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Licence notice has been replaced in all notebooks: the english term redacted was misused in it.

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%% Cell type:markdown id: tags:
# [Getting started in C++](/) - [Getting started with the tutorial](/notebooks/getting_started_with_tutorial.ipynb)
%% Cell type:markdown id: tags:
<h1>Table of contents<span class="tocSkip"></span></h1>
<div class="toc"><ul class="toc-item"><li><span><a href="#About-the-choice-of-a-Jupyter-notebook" data-toc-modified-id="About-the-choice-of-a-Jupyter-notebook-1">About the choice of a Jupyter notebook</a></span></li><li><span><a href="#When-the-notebook-is-not-enough..." data-toc-modified-id="When-the-notebook-is-not-enough...-2">When the notebook is not enough...</a></span></li><li><span><a href="#Few-guidelines-about-Jupyter" data-toc-modified-id="Few-guidelines-about-Jupyter-3">Few guidelines about Jupyter</a></span><ul class="toc-item"><li><span><a href="#Restarting-the-kernel" data-toc-modified-id="Restarting-the-kernel-3.1">Restarting the kernel</a></span></li></ul></li><li><span><a href="#Very-basic-C++-syntax-(in-notebook-and-in-general)" data-toc-modified-id="Very-basic-C++-syntax-(in-notebook-and-in-general)-4">Very basic C++ syntax (in notebook and in general)</a></span><ul class="toc-item"><li><span><a href="#Semicolons" data-toc-modified-id="Semicolons-4.1">Semicolons</a></span></li><li><span><a href="#Blocks" data-toc-modified-id="Blocks-4.2">Blocks</a></span></li><li><span><a href="#Input-/-output" data-toc-modified-id="Input-/-output-4.3">Input / output</a></span></li><li><span><a href="#Comments" data-toc-modified-id="Comments-4.4">Comments</a></span></li></ul></li></ul></div>
%% Cell type:markdown id: tags:
## About the choice of a Jupyter notebook
This notebook uses up [xeus-cling](https://xeus-cling.readthedocs.io/en/latest/), a special instance of Jupyter able to run C++ code based upon xeus (tool to build Jupyter kernels for any language) and cling (a creation from CERN to be able to run C++ as an interpreted language).
The reasons for these choices is to really access directly to handle C++ code without the hassle of explaining how to compile and run stuff, which is an especially cumbersome way to start with this (or any really...) language.
This is not to say this tutorial will ignore entirely these topics (see the dedicated [chapter](/notebooks/6-InRealEnvironment/0-main.ipynb), just that we will first focus on C++ code. However keep in mind that this notebook's fancy interpreter is not a typical C++ environment.
Jupyter Xeus-Cling is still under active development: you should really get a recent version and keep it up-to-date. Some examples in this lecture didn't work at first and were properly dealt with with a version one month later!
## When the notebook is not enough...
As we shall see repeatedly, Xeus-cling notebooks are far from being full-proof: some stuff that are perfectly acceptable C++ aren't accepted in them, and some others required work-arounds. When such an issue appears:
* It will be indicated explicitly in the notebook if a specific work around is used. We do not want you to take Jupyter work-arounds as a legit advice on how to write proper C++.
* If Jupyter can't deal with the code, we will use [Coliru](https://coliru.stacked-crooked.com/). Coliru is a C++ online compiler; others are listed [here](https://arne-mertz.de/2017/05/online-compilers/) ([Wandbox](https://wandbox.org/) deserves a shout out as it enables testing the same code with a great variety of compiler versions).
%% Cell type:markdown id: tags:
## Few guidelines about Jupyter
You might not be familiar with Jupyter notebooks, so here are few tips to run it smoothly (the _Help_ menu will help you find more if you need it).
In a Jupyter notebook the content is divided into _cells_, in our case we are using two kind of cells:
* Markdown cells, such as the ones into these very words are written.
