Practical 1.0 – Lua Video Lecture - Torch A Practical Hands On Tutorial

the following practical will give you an
overview on the Lua scripting language
l1 pronounced Lua means moon in
Portuguese it is a powerful efficient
lightweight embeddable scripting
language which allows you to program
with different paradigms like procedural
programming object-oriented programming
and functional programming the tutorial
is divided in the following chapters we
start to get acquainted with the
terminal how to open it we'll see the
torch ripple read evaluate print loop to
have an interactive session with the Lua
scripting language we learn how to write
comments how to use numbers operators
and mathematical functions on these
numbers how to write strings we will
learn about needle and the cout function
the while and for loops and the tables
moreover we see how to write functions
how to quit our wrapper and at the end
how to write modules and local variables
in scripting files to open a terminal on
OS X we can press command + space to
open the spotlight search then we can
type terminal and press ENTER to open
the terminal on ubuntu machine we can
press ctrl alt and T a terminal will
open to have the same settings I'm using
on this machine you can go on this
website and follow the instructions if
you're using the virtual machine we
provide it for the class all the
settings are already on your machine to
learn some luan syntax we'll be using
the torch wrapper which is a read
evaluate print group interactive
top-level interface the tutorial can be
found at this location
to open torch wrapper we can simply type
th to type a comment I can simply type
double - so this is a comment and press
Enter
by pressing ctrl L we can clear the
screen to insert the multi-line comment
we can type - - open square bracket type
our multi-line comment and then we can
close it in a similar way with - - close
close and press Enter
we are going to be talking now about
variables and numbers all numbers in lua
are 64 bit doubles so basically our code
like react we don't have integers we
don't have float we don't have other
types so for example number equal 42 and
if I press number and press ENTER we
have 42 we have the 5 standard
operations on numbers summation
subtraction multiplication division and
the reminder for the integer division
and we have also a mathematical library
we can access by typing math dot
pressing twice the tab character we have
a list of more advanced functions for
example we can compute the absolute
value of 5 - 7 on expression which is
evaluating 2 - 2 and then we have the
absolute values of Manion - 2 which is
plus 2 so strings for example we can
have s equal the C string or we can have
for example T equal double quotation
doable
what which is exactly the same of the
previous one or one more way to write a
string is going to be using the square
bracket operator so we can write a
square bracket square bracket multi-line
string definition and then we can print
them by just typing X Enter or T enter
or you enter and we notice that we have
inserted a empty line at the end because
the last couple of square brackets were
on the new line below we can undefined
variables by assigning to them the
needle but we can kill the last string
for example by typing u equal new if we
check now the not the variables that we
have defined within function ho we can
see that we have just T num s and result
which is the output of the last comment
ctrl L once more to clean the screen
oops
so we can type while and then we have a
condition which has to be evaluated for
example now below a lower than 50 do now
we can help we can type the body of the
while loop for example num equal now
plus 1 and then type end and then we can
verify that num now it's 50 so we
someone to numb from 46 it was its
previous value up to the point in which
it is 49 and we see someone so we end up
to 50 in a similar way we can also write
a for loop for example for I
one one two five though and then we can
do IO dot right you can write i dot
space and then we do the output are
going to be the numbers from one to five
in a similar way we can also write a
four dog with a one more extra parameter
which allow us to write for example the
numbers in reverse order for example for
I equal five to one and the increment is
going to be minus one do again I all
right i concatenate a space and then end
and then we have the reverse sequence
another common way to use for loop is
with arrays we can type for index and
value in I pairs of the following array
so a b c d do print index and value and
so we have that the indexes of these
lists of values are 1 2 3 and 4 and the
corresponding values are a b c and
string T in a similar way we can do a
for loop over a dictionary or a generic
table and it's very similar so it's
going to be so for key and the value
in pairs so there is no I in this case
and then here we have our dictionary a
equals 3 P equals 5
C equal 8 D equal to print key and the
value and so it prints all the keys a B
D and C with their corresponding values
3 5 2 & 8 but it's important to notice
there is no order so in a table there is
no order where is in array which is that
table in which the key are corresponding
to integer starting from 1 up to the
length of T of the table will have will
maintain order when used with the I
pairs function therefore we have seen
there are two types of tables there is
the list or array version and there is
the dictionary so let's start with the
first one we have table 1 equal I can
write here a list of values for example
a string a number a boolean true I can
write again number and one more string
and then I can see what T 1 contains by
typing T 1 and press ENTER we can access
different values of a table which is a
list by typing T 1 and then I put the
number for example number 4 and then I
access the fourth element which is equal
to 3 and I can even type T 1 of 4 and I
can assign a new value value for example
false and then if I
print t1 we can see that the fourth
value has been changed from three to
false the dictionaries that are the most
generic table we can think of to define
a dictionary it is very similar to the
way we define this table so we have to
specify the key in this case so we can
call for example key ABC and we can have
equal to 4 then key d e f equal 9 key T
number 3 equal to a string and then we
control season so if I type t2 now we
have that and to access this different
values I can type t2 top top top and I
have just the string version of the key
so I can do ABC press ENTER and we had
the number 4 or t2 dot tab tab
DF enter number 9 but to access numeric
positions I had to use a square bracket
operator number 3 in the same way I
could also access the string operators
