Tools and Environment Setup
*********************************************

.. epigraph::

    The purpose of a programming language is to help express ideas in code.

    -- Bjarne Stroustrup

Operating System
=================================

All the examples in this book works in `Ubuntu 20.04.4 LTS <https://releases.ubuntu.com/20.04/>`_.

.. code-block:: bash

  $ lsb_release -a
  No LSB modules are available.
  Distributor ID:	Ubuntu
  Description:	Ubuntu 20.04.4 LTS
  Release:	20.04
  Codename:	focal


C++ Standard and Compiler
=================================

In the past 10 years, C++ has drmatically changed. The C++ standard has evolved from C++98 to C++20 in order to embrace more modern programming language features. We will use C++20 as the standard for this book. However, in the interview, the focus is on data structure and algorithm. Unless there is a special requirement for candidates' C++ familiarity, C++98 is enough for the candidate to do the work.

GNU g++, LLVM clang++, Microsoft Visual C++ are popular C++ compilers. In this book, clang++ 15.0.0 is used though it is still under development at the time of this writing [#]_.

.. code-block:: bash

  $ clang++ --version
  Ubuntu clang version 15.0.0-++20220225053013+37e84d9be06d-1~exp1~20220225173121.170
  Target: x86_64-pc-linux-gnu
  Thread model: posix
  InstalledDir: /usr/bin

To install clang 15, add the two lines in `/etc/apt/sources.list`:

.. code-block:: bash

  deb http://apt.llvm.org/focal/ llvm-toolchain-focal main
  deb-src http://apt.llvm.org/focal/ llvm-toolchain-focal main

Add gpg key to retrieve the archive signature:

.. code-block:: bash

  wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key|sudo apt-key add -

Then use apt to update and install all necessary packages:

.. code-block:: bash

  sudo apt update
  sudo apt install clang lld lldb clang-format clang-tidy -y

Not all the language and library features in C++ standard are supported by compilers.


.. rubric:: Footnotes

.. [#] The latest updates are available at LLVM Debian/Ubuntu nightly packages (https://apt.llvm.org)


C++ REPL
=================================

C++ is a static language, in contrast with dynamic langues like Python. This makes it easy to write high performance program but sometimes hard to learn. Fortunately there is a REPL(read–eval–print loop) tool **cling** to help. A read–eval–print loop (REPL), also termed an interactive toplevel or language shell, is a simple interactive computer programming environment that takes single user inputs, executes them, and returns the result to the user; a program written in a REPL environment is executed piecewise. **cling** is developed by CERN(European Organization for Nuclear Research) and hosted under its `ROOT project <https://root.cern.ch/download/cling/>`_, where you can download Linux and Mac version.

At the time of this writing, the latest version of **cling** is `cling_2020-11-05_ROOT-ubuntu2004.tar.bz2`. Run the following command to install **cling**:

.. code-block:: bash

  wget https://root.cern.ch/download/cling/cling_2020-11-05_ROOT-ubuntu2004.tar.bz2
  tar xjvf cling_2020-11-05_ROOT-ubuntu2004.tar.bz2
  sudo mv cling_2020-11-05_ROOT-ubuntu2004 /usr/local
  export PATH=/usr/local/cling_2020-11-05_ROOT-ubuntu2004/bin:$PATH
  exec $SHELL

Now you can run **cling** in the terminal and try some C++ code.

.. code-block:: bash
  :linenos:

  $ cling -std=c++2a

  ****************** CLING ******************
  * Type C++ code and press enter to run it *
  *             Type .q to exit             *
  *******************************************
  [cling]$ #include <iostream>
  [cling]$ #include <fstream>
  [cling]$ #include <cstdlib>
  [cling]$ #include <filesystem>
  [cling]$ namespace fs = std::filesystem;
  [cling]$ fs::current_path(fs::temp_directory_path());
  [cling]$ fs::create_directories("sandbox/1/2/a");
  [cling]$ fs::create_directory("sandbox/1/2/b");
  [cling]$ fs::permissions("sandbox/1/2/b", fs::perms::others_all, fs::perm_options::remove);
  [cling]$ fs::create_directory("sandbox/1/2/c", "sandbox/1/2/b");
  [cling]$ std::system("ls -l sandbox/1/2");
  total 12
  drwxrwxr-x 2 henry henry 4096 Feb 27 10:19 a
  drwxrwx--- 2 henry henry 4096 Feb 27 10:19 b
  drwxrwx--- 2 henry henry 4096 Feb 27 10:19 c

- Line 1 is to launch `cling` REPL environment.
- Line 12 is to change current directory to system's temporary folder
- Line 13, 14 and 16 are to create folders
- Line 15 is to change permissions of one folder
- Line 17 is to run a system command to print the contents of one folder

The code snippet above shows the new standard does make the programmer life easier. With built-in filesystem library support in modern C++, we don't have to use third-party library to write cross-platform code.