* Code cells, which are running code. In these notebooks the chosen kernel is C++17, so the code is C++17 which is interpreted by cling.
There are two modes:
* Edit mode, in which you might change the content of a cell. In this mode the left part of the cell is in green.
* Command mode, in which you might take actions such as changing the type of a cell, create or delete a new one, etc...
To enter in edit mode, simply type on 'Enter'.
To enter in command mode, type 'Esc'.
To execute a cell, type 'Shift + Enter'. For a markdown cell it will edit nicely the content by interpreting the markdown, and for a code cell it will run the code.
In command mode, several handy shortcuts are there; I would recommend especially:
* `a` (add a cell above)
* `b` (add a cell below)
* `x` (cut a cell)
* `M` (change cell mode to Markdown)
The complete list is available in _Help_ > _Keyboard_ shortcut.
If for some reason the code in the notebook seems stuck, you might try to restart the kernel with one of the restart option in the _Kernel_ menu.
### Restarting the kernel
Sometimes something that should work doesn't... In this case try restarting the kernel: it might fix your issue!
%% Cell type:markdown id: tags:
## Very basic C++ syntax (in notebook and in general)
### Semicolons
In C++ most instructions end by a semicolon `;`. If you forget it, the underlying compiler doesn't understand the syntax.
%% Cell type:code id: tags:
``` C++17
{
int foo = 5 // COMPILATION ERROR!
}
```
%% Cell type:code id: tags:
``` C++17
{
int foo = 5; // OK
}
```
%% Cell type:markdown id: tags:
Spaces, end lines and tabulations act as word separators; utterly unreadable code as the one below is perfectly fine from the compiler standpoint:
%% Cell type:code id: tags:
``` C++17
# include <string>
{
int nombre ; nombre = 1
; std::string nom;
nom=
"truc" ;
nombre = 2
;
}
```
%% Cell type:markdown id: tags:
### Blocks
You may have notices the braces `{` and `}` in the examples above. They are here a technicality to make cling interpreter work better in a Jupyter environment, but that is also useful in a real C++ code.
What is between both braces constitute a **block**; variables that are initialized inside are destroyed once the closing brace is past (don't worry - we will come back to that [later](/notebooks/1-ProceduralProgramming/5-DynamicAllocation.ipynb#Stack)).
This is an incredibly useful feature: you may ensure this way that a variable is not kept any longer than necessary. We will come back to this feature (called the **locality of reference** later); let's see why they are useful in a notebook environment.
In C++, at a given scope a same variable can't be defined twice. So for instance if I defined twice a same variable, the compiler will yell about redefinition of a variable:
%% Cell type:code id: tags:
``` C++17
int i = 2; // Should be fine on the first call.
// But if you have executed it already, a new attempt will make the compiler yell.
```
%% Cell type:code id: tags:
``` C++17
int i = 1; // If the cell above was run, compilation error as `i` is already defined.
```
%% Cell type:markdown id: tags:
The only way to circumvent this is to restart the kernel... and you may then run only one of this cell, and only once.
So you might now see the usefulness of the braces: by putting them I define the variables in a cell in a block, and it is only defined inside this block. Same variable may this way be defined in different cells:
%% Cell type:code id: tags:
``` C++17
{
int i = 2; // Fine
}
```
%% Cell type:code id: tags:
``` C++17
{
int i = 1; // Also fine
}
```
%% Cell type:markdown id: tags:
### Input / output
Inputs and outputs aren't directly a part of the language itself, but are in the standard library (often abbreviated as STL for Standard Template Library even if some purist may yell and explain it's not 100 % the same thing...). You therefore need to __include__ a file named `iostream`; doing so will enable the use of the input / output facilities.
%% Cell type:code id: tags:
``` C++17
{
std::cout << "Hello world!" << std::endl; // Should fail (unless you run a cell that includes iostream before)
}
```
%% Cell type:code id: tags:
``` C++17
#include <iostream>
{
std::cout << "Hello world!" << std::endl; // Should work: std::cout and std::endl are now known.
