hey CS dojo its YK here welcome to video
number two of my Python tutorial for
absolute beginners in this video first
of all I'm gonna answer a question I
received on my last video and then I'm
gonna give you some additional commands
you can use interpreter notebook for
example for adding and deleting cells
and remember that you don't necessarily
have to use to put a notebook to follow
this course you can use for example part
charm which is a popular IDE for python
after that I'm gonna cover if-else
statements how to use them and what they
are and by the end of this video you'll
be able to create a simple BMI
calculator which can tell if a person is
overweight or not given the person's
height and weight I'm gonna put an
outline of this video below so you don't
have to watch the whole thing if you
don't want to and you can go to CS dojo
io / Python - to download the sample
files I create in this video alright
first of all let me answer a question I
received on my last video
domani Ostrom sorry for the
pronunciation says currently I'm
learning web double mint what do you
think about it should I keep going or
start another language like Python so I
wanted to answer this particular
question because I thought some other
people might have the same question
maybe you started learning C++ or Java
already and you're wondering should I
keep going with it or should I take this
course for Python instead I would say
keep going with whatever you're doing as
long as you enjoy and then if you want
you can take this course at the same
time because some of the concepts I'll
teach are going to be the same
regardless of which language you're
using so for example the method for
swapping two variables I talked about in
the last video you can use that in C++
or Java or in any other language you're
learning okay with that out of the way
let's now talk about Geppetto notebook
first of all launch Jupiter notebook
through anaconda navigator or with any
other method you'd like to use and then
navigate to desktop and let's create a
new folder here I'm gonna call this
python tutorial to navigate to that
folder within desktop and click new
at the top right corner click Python 3
and let's call this file if-else
statements click rename and then I'm
just gonna maximize my browser window so
the only command we learned in the last
video was this button right here for
running or executing yourself once I
click it it executes or runs all the
commands within the given cell this
green box right here so if we have hello
one here and then hello to here and the
print function will execute both of
these lines and you might have noticed
that after running this cell it
automatically adds a new cell below it
and what if we wanted to delete the cell
for that you can use this button that
looks like scissors and for adding cells
you can use this button right next to it
that looks like a plus button and you
can add as many cells as you want and
one interesting thing here is that you
can execute these cells in any order you
like so let's say you have print a in
this cell and a equals 1 in this cell
you might say well if we try to execute
this cell it will give us an error let
me actually try it again because a is
not defined yet so it'll give us an
error saying name error name a is not
defined but before executing this cell
print a if we execute this cell right
below it to define a to be 1 it won't
give us an error anymore ok let me now
delete a bunch of cells here with this
scissors button and let's add one cell
below the first cell and let's say we
have B equals to here and let's run it
with the Run button right here and of
course this way B is defined to be 2 now
what happens if we delete the cell with
this one right here and if we try to
print be right here with print
parentheses B what happens is actually
we can still print it because even
though the cell containing this
statement B equals 2 is deleted the
state of this Jubran notebook
is actually still preserved so when I
say the state of this Jupiter notebook
what I mean is pretty much all the
variables that are stored in this
Jupiter notebook and the state of this
Jupiter notebook is not stored on this
browser window because like I say in the
last video this browser window is just a
user interface for the Jupiter notebook
the state of distributor notebook is
stored in something called the kernel of
this Jupiter notebook that's basically
represented by this command-line
interface this might look slightly
different if you're using Windows and
the kernel is what execute your code
once you click the Run button and it
also stores all the variables that
you've defined so to clear out the state
of this Jupiter notebook or to clear out
all the variables you have defined you
need to click colonel at the top and
then click restart actually I like to
use the command below restart and clear
output because it restarts the kernel
and clears out everything that's been
printed so this kernel has been
restarted now and if we try to print B
now it will give us an error because
name a is not defined yet also remember
that there is one kernel for each
notebook file so if we had another
notebook file and if we restart the
kernel for this notebook file that won't
affect the other notebook at all because
that notebook will have a separate
kernel okay so just to recap we've
learned how to use for commands in
Tibetan notebook the first is running a
cell and then deleting cells adding
cells as well as restarting the kernel
and clearing the output okay let's now
delete all the cells we have here and
restart the kernel and clear the output
and let's dive into our main topic today
if-else statements let me give you a
quick example here a is equal to 1 and B
is equal to 2 and what if we wanted to
show a message that something like a is
less than B by printing this string only
if
is actually less than B we can do this
with if a less than B Colin so that's if
space a space less than space B Colin
enter into a new line and print a is
less than B and you need to indent this
new line by four spaces so that's one
two three four you could actually use a
different number of spaces for example
three spaces or five spaces but using
four spaces is a convention in Python so
let's stick with four spaces in this
video so these two lines if a less than
B print a is less than B this whole
thing is called an if clause and after
this if clause you could have more code
for example print not sure if a is less
than B and so this if condition only
applies to this line in this example and
Python knows that because of the colon
sign here as well as the four spaces
that we have before this print mine so
let's try running the cell because a is
currently us than B we should be able to
print a is less than B and we do and
let's see what happens if a is three in
that case a is not less than B so this
cell should only print this line right
here not sure if a is less than B and
that's indeed the case and it's actually
possible to have multiple lines within
the if clause so instead of just having
print a is less than B we're gonna write
print double quotes a is definitely less
than B and in this statement Python
knows that these two lines are part of
the if clause because both of these
lines have four spaces in front of them
so let's see if that works by changing a
to one again and since a is less than B
we should be able to print these two
lines again so we see that a is less
than B and a is definitely less than B
so in a different language you might see
