open Data, you will be looping over events
+and may find yourself making selections based on certain physics criteria.
+To that end, you may want to familiarize yourself with the C++ syntax for
+[loops](https://www.w3schools.com/cpp/cpp_for_loop.asp)
+and
+[conditionals](https://www.w3schools.com/cpp/cpp_conditions.asp).
+
+::::::::::::::::::::::::::::::::::::: keypoints 
+
+- We must compile our C++ code before we can execute it.
+
+::::::::::::::::::::::::::::::::::::::::::::::::
+