}
```
%% Cell type:markdown id: tags:
- `std::cout` is the symbol to designate the standard output (i.e. your screen...)
- `std::endl` is the symbol to clean-up the stream and go to next line.
The operator `<<` is used to indicate what you direct toward the stream; here std::cout << "Hello world!" tells to redirect the string toward the standard output.
We will see that a bit more in detail in [a later chapter](/notebooks/1-ProceduralProgramming/6-Streams.ipynb), but printing something is really helpful early on hence this brief introduction here.
%% Cell type:markdown id: tags:
### Comments
There are two ways to comment code in C++:
- `//` which comments all that is after this symbol on the same line
- `/*` ... `*/` which comments everything in the between the symbols.
%% Cell type:code id: tags:
``` C++17
{
int i = 0; // Everything after // is commented until the end of the line
/*
commented...
also commented...
*/
int j = 5; // no longer commented
/*
// This type of comment might be in the other style
*/
}
```
%% Cell type:markdown id: tags:
© _CNRS 2016_ - _Inria 2018-2019_
_This notebook is an adaptation of a lecture prepared and redacted by David Chamont (CNRS) under the terms of the licence [Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0)](http://creativecommons.org/licenses/by-nc-sa/4.0/)_
_The present version has been redacted by Sébastien Gilles and Vincent Rouvreau (Inria)_
_This notebook is an adaptation of a lecture prepared by David Chamont (CNRS) under the terms of the licence [Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0)](http://creativecommons.org/licenses/by-nc-sa/4.0/)_
_The present version has been written by Sébastien Gilles and Vincent Rouvreau (Inria)_
......
%% Cell type:markdown id: tags:
# [Getting started in C++](/) - [Procedural programming](/notebooks/1-ProceduralProgramming/0-main.ipynb)
%% Cell type:markdown id: tags:
* [Variables, initialisation, affectation](/notebooks/1-ProceduralProgramming/1-Variables.ipynb)
* [Condition and loops](/notebooks/1-ProceduralProgramming/2-Conditions-and-loops.ipynb)
* [Predefined types](/notebooks/1-ProceduralProgramming/3-Types.ipynb)
* [Functions](/notebooks/1-ProceduralProgramming/4-Functions.ipynb)
* [TP 1](/notebooks/1-ProceduralProgramming/4b-TP.ipynb)
* [Dynamic allocation](/notebooks/1-ProceduralProgramming/5-DynamicAllocation.ipynb)
* [Input/output](/notebooks/1-ProceduralProgramming/6-Streams.ipynb)
* [TP 2](/notebooks/1-ProceduralProgramming/6b-TP.ipynb)
%% Cell type:markdown id: tags:
© _CNRS 2016_ - _Inria 2018-2019_
_This notebook is an adaptation of a lecture prepared and redacted by David Chamont (CNRS) under the terms of the licence [Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0)](http://creativecommons.org/licenses/by-nc-sa/4.0/)_
_The present version has been redacted by Sébastien Gilles and Vincent Rouvreau (Inria)_
_This notebook is an adaptation of a lecture prepared by David Chamont (CNRS) under the terms of the licence [Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0)](http://creativecommons.org/licenses/by-nc-sa/4.0/)_
_The present version has been written by Sébastien Gilles and Vincent Rouvreau (Inria)_
......
