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So, in the previous video, we started
talking a little bit about program flow.

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So, let's now continue on with that, and
actually talk about the if statement.

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So, what I've done is,
I've created a new project in IntelliJ.

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And I've called it ifprogramflow.

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And I'm going to right-click that now,
click on New, select Python File, and

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I'm just gonna call it ifprogramflow.

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Making sure that I'm
actually clicked in there.

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Okay, so we've seen how we can
store values in variables,

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and we can then, print them out.

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And we've also looked at how to
get input from the keyboard.

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So, we now start getting
Python to make decisions

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based on the values of variables, and
the input received from the keyboard.

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As a quick recap,
just from the following program,

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just to see where we were
at in the previous video.

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So, we can type name Equals input.

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Please enter your name.

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Now, we can type age equals input.

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How old are you, placement field, and

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we're going to put format name,
you print age.

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So, notice how we can apply
formatting to any string,

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even the ones that we're going to be
using as a prompt to the input function.

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So, in the first line we've
created a variable called name.

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And we're going to accept the name
that's been entered by the user

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from the keyboard, and
put that into the variable called name.

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Then, the next line, line three,
we've got an age, we'll ask it to,

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for the user to put how old they are,
and we're using a replacement field,

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to actually format, and print the name
that was entered on line two, and

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share that on the string, on the screen,
along with the prompt to enter their age,

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and on the fourth line, line four,
we actually printed out.

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So, if we run this,
please enter your name.

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Tim.
How old are you Tim?

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I'll say I'm 21, I certainly wish
that I was Press Enter > 21.

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So, we can say that's
a bit of a recap we're at.

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Now, the input function
returns a string data type.

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So, in order to actually convert that 21.

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That number that I actually
typed into a number.

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If we want age to represent a number,
rather than a string,

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we need to actually convert it.

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We do that using the int function.

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Before making some decisions
based on the person's age.

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So, what we can do is actually
change this, to put it int,

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which again converts,
what's been typed into a stream.

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Then, we actually

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have to add a bracket right on the end
of the line to make that valid.

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When we go to run it, we will see
that there aren't any differences, so

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it will do that same thing.

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The only thing is we've changed it now, so
that the data type, because we've selected

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the int, it's going to be a number Instead
of what it was before which was a string.

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So now, that we've done that what
we can do is we can actually get

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the computer to actually try, and
do some processing based on that age.

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So, lets do that, so I'm gonna put if,
if age is greater than or equal to,

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that's what those two symbols are that
means it greater than or equal to 18.

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Then we put a colon and
we press Enter to go to the next line.

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Then we're going to print ("You
are old enough to vote.").

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So, if we run that.

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Before we do that, let's actually type
in something that's not a number, and

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see what happens.

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We're gonna type in the words twenty one.

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We actually got an error.

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And the error is that it actually,
basically just is expecting a number, but

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it actually receives some text, and
it's got a little bit confused.

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So, keep that in mind, that if you specify
something like we've done here, and

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an input,
it's gonna assume that that is the case.

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But don't worry too much about that,
because we're gonna be covering

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error handling, and
input validation in the course.

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But for now, to be valid,

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we need to make sure that we're
only entering valid numbers.

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So, we're gonna go back, and do it again.

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Tim?

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Twenty-one, and you can see we've got
the message printed "You are old enough

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to vote." And, once we've got
the age in the numeric format,

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we can actually use the greater than,
and the equal, as we've used here,

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to actually check to see whether
it is greater or equal than 18.

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And we're doing that using an if
statement, with a condition to check.

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So, if the condition is true, in this case
the age was greater than, or equal to 18,

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then, we're printing the message
that you're old enough to vote.

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If the age is less than 18,
nothing happens.

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It actually would go into the next line.

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Now, that's a little but unfriendly,
now what we can do we can code you for

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that we can put in
something called an else.

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Press back, press space will
delete press else or top else.

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.
And press Enter.

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Print.

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Please come back in, and
we're gonna put our replacement field.

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Let's use .format.

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Let's actually get the computer to decide,
how many years I've gotta come back with.

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And we can do that.

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We can actually put the number
of years which was 18,

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less the current age that was typed in,
so 18 take whatever.

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So, if I've actually typed in the age
of 15, it would be 18 take 15 is 3.

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So, if we actually run that now, firstly,

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I'm putting my name, I'll put 21 we still
get the same message, and notice how this

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line here is basically ignored, and
that's because the first part was correct.

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So, how the program works is, it comes to,
and looks for this condition, and

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if that is true, in other words, in this
case what was typed in is greater than or

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equal to 18, then that line is executed.

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And then, that line is executed, not
the next line with the else isn't it, and

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then, the actual code jumps past the else,
and the print and goes on to next part.