To save the typing, we can set alias:

.. code-block:: bash

  alias cling='cling -std=c++2a'

And put all the common C++ header files into one `sein.hpp`:

.. code-block:: c++

  #pragma once
  #include <iostream>           // Stream
  #include <fstream>
  #include <vector>             // STL
  #include <string>
  #include <list>
  #include <map>
  #include <unordered_map>
  #include <set>
  #include <unordered_set>
  #include <forward_list>
  #include <algorithm>
  #include <functional>
  #include <tuple>
  #include <deque>
  #include <array>
  #include <stack>
  #include <queue>
  #include <bitset>
  #include <iterator>
  #include <memory>             // Memory Management
  #include <chrono>             // Time
  #include <cassert>            // Assert
  #include <exception>          // Exception
  #include <thread>             // Multithread
  #include <mutex>
  #include <future>
  #include <condition_variable>
  #include <cmath>              // Numerics
  #include <filesystem>         // File System
  using namespace std::chrono;
  using namespace std;
  namespace fs = std::filesystem;

In cling, we can just include `sein.hpp` and using the classes and functions directly.

.. code-block:: c++
  :linenos:
  :emphasize-lines: 2

  $ cling
  [cling]$ #include "sein.hpp"
  [cling]$ vector<string> v={"1","2"};
  [cling]$ v.size()
  (unsigned long) 2
  [cling]$ v
  (std::vector<std::string> &) { "1", "2" }
  [cling]$ string s="hello__world";
  [cling]$ size_t p= s.find("__");
  [cling]$ p
  (unsigned long) 5
  [cling]$ s.substr(0,5)
  (std::basic_string) "hello"
  [cling]$ s.substr(5+2)
  (std::basic_string) "world"


.. note::

    - use 'c++98' or 'c++03' for 'ISO C++ 1998 with amendments' standard
    - use 'gnu++98' or 'gnu++03' for 'ISO C++ 1998 with amendments and GNU extensions' standard
    - use 'c++11' for 'ISO C++ 2011 with amendments' standard
    - use 'gnu++11' for 'ISO C++ 2011 with amendments and GNU extensions' standard
    - use 'c++14' for 'ISO C++ 2014 with amendments' standard
    - use 'gnu++14' for 'ISO C++ 2014 with amendments and GNU extensions' standard
    - use 'c++17' for 'ISO C++ 2017 with amendments' standard
    - use 'gnu++17' for 'ISO C++ 2017 with amendments and GNU extensions' standard
    - use 'c++2a' for 'Working draft for ISO C++ 2020' standard
    - use 'gnu++2a' for 'Working draft for ISO C++ 2020 with GNU extensions' standard
    - use 'c++2b' for 'Working draft for ISO C++ 2023 DIS' standard
    - use 'gnu++2b' for 'Working draft for ISO C++ 2023 DIS with GNU extensions' standard

Advice
=================================

C++ is a hard language in terms of coding speed and convenience. As an interviewer, I normally give bonus points to the candidates if they can finish the same coding question with C++ while other candidates use other advanced languages like Python, Java or Go. My interview experience shows that hardcore programmers prefer C++ as their coding languages in the interview, and they normally have very good performance in the interview. It is not because C++ is a better language. It is just because it is hard, as there are not many handy helper functions available in C++. For example, splitting string is a simple task and can be done in Python with one line as below:

.. code-block:: python
  :linenos:  

  >>> s = "hello__world"
  >>> s.split('__')
  ['hello', 'world']

In C++, you have to write a function with many lines. The following is one of the possible implementations with eleven lines of code:

.. code-block:: c++
  :linenos:

  vector<string> split(string s, const string& delimiter){
    vector<string> r;
    size_t pos = 0;
    while ((pos = s.find(delimiter)) != string::npos) {
        r.push_back(s.substr(0, pos));
        s.erase(0, pos + delimiter.size());// or s=s.substr(pos+delimiter.size())
    }
    if(!s.empty())
      r.push_back(s);
    return r;
  }

To finish the same task, the C++ candidate have to understand the usage of `string::find`, `string::substr`, `string::erase`, `string::npos` and `vector::push_back`.

Moreover, C++ requires candidates to consider memory management, which is automatically taken care of by Garbage Collector in advanced languages. So if you want to stand out, be a hardcore programmer and get the bonus points, C++ is a good choice to show you skills and help you crack the data structure and algorithm problems in the competitive interview.


Reference
=================================

- Jupyter Xeus `Xeus: a C++ implementation of the Jupyter kernel protocol <https://github.com/jupyter-xeus>`_
- `Interactive C++ with Cling <https://blog.llvm.org/posts/2020-11-30-interactive-cpp-with-cling/>`_