%% Cell type:markdown id: tags:
# [Getting started in C++](/) - [Procedural programming](/notebooks/1-ProceduralProgramming/0-main.ipynb) - [Predefined types](/notebooks/1-ProceduralProgramming/1-Variables.ipynb)
%% Cell type:markdown id: tags:
<h1>Table of contents<span class="tocSkip"></span></h1>
<div class="toc"><ul class="toc-item"><li><span><a href="#Ordinary-variables" data-toc-modified-id="Ordinary-variables-1">Ordinary variables</a></span><ul class="toc-item"><li><span><a href="#Declaration" data-toc-modified-id="Declaration-1.1">Declaration</a></span></li><li><span><a href="#Initialisation" data-toc-modified-id="Initialisation-1.2">Initialisation</a></span></li><li><span><a href="#Affectation" data-toc-modified-id="Affectation-1.3">Affectation</a></span></li><li><span><a href="#Increment-and-decrement-operators" data-toc-modified-id="Increment-and-decrement-operators-1.4">Increment and decrement operators</a></span></li><li><span><a href="#Comparing-values" data-toc-modified-id="Comparing-values-1.5">Comparing values</a></span></li></ul></li><li><span><a href="#References" data-toc-modified-id="References-2">References</a></span></li><li><span><a href="#Pointers" data-toc-modified-id="Pointers-3">Pointers</a></span><ul class="toc-item"><li><span><a href="#nullptr" data-toc-modified-id="nullptr-3.1"><code>nullptr</code></a></span></li></ul></li><li><span><a href="#Constant-variables-and-pointers" data-toc-modified-id="Constant-variables-and-pointers-4">Constant variables and pointers</a></span></li><li><span><a href="#Arrays" data-toc-modified-id="Arrays-5">Arrays</a></span><ul class="toc-item"><li><span><a href="#Arrays-and-pointers" data-toc-modified-id="Arrays-and-pointers-5.1">Arrays and pointers</a></span></li></ul></li></ul></div>
%% Cell type:markdown id: tags:
## Ordinary variables
%% Cell type:markdown id: tags:
### Declaration
%% Cell type:markdown id: tags:
To be usable in a C++ program, a variable must be declared. This declaration shall include at least the type of
the variable, followed by its name and a semicolon.
%% Cell type:code id: tags:
``` C++17
#include <iostream>
{
int number; // integer variable
double real; // floating-point variable
std::cout << number << std::endl;
std::cout << real << std::endl;
}
```
%%%% Output: stream
32766
1.37345e-70
%% Cell type:markdown id: tags:
### Initialisation
%% Cell type:markdown id: tags:
Although not mandatory, it is **strongly** recommended to give a
initial value to your variables, as an expression between brackets.
Not providing an initial value may lead to unexpected behaviour. For instance you can't make hypothesis upon the values of `number` and `real` in the cell above: you might end-up with any value... and someone else might get other values on his computer!
If you give braces without values, a predefined and associated value
to the type is used (usually a form of 0).
%% Cell type:code id: tags:
``` C++17
{
int nb1 { 1 }; // integer variable set with the value 1
int nb2 {}; // same as int nb2{0};
double pi { 3.14 }; // real variable
}
```
%% Cell type:markdown id: tags:
C++ actually supports many other historical forms
of initialization, which you will encounter everywhere, including in this tutorial,
with brackets and/or an equal sign. There are some subtle differences
between each other... that you can ignore most of the time.
%% Cell type:code id: tags:
``` C++17
{
int a = 5;
int b(5);
int c = { 5 };
}
```
%% Cell type:markdown id: tags:
In all cases, even if there is an equal sign, it is important to remember
that it is an initialization, not an assignment (this will be
important when we will define our own types).
%% Cell type:markdown id: tags:
### Affectation
A new value is stored in an existing variable using the operator
of assignment `=`. The name of the variable is on the left; the expression
on the right of the `=` sign is evaluated, and its result is assigned to the variable.