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But otherwise, so if the age entered
isn't greater than a towards at 17 or

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less, we get the second part.

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And just to confirm that,
we can run that again.

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Tim, if I put in 15,
please come back in three.

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And I should go back to change that,
and put that in years, like so.

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If we actually run that again, Tim 15.

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Please come back in three years.

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Now, the other thing to note
here is the block of code

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that's within these conditions can have
more than one line of code to execute.

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So, in other words we can come back, and
put, you're old enough to vote, print,

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please put an X in the ballot box or
more correctly print x in the box.

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Else, we'll leave that as it is.

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Now, if we run that again.

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Tim, 15.

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We get the else only,
please come back in three years.

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But if actually run it again,
and type in Tim.

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21.

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You're old enough to vote,
please put an X in the box.

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So basically,
the code that's in this block of code,

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all of that is going to be executed
if this condition above it is true.

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So likewise, we could've done
a similar thing for the else.

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We could put extra code down here, and

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provided it's indented correctly,
that's going to run,

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if that else condition is actually true
in this case, if h was less than 18.

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Now, sometimes you might need more
than 2 cases, so what Python does,

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it let's us test conditions by letting us
the L if which is an abbreviation for.

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So, let's actually come up with another
example, and move on from this one,

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so I'm going to comment that out.

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And I've commented on that.

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You noticed that at the bottom it
actually tells you the command I used.

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Command slash which is
control slash on Windows or

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Linux, and that gives you a way to
actually comment out a locator once.

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So, if you want to instead of just
deleting something instead of

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comment it out, so that it's not going to
be executed, that's a way of doing it.

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Let's type another example in.

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I'm gonna print, please guess a number,

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between 1and 10.

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We're going to set guess,
the variable guess,

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to convert the input to an int, and
get that number from the keyboard.

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Now, we're going to do some
testing using the if statements.

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So, we're going to put if,
guess is less than 5, print.

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Please guess, higher.

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Guess equals input, and

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we need to add a [INAUDIBLE] to make
it valid, converting it to a number, so

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we can do these if comparisons, and then,
we're gonna put if guess is equal to 5.

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Now, notice when I'm putting equal to 5,
I've had to put two equals.

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If you use one equal sign,
like the line above, guess equals int,

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we're actually setting the value for
the guess variable.

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But two equals together,
that's actually doing a test.

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We're actually saying, does the guess
variable have a value of 5 to it.

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So, if that's the case we're
going to put another colon,

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press enter, print,
well done, you guessed it.

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And pressing backspace again, else print,

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sorry you have not guessed correctly.

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So, I can see what's happened there,
if your guess is less than five,

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you're gonna print please guess higher.

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If your guess equals input, if your
guess equals five, print well done,

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you've guessed it.

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Else, print sorry,
you have not guessed correctly.

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But, what we're also gonna do
is we're going to embed that, so

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we're going to press backspace, backspace,
type elif which stands for else if,

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guess greater than five, you're gonna
type print, please guess lower.

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Guess equals input.

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Guess equals int input.

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If guess is equal to five,
the two equals again, print, well done.

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You guessed it.

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Else, print sorry you have
not guessed correctly.

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And so, and informally,
and here we can put else,

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print you got it this time.

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So, you can see we've got
quite a few ifs there, and

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an elif, an else if and then,
an else down at the bottom.

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So, what's actually happening here, we're
doing if the guess, that the first number

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that they typed is less than five, then
it will go into, and execute this code.

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Otherwise, getting back to
the start again, that wasn't true,

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so in other words,

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what they type was greater than five,
then this code down here will be executed.

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We'll do a test here to see whether
the guess is greater than five,

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and if it was it's going
to execute that code.

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And finally, the third level, else,
will be if you typed five for

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the first time when you get that error, or

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get that message to say that you
got it right the first time.

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So, the indentation level is very
important to actually get an idea,

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of what's actually working
because this level of if,

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is completely different to this level.

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This second level was only
going to be executed.

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The codes only going to go in there,
if this condition was true.

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So, let's actually run it using that code,

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and see how the code's
actually going to work.

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So, let's do the first test, and
we type in the number that's exactly five.

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You got it first time.

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So, what happened was, this code was
executed, guess is less than five,

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and that didn't happen, because guess was
equal to five it's not less than five.

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So, the computer jumps into this line of
code, and says okay, was the guess that

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was entered greater than five, or no in
this case it wasn't because we typed five.

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So therefore, it jumped to the third one,

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which was else, and printed that list,
cuz there wasn't a condition.

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Else, without any statement means just
actually print whatever else is there.

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So, running it again.

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This time if we type a number,
let's say typing a number that's 4.