an if clause with parentheses
and with curly brackets but in Python
we're using a column as well as four
spaces in front of each line you need to
be really careful about the number of
spaces in front of each line because if
we had for example four spaces in front
of the first line and five spaces in
front of the second line it will give us
an error so the error says indentation
error unexpected indent let's fix that
by deleting one space here and now we
have four spaces in front of each line
so it's gonna work again okay let's take
a look at a few more examples here the
first one is this C is equal to three
and D is equal to four if we write C
less than D :
four spaces print double quotes C is
less than D and let's add something
outside this if block by writing print
outside the if block and once we write
this will print this line C is less than
D only when C is less than D what if we
wanted to add a new line here that says
C is not less than D and what if we
wanted to print it only if C is not less
than D we can do that by writing else :
right after the if clause and note that
there's no spaces before this else line
and insert four spaces before this line
to show that this line is part of the
else Clause we could also have multiple
lines within the else clause and write
print double quotes
I don't think C is less than D so this
whole block says if C is less than D
then print C is less than D and if
that's not the case or else print these
two lines instead so currently C is
definitely less than D so we should see
this line printed C is less than D and
this line outside the if block and we
see that C is less than D
outside the if block and once we change
the value of C to five so that C is not
less than D we should be able to see
these two lines instead and we do see
it's not less on T and I don't think C
is less than D take a look at another
example here E is equal to 7 and F is
equal to 8 so we're gonna write pretty
much the same thing after that so if E
is less than F then print is less than F
and else : print is not less than F so
here what if we wanted to deal with
three cases instead of only two cases
for example what if we wanted to deal
with the case where is less than F and
is equal to F and is greater than F
separately so those three cases we can
do that with all F which stands for else
if so write L if e double e cosine F
newline print parenthesis double quotes
is equal to F and let's change what's in
the else Clause to e is greater than F
so this block now says if E is less than
F then print is less than F and if
that's not the case check the next
condition e is equal to F and not here
that we're using the double e cosine
instead of a single a cosine so the
difference is that when we use the
single a cosine for example right here
we're assigning what's on the right-hand
side to the left-hand side so in Python
and in many other languages the single a
cosine is an assignment operator now if
you want to check if e is equal to F
similar to the way we can check if a is
less than F we need to use the double e
cosine so if you're not sure about which
one to use just remember that in this
line
we're not assigning F to e we're
evaluating is
that if E is equal to F so this whole
block says if E is less than F print
this line and if that's not the case
check the next condition if E is equal
to F then print this line right here
print E is equal to F and if none of
these conditions are true so it's not
true that E is less than F and E is
equal to F then else print is greater
than F because that's the only
possibility that's left here now let's
just make sure this whole block works
currently E is less than F so we see is
less than F and what if is equal to F so
E and F are both equal to 8 then we see
is equal to F right here and what if E
is greater than F so is 10 and F is 8 we
see a is greater than F right here
and actually after the if Clause we
could have multiple L if clauses so
after the first L if we can write well
if let's say e greater than F plus 10 so
this just means e is greater than EF
by more than 10 so right now he is not
greater than F by more than 10 we should
still see he is greater than 10 once we
run this cell and we do and once we
change e to for example 20 we'll see he
is greater than F by more than 10 so
there's actually another way to deal
with 3 or more separate cases without
using the LF clauses so to show you
let's define new variables here H and G
7 and 8 so we're gonna write if G is
less than H :
print G is less than H and so far it's
the same but after this we're gonna
write else instead of LF and within this
else clause we can create a new if
clause and say if G is equal to
the EE cosine print G is equal to H and
after that else colon print G is greater
than H so let's see how this block works
first we are checking if G is less than
H and if that's the case we print G is
less than H and if that's not the case
we'll go to the else clause and there
are four lines in the else clause and if
you look at this line the if clause
there are four spaces in front of it and
if you look at this print line that says
G is equal to H that's within the if
clause within the else clause so there
are four spaces here and then there are
four additional spaces here before the
print line so there are eight spaces
total here and similarly for else there
are four spaces here because this else
Clause is within this else clause and
then there are eight spaces before the
print line here because there are
already four spaces before the else
clause and then there are four spaces
after that to show that this print line
is within this else clause okay let's
make sure this works right now G is less
than H so we should see G is less than H
we do and if we have G being equal to H
we should see G is equal to H and we see
that and if G is greater than H we
should see this line being printed and
we do okay let's now create a simple BMI
calculator BMI is basically a number
that can tell if a person is overweight
or not depending on their height and
weight so let's say hypothetically
speaking we have a person named YK and
his height in meters again
hypothetically speaking is 2 meters and
his weight in kilograms is 90 now if you
only know someone's height in feet and
inches and weight in pounds just use an
online calculator to convert it so you
can just search for
feet and inches to meters and you should
be able to find a good online calculator
for that okay now the formula for
finding the BMI is the person's weight
in kilograms divided by this person's
height in meters squared and to express
height in meters squared you could
either write height m times height M or
you can write height m double star two
and this double star just means to the
power off and let's print this person's
BMI here with print double quotes
BMI and then BMI so this person would be
not overweight if this person's BMI is
less than 25 so in that case if BMI less
than 25 let's print this person's name
is not overweight and else if BMI is
greater than or equal to 25 we're going
to print this person is overweight let's
see if YK is overweight here okay he's
not but what if he had 110 kilograms of
weight then his BMI would be twenty
seven point five and YK would be
overweight okay if you'd like to support
me so I can keep making videos like this
full-time just go to CS dojo io / PT to
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some sample files from this video go to
CS dojo da yo / Python - I'm YK from CS
dojo and I will see you in the next
video