iOS 11 Developer Course Tutorial - App (Is It Prime Part I) Part 19 Video

congratulations or make it to the end of
section 3 of the complete iOS developer
course you're now hopefully familiar
with variables arrays and dictionaries
if statements for loops while loops
classes objects and these mysterious
optionals now we're gonna put it all
together to make a slightly math she
focused app called is it prime so this
app is gonna tell you when you put in a
number whether or not that number is
prime now if you're not sure a prime
number is it's pretty simple as it says
here a prime number is a whole number
greater than 1 that has only 2 factors 1
and itself
so the first prime number is 2 3 is a
prime as well 4 is not because 2 goes
into it 5 is prime 6 is not because 2 &
3 go into it 7 is prime 8 9 and 10
aren't etcetera etcetera so we need to
look at the factors or dividers of a
number and if it has any numbers that
divide it that aren't one or itself then
it's not prime so your first challenge
then and this will be a challenge is to
write some code where we have let's say
a number which maybe is 643 and then
your code will work out whether or not
that number is prime so you'll likely
need to use loops you'll need to use if
statements and a fair bit more of what
you've learned about in this section but
that's all you need to do so we should
have a true or false statement at the
very end to tell us whether or not this
number is prime there is one new little
trick that you'll need to figure out to
write this code and I have left this out
on purpose so you'll need to do a little
google to figure out how to work out if
one number divides exactly into another
so that's one thing we haven't done in
Swift before but you should be able to
find the answer very easily on the web
so I'm going to leave that for you to
figure out as always we're at early
stage in the course so if you don't feel
confident to write the whole thing
yourself
feel free to just give it a try and then
follow along with the
but do have a go you might surprise
yourself with how much you can do best
of luck
all right here we go then there's a
number of ways as is often the case that
you can solve this but what I'm going to
do is use what I mentioned at the
beginning which is if a number has any
factor which isn't one or itself then
it's not prime so I'm going to start by
assuming that the variable is prime so
I'll set is prime is equal to true and
then I'm gonna set our counter variable
I'll call it I equal to 2 so that will
be the first number that we're going to
attempt to divide into our number and
then we'll use a while loop which will
keep going
as long as I is less than our number we
don't want I to actually reach our
number because we know that 643 divides
into 643 but that doesn't count it would
still be prime if that was the case so
we want to keep I just less than our
number and let's not forget to add one
two I each time we go through the loop
otherwise it will just go on forever and
ever with I being too and now comes that
bit I mentioned at the beginning where
we have to tell whether I divides into
our number or not and the way we do that
is we look for number and then we use
the percent symbol and I to work out the
remainder when I divides into number so
if that remainder is zero that means I
divides into the number exactly which
means that it can't be prime so if
that's the case we'll set is prime
to be false and just to show you what I
would have searched for if you didn't
manage to figure that out on the
internet I would go for something like
Swift how to tell if one number divides
another something like that so just ask
what what it is you're trying to do and
but Swift at the beginning here we go
now that first result actually looks
very promising but it doesn't have the
words Swift in it so I'm going to
actually use this one check if an
integer is divisible by another integer
this takes us to the wonderful stack
overflow which you'll see many times in
your coding journey and there we go you
see this is exactly the answer we were
looking for so if number two percent
number one is zero that means the
remainder when number one divides into
number two is zero which means it goes
exactly so that's actually all the code
that we need so let's then print
here's prime and we should get the
answer first off whether the 643 is
actually prime and the answer is true
so 643 is a prime number so let's check
through a few numbers that we know and
make sure it gets the right answers are
those no ha so we have our first problem
already one is not a prime number it's a
bit of a special case so I'm actually
going to cheat slightly and just put
that in as a manual test so if number
equals one then we'll set his prime
equals false you may be able to find a
more elegant way of dealing with that
but one is our only exception so I think
it's reasonable to put that in there to
keep our code nice and clean so let's
try number equals two and this should
return true and that's true so looking
good three should be true as well and it
is excellent for should be false because
two goes into it and it is yep so this
is looking promising five is true as it
should be six is false excellent let's
just try a larger one seventeen is true
excellent so we now have our core code
to work out whether an number is prime
or not and I hope you've also seen the
advantage of in some cases setting up
the core of your app in a playground or
a separate coding environment because
that means that you can afford to make
errors a lot quicker and figure out how
that bit works without having to compile
and run the app every time to see if
your code is right so for some apps this
can be a really useful diversion to get
this bit done first and then import it
over into a main Xcode project but
that's of course what we need to do now
and we'll be doing that part of the app
in the next video