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And it says, Please guess higher, so
you can see, is guess less than 5?

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Well yes, cuz it's 4.

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If guess is equal to 5,
print Well done, you guessed it.

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So if we type 5 in here now it should come
up and say, Well done, you guessed it.

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But if we run it again, and

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put four, please get's higher,
this time we're going to guess 890.

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Sorry you've not guess correctly.

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And lastly, we'll do a test and we'll say,
we'll start with ten, please guess lower.

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Your top five.

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Well now, you've guessed it.

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And run it again.

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12, please guess lower.

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11, sorry you've not guessed correctly.

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So, you can see, I hope you're now getting
an idea about how you can actually,

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how this code works, and how blocks can
be quite complex including further if and

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else blocks, and much more contained
within it, you saw how when testing for

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equality we don't use
a single equal symbol.

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We use the two symbols to actually check
to see whether value has a certain value.

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And of course a single equal is if you're
assigning a value as in this line here.

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Assigning the variable guess,
the value of whatever of was

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typed by the user converting
it automatically to a number.

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Now, and if statement can also include
many else ifs, elifs, and as you can see,

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I've already used one there.

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I've already used one elif,
and then at the last line,

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we used an else, which is sorta the final
test that we can actually have.

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If all else files,
it's gonna go to that part of the code.

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Now, there are much better ways of
writing code than what we've shown here,

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this is really just
an example to get you going.

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And you will see better
ways of doing this shortly.

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One of the bad things that we've done here
is we're actually duplicated code, so

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we've actually added this code here for

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example, we've duplicated there,
and we've duplicated it there.

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And we wouldn't want to do that in
a real-world scenario, but this again,

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just to get us going.

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If you ever see something like this
when you're programming though,

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that's probably a good indication
that the code that you're working on

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could be improved or
reducing that duplication.

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So, we saw that in a previous bit of code
this one up here, greater than or equal,

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you can actually test for
that using a greater than or

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equal sign as you saw there.

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We can test for
equality with the two equals.

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We can test for

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greater than which is the greater than
sign likewise with the less than.

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And there's a couple
of other ones as well.

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which we'll see as we go through.

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There's a not equal which
would be similar to that.

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Which would be like this.

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If you want to check something
that is not equal to.

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And there's also a less than and equal
which works just like a greater than does,

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but obviously,
it's the other side comparison,

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making sure our value is either
less than or equal to that value.

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So now,
we actually know how to actually check for

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nautical as I showed you briefly,
by doing something like that,

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we can actually change our code,
and make it a lot more efficient.

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So, what we could actually
do here is we could,

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instead of putting if guess is less than
5, we could put if guess not equal to 5.

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And the next time we could
put if guess less than 5,

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and then, we could indent that
to put please print higher.

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Else, print please guess lower.

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Please, guess lower.

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And what we can do then,
is we can put guess equals int, input.

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Then, put if guess, is equal to 5,

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print, well done you guessed it,

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else, print("Sorry,

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you have not guessed correctly").

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And then, removing this else if now,
the elif, which we no longer need.

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Deleting that, we can actually just
print("You got it first time").

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So, you can see we've now
consolidated that code.

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We're doing a test at the start.

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So, if guest is not equal to 5,

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so if any value other than 5 was entered,
we're gonna start processing down here.

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But if 5 was entered,
we're gonna go straight down to here,

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just how you got it first time.

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They were doing a test say if guess is
less than five, the message would say,

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please guess higher, because we
know the number needs to be higher.

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Else, at this point the number has to be
higher, so we print a message that says,

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please guess lower.

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And will ask for an input again, and
if the guess is correct, and equal to 5,

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we print a message that say, what we've
done you guessed it, should be guessed.

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00:15:40,880 --> 00:15:43,930
Otherwise, we put in summary
you've not guessed correctly, and

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00:15:43,930 --> 00:15:46,760
this funnel else,
is only called at this level

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which would be if they actually
got it right the first time.

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00:15:49,090 --> 00:15:51,910
So, let's just run that, and
confirm that it still works.

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So, I enter number 5 first.

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00:15:53,320 --> 00:15:54,600
He got it right, first time.

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00:15:54,600 --> 00:15:58,870
So, the first test to see whether it's
not equal to 5 doesn't go into there, so

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therefore, it has to be equal
to 5 because we've tested for

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not equal here, so we're getting that
message correctly, gonna run this again,

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went to the number four,
please gets higher.

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Which you would expect it to do.

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00:16:09,840 --> 00:16:13,350
Now, when I try another number this time
we'll say six, sorry it could not guess

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00:16:13,350 --> 00:16:19,030
correctly, run it again four,
this time we'll actually guess five.