%% Cell type:code id: tags:
``` C++17
#include <iostream> // for std::cout and std::endl
{
int a {}, b {}, c {} ; // default initialization; set the values to 0
std::cout << "Default initialization: a = " << a << ", b = " << b << " and c = " << c << std::endl;
a = 4;
b = 7;
c = a + b;
std::cout << "After affectations: a = " << a << ", b = " << b << " and c = " << c << std::endl;
}
```
%%%% Output: stream
Default initialization: a = 0, b = 0 and c = 0
After affectations: a = 4, b = 7 and c = 11
%% Cell type:markdown id: tags:
Affectations may be chained:
%% Cell type:code id: tags:
``` C++17
{
int a {}, b {}, c {};
a = b = c = 5;
std::cout << "a = " << a << ", b = " << b << " and c = " << c << std::endl;
}
```
%%%% Output: stream
a = 5, b = 5 and c = 5
%% Cell type:markdown id: tags:
It is also possible to define (slightly) more advanced operators of assignments that modify the value currently stored by a simple operation:
%% Cell type:code id: tags:
``` C++17
#include <iostream>
{
int a {}, b {}, c {} ; // default initialization; set the values to 0
a += 4; // add 4 to the current value of a
std::cout << "a = " << a << std::endl;
a *= 7; // multiply current value of a by 3
std::cout << "a = " << a << std::endl;
a /= 5; // divide a by 5 and assign the quotient to a
std::cout << "a = " << a << std::endl;
}
```
%%%% Output: stream
a = 4
a = 28
a = 5
%% Cell type:markdown id: tags:
### Increment and decrement operators
Finally, C++ also provides a shortcut when value is either incremented or decremented by adding `++` or `--` before or after the name of the variable.
* If the sign is placed before the variable, it is a **pre-increment**.
* If the sign is placed after the variable, it is a **post-increment**.
An example is the better way to explain the difference between both:
%% Cell type:code id: tags:
``` C++17
#include <iostream>
{
int a = 5;
int b = 3;
a++; // increment a by 1.
++a; // same, both are actually equivalents here.
int c = a + b;
std::cout << "a = " << a << ", b = " << b << " and c = " << c << std::endl;
c = a-- + b; // first a + b is evaluated, and only then a is decremented.
std::cout << "a = " << a << ", b = " << b << " and c = " << c << std::endl;
c = a + ++b; // first b is incremented, and only then a + b is evaluated.
std::cout << "a = " << a << ", b = " << b << " and c = " << c << std::endl;
}
```
%%%% Output: stream
a = 7, b = 3 and c = 10
a = 6, b = 3 and c = 10
a = 6, b = 4 and c = 10
%% Cell type:markdown id: tags:
Honestly it's usually better to remove any ambiguity by separating explicitly both operations:
%% Cell type:code id: tags:
``` C++17
#include <iostream>
{
int a = 7;
int b = 3;
int c = a + b;
std::cout << "a = " << a << ", b = " << b << " and c = " << c << std::endl;
c = a + b;
--a; // equivalent to a-- but for reasons related to the standard library I advise you
// to rather use the pre-increment form when both are equivalent.
std::cout << "a = " << a << ", b = " << b << " and c = " << c << std::endl;
++b; // same: equivalent to b++;
c = a + b;
std::cout << "a = " << a << ", b = " << b << " and c = " << c << std::endl;
}
```
%%%% Output: stream
a = 7, b = 3 and c = 10
a = 6, b = 3 and c = 10
a = 6, b = 4 and c = 10
%% Cell type:markdown id: tags:
### Comparing values
As shown above, `=` is the affectation operator. To compare two values, the symbol to use is `==`.
Other arithmetic operators are:
| Operator | Effect |
|:------------- |:-------------------------------------------:|
| `a == b` | `true` if a and b are equals |
| `a != b` | `true` if a and b are different |
| `a < b` | `true` if a is less than b |
| `a > b` | `true` if a is greater than b |
| `a >= b` | `true` if a is greater than or equal to b |
| `a <= b` | `true` if a is less than or equal to b |
These operators are defined for most ordinary types and may be defined for your own types (we'll see that [later](/notebooks/3-Operators/2-Comparison.ipynb)).
%% Cell type:markdown id: tags:
## References
A reference is a variable that acts as a kind of alias, and provides another name for the same value.
When defining a reference, it must be immediately initialized by
indicating on which variable it should point; it cannot be changed after that.