by the square bracket and ABC so let's
see how we can append and add new
elements to lists and dictionaries so
let's start with t1 we can do table top
top insert to table 1 the element 8
and then if I print t1 we will have the
new element in position 6 containing the
value 8 to note the size of a table we
can use the hash operator T 1 with the
hash in front will give us the length of
our list so for adding a new element to
a list we can do t1 and we say size of
t1 plus 1 and then we put our new value
and then we have t1 if we do type t1 of
number 9 and we assign a value this on
t1 is no longer a list but becomes a
generic table and I pairs won't work on
a sequence of numbers that are not
contiguous for dictionaries so let's
take out t2 we can define new new items
simply putting a dot the name of the new
key and then we can assign a new value
and then if I do too we can see here
it's assign a new value let's use now
functions and to determine how to see
how what works with conditional
statements so I can define my function
it's called F which takes a input
parameter X and does the following so if
X then
trained through and then X itself else
print photos and seal X itself so you
can see what it is equal to we close the
if and there we close the function so of
course if we send to f2 it's gonna
evaluate true and it's true and then if
we send false it's going to ablate false
and then it's false
now interestingly we can also try to
send values that are not boolean so
everything that is not needed which is
going to be evaluated is false a swing
is considered true so f of a numeric
value it's true F of string it's also
considered true and F of a table to put
a simple list it's also considered true
we can exploit this language feature to
implement default values for function
parameters let's create a new function
function G of X and which is going to be
simply printing X if it's defined or we
just pretty fine a value for you so X
equal x or let's put the string on the
find here what happens is that if X is a
value so if it's anything that is not
nil or false
then X is going to still be is going to
be X itself if instead we send an eel or
a false value the or will simply return
the second argument so if I do here
print X and you can see if I call G with
a value it's going to print the value
even a string or if I send nothing means
I send a needle side which is the same
or I send a false but that's not how
it's used usually it's going to be
taking the value we have just predefined
here similarly we can define in line if
statement so let's create a one more
function a function H of X and in this
case I would like just to print so if X
is greater than zero
and we're gonna output plus or we are
gonna put a minus so the function
between the parentheses says if X is
greater than zero then the first one is
going to be the output of our of our
evaluation of statement otherwise if
this is false with the end is also be
false and as we saw before the or would
be outputting the second part so let's
try end so H of
three is going to be a plus in age of
minus nine it's a - functions can also
return functions for example write our
function after X which returns another
function so functions are first citizens
in dual which doesn't have a name and
recall on the parameter Y return X plus
y and and then we end the other function
so if we call other with a value of five
a is a function still so we can call a
with the number two and this one is
going to be evaluating to number seven
okay so now we see the last part which
is quite important and are the modules
how to play with external files so we
can quit the triple by typing ctrl D and
then yes Y for yes we'll use T max in
order to split the windows in two part
let's see the into deep code which is
directory well aware where I will put my
files and let's start with my module so
I call this module mod dot one
this is my first demo module so I start
with local M equal game t-table local
it's necessary so that I will not be
polluting the whole code with my
variables but I will keep them private
to this file so let's define the first
helper function say my name which is
simply printing my name and then a
function which is going to be inside the
table above so m dot say hello which
print hello there and then say my name
and and then we have the D function then
actually the module returns the table M
and at it so I can save I split my
windows call it control a and quotation
so control a plus rotations and here we
can call our wrapper torch control L for
cleaning the screen not equal require
Bob and then if I print mode we see it's
a table with a function say yellow so
turn a K to go up so say nay say my name
it's a local function which is local to
only the mod dot file and and therefore
does not appear below control AJ to go
below we can try to call say low mod dot
say hello and then yes it's going to
print out what we expect if I try to
call the other function mode say my name
we will get a attempt to call field say
my name a nil value because it's
undefined
because it's local to the mod file
let's go above control a ok let's go at
the end sorry
so here before returning
let's put also a print own code and then
save wake up below control a.j so we can
do require mod as we did before and well
nothing happened we don't have the print
all code executed anywhere this is
because load require does cache the
files so once it's it's run once we
won't run again the decode if you'd like
to run the new code we already see we
just saved we had to use to file and
then we had to put the name of the file
run to the one again here we can call
mod one and then all code loaded cool if
I do again the same perhaps I change
here mod 2 still oak all called loaded
but if I do mod 3 equal require
Maude it won't print all code loaded in
actually mode 3 it's the previous mode
we have ran the first time if we'd like
to run to load code but do not run at
the same moment we can do in this way we
can do f45 equal load file and then I
can say it's like the do file but it
doesn't do it just load it some main
memory so mod dot Rho and so f it's a
function if I run the function done it's
actually executing the code the last the
last interesting load file load string
load command is this one it's called
load string and here we can write a
command so 1 2 3 and load string so G is
a function if I call G G is going to be
evaluating the string we send to G as a
general reminder of good practice always
use local variables in your script
unless you're you are in the triple and
you're coding with the interactive
interface which does not allow you to
properly use local variables local
variables are mandatory in order to do
not pollute the scope of the whole
program