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00:16:19,030 --> 00:16:22,390
Well, now you've guessed it, and that's
because this bit of code was executed

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00:16:22,390 --> 00:16:25,690
because the second time we actually
asked for the input which was there.

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00:16:25,690 --> 00:16:27,950
Just doing another test to see
whether we've got it right.

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00:16:27,950 --> 00:16:31,290
And finally,
we'll hit a number like eight.

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Please guess lower.

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00:16:32,560 --> 00:16:34,710
Five.
Well, now you've guessed it, and

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00:16:34,710 --> 00:16:36,570
the last one we'll actually do a test for
eight.

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00:16:38,280 --> 00:16:41,290
And so six,
sorry you have not guessed it correctly.

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00:16:41,290 --> 00:16:44,230
So, you can see this really highlights
the importance of indenting your

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00:16:44,230 --> 00:16:47,620
code in Python,
because the second guess, this one here,

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is not at all dependent on this other code
that's in this first block, so it's really

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00:16:51,970 --> 00:16:56,010
important to structure your code, to put
your code in the right indentation level,

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00:16:56,010 --> 00:17:00,290
and that helps you ensure that you're
executing the code in the right scenarios.

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00:17:00,290 --> 00:17:04,410
So, for example, here that's only going
to execute is guests is less than five.

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00:17:04,410 --> 00:17:07,460
This line's only going to be executed
if guests is greater than five,

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00:17:07,460 --> 00:17:12,109
but in this case, here, this code is
going to be executed no matter what,

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00:17:12,109 --> 00:17:14,720
because we haven't got any tests, and
it's not indented to the next level.

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00:17:14,720 --> 00:17:18,119
Indented, it's going to be, running
the code at the same level, if you will.

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00:17:18,119 --> 00:17:19,380
One of the other things
we could've done there.

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00:17:19,380 --> 00:17:26,099
We could've done something like
# guest must be greater than 5.

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00:17:26,099 --> 00:17:30,490
Which will tell us that, because in
this code here we can see that, okay,

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00:17:30,490 --> 00:17:32,240
that's at less than 5 test.

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00:17:32,240 --> 00:17:35,200
Sometimes with just an else it can
be a good idea to put a comment,

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00:17:35,200 --> 00:17:38,970
just to spell out what the scenario is,
what this else is gonna be called for.

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00:17:38,970 --> 00:17:42,420
In this case It's only going to be
executed if guess is greater than five, so

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00:17:42,420 --> 00:17:44,270
that can be a good place to put a comment,

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00:17:44,270 --> 00:17:46,190
just to confirm your
understanding of the actual code.

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00:17:46,190 --> 00:17:50,420
And it also helps yourself certainly, but
also other programmers in the future,

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00:17:50,420 --> 00:17:52,260
who are actually reading that
to figure out what's going on.

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00:17:52,260 --> 00:17:56,420
And again, just a reminder that comments
are ignored by the Python interpreter, and

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00:17:56,420 --> 00:17:58,560
they're there purely
to document your code.

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00:17:58,560 --> 00:18:01,360
So generally, if you use comments to
explain why you've done something in

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00:18:01,360 --> 00:18:05,630
a certain way, or as a reminder of
certain conditions, or variable states,

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00:18:05,630 --> 00:18:08,360
that may not be obvious to someone
who's reading it for the first time.

324
00:18:08,360 --> 00:18:10,460
And remember that,
this is very important in comments,

325
00:18:10,460 --> 00:18:12,610
if you come back to tomorrow for
a code weeks or

326
00:18:12,610 --> 00:18:16,360
months after you wrote it, that
documentation can really come in handy.

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00:18:16,360 --> 00:18:19,500
And just a final thing about comments
before we end this video is that

328
00:18:19,500 --> 00:18:22,010
Python does provide a way to
have multiline comments, and

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00:18:22,010 --> 00:18:25,470
we'll be covering that in the section
on documentation, and doc strings, and

330
00:18:25,470 --> 00:18:26,970
you can find out more about it there.

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00:18:26,970 --> 00:18:29,820
For now, if you want a comment to span
several lines just start each line

332
00:18:29,820 --> 00:18:30,730
with a hash.

333
00:18:30,730 --> 00:18:33,060
That you've seen at the top
of this code here now.

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00:18:33,060 --> 00:18:34,800
Okay, so
we're gonna end this video here now.

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00:18:34,800 --> 00:18:37,700
When we come back in the next video,
we're gonna start looking at

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00:18:37,700 --> 00:18:40,960
more complicated conditions than the
simple ones that we've used so far, and

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00:18:40,960 --> 00:18:45,470
show you how Python provides the means
to produce quite complex expressions.