The syntax is to add a `&` character just after the type:
%% Cell type:code id: tags:
``` C++17
#include <iostream>
{
int a { 2 };
int b { a };
b = b + 1;
int& c { a }; // c is a reference to a
std::cout << "Initial values : a = " << a << ", b = " << b << " and c = " << c << std::endl;
b = b + 5;
std::cout << "After b is modified: a = " << a << ", b = " << b << " and c = " << c << std::endl;
c = c + 1 ; //
std::cout << "After c is modified: a = " << a << ", b = " << b << " and c = " << c << std::endl;
a *= 3;
std::cout << "After a is modified: a = " << a << ", b = " << b << " and c = " << c << std::endl;
}
```
%%%% Output: stream
Initial values : a = 2, b = 3 and c = 2
After b is modified: a = 2, b = 8 and c = 2
After c is modified: a = 3, b = 8 and c = 3
After a is modified: a = 9, b = 8 and c = 9
%% Cell type:markdown id: tags:
Reference is a purely C++ concept that doesn't exist in C.
%% Cell type:markdown id: tags:
## Pointers
A pointer contains the address of another variable. It declares itself by slipping
a `*` character before the name. It can be initialized or not with the address
of another variable. To explicitly extract the address of this other variable,
we use the symbol `&`.
Later, either you want to modify the address stored in the pointer, to which
in this case we use the pointer name directly, or we want to modify the variable
pointed, in which case the name is prefixed with `*`.
We can therefore see the pointer as a kind of redefinable reference; pointers are a feature from C language.
%% Cell type:code id: tags:
``` C++17
#include <iostream>
{
int a { 2 }, b { 10 };
int* p {&a}; // define a pointer p which is initialized with the address of a
std::cout << "Initial values: " << std::endl;
std::cout << "\t p: address = " << p << ", value = " << *p << std::endl;
std::cout << "\t a: address = " << &a << ", value = " << a << std::endl;
std::cout << "\t b: address = " << &b << ", value = " << b << std::endl;
std::cout << std::endl;
*p = *p + 1; // increment the integer value present at address p
std::cout << "After value pointed by p is incremented:" << std::endl;
std::cout << "\t p: address = " << p << ", value = " << *p << std::endl;
std::cout << "\t a: address = " << &a << ", value = " << a << std::endl;
std::cout << "\t b: address = " << &b << ", value = " << b << std::endl;
std::cout << std::endl;
p = &b; // change the address pointed by p
std::cout << "p now points to the address of b:" << std::endl;
std::cout << "\t p: address = " << p << ", value = " << *p << std::endl;
std::cout << "\t a: address = " << &a << ", value = " << a << std::endl;
std::cout << "\t b: address = " << &b << ", value = " << b << std::endl;
}
```
%%%% Output: stream
Initial values:
p: address = 0x7ffeea882974, value = 2
a: address = 0x7ffeea882974, value = 2
b: address = 0x7ffeea882970, value = 10
After value pointed by p is incremented:
p: address = 0x7ffeea882974, value = 3
a: address = 0x7ffeea882974, value = 3
b: address = 0x7ffeea882970, value = 10
p now points to the address of b:
p: address = 0x7ffeea882970, value = 10
a: address = 0x7ffeea882974, value = 3
b: address = 0x7ffeea882970, value = 10
%% Cell type:markdown id: tags:
### `nullptr`
A pointer can also designate no variables if initialized with the special value
`nullptr`. It is then a mistake to try to access its pointed value (as we shall see later, an [`assert`](/notebooks/5-UsefulConceptsAndSTL/1-ErrorHandling.ipynb#Assert) is a good idea to ensure we do not try to dereference a `nullptr` value).
It is strongly recommended to initialize a pointer when creating it: if you define an uninitialized pointer, it points to an arbitrary area of memory, which can create undefined behaviors that are not necessarily reproducible.
If the pointed area is known at initialization and never changes throughout the program, you should consider a reference rather than a pointer.
%% Cell type:code id: tags:
``` C++17
#include <iostream>