Hello, welcome in 
this third section 
about programming 
dedicated to bugs. 
I'm sure you've come already come across one,
I'll explain some of them.
The first one is the USS Yorktown, 
which is an american warship
launched in 96 with a Navy's Smart Ship
program, which means
"Intelligent Ship".
During trials, 
a crew member made a mistake
when inputting 
entry values into the machine,
he entered 0 for a value
that was not supposed to be 0.
the consequence of that 
was that a division by 0
wormed its way inside the program
and that totally crashed 
the operating system, 
and as a consequence
nothing worked on the boat anymore, 
particularly the engines,
which is a bit surprising,
so the boat got totally
stranded in the middle of the ocean.
You can get some more details
on that webpage,
so what you need to remember,
is that entry values must be checked
because one can never be too sure
that the end user is aware 
that there are forbidden values.
That kind of unexpected value
also provides a gateway
for computer attacks.
The 
Ariane 5 satellite launcher is another example.
That satellite launcher 
is the replacement of Ariane 4,
which worked really well 
with a certain program.
And what they did was 
copy-paste the Ariane 4 program
and put it into Ariane 5.
The thing is Ariane 5 is much 
bigger, much heavier than Ariane 4,
so some maximum values
that were correct for Ariane 4
were not correct anymore
for Ariane 5.
So when they first launched Ariane 5,
those maximum values were exceeded,
and when maximum authorised values
are exceeded in computer sciences,
for an integer 
or a floating number,
strange things happen,
particularly for floating numbers,
floating-point numbers, 
some very high values 
can become very small.
For integers, some positive 
integers can become negative,
and because of that, 
a totally ridiculous value appeared
and the satellite launcher 
started going sideways
because of that wrong value, 
so the launcher had to be destroyed 
to keep it from falling on the houses.
So we can conclude that 
great care has to be taken 
of the variables we use
and to their type to make sure
maximum and minimum values
are not exceeded.
So what is a bug?
Actually, a bug is what happens
when the end user does not get what he wants.
Bugs can occur for many reasons.
It can be a programming error,
which means
the programmer made a mistake.
Forget about a special case
and use a typography,
having a capacity overload
like what I just talked about,
illicit memory access,
which means accessing
a memory space that 
was not intended to be accessed,
and so that was not initialised
with a correct value,
or a whole lot of other mistakes.
There's a more insidious kind of bug,
but just as regular,
which is communication failure,
meaning that there was a misunderstanding
between the person who ordered the program 
and the person who programmed it,
on entry values maybe,
or on what the program was supposed to do,
it can be many things.
There can also be a communication failure
between two programmers, 
because today's programs 
are not written
by a single person,
but by thousands of people
who must interact
and have to understand what the piece
of program coming from the others does.
To show you how big programs are,
I showed you a little 
example with a drawing 
representing the number 
of lines of code 
for some programs.
You can find that drawing in a 
higher resolution at this Web address,
and I'll explain some of them to you.
A pacemaker roughly 
amounts to 100,000 lines of code,
so you can see it's already a hard
for just one person to write it,
and it's only a small program 
compared to what 
I'm about to tell you.
Inside the Hubble space telescope,
there are 2 million code lines,
so that's quite a lot.
Another bigger example
would 5 million code lines,
that's for the World of Warcraft servers,
or the on-board code of a Boeing 787.
Even bigger,
Facebook represents
approximately 60 million code lines.
So you can imagine it's very complicated
to be sure there are 
no errors in 60 million code lines,
and that's why sometimes your programs
don't give good results, or crash.