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SPEAKER 1: Let's talk about water.

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In this problem, we ask the user to tell
us how long in minutes their shower was

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and then we calculate the equivalent
number of bottles of water

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that their shower used.

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>> So what are our to-do's
for this problem?

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First, we're going to want
to prompt and then validate

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the input that the user gives us.

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Once we ensure that we
have valid user input,

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we're going to calculate the
equivalent number of bottles of water

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that the user had during
their shower, and then we're

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going to tell them by
printing out that number.

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>> So let's tackle our first
task, prompting and validating

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the user input.

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For this we've written a
function for you called

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get_int located in the CS50 Library.

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get_int ensures that the user inputs
an integer, so any positive numbers,

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negative numbers, or
zero are all accepted.

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But if the user inputs anything else,
so any combination of letters or numbers

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or decimal numbers, then the
user will be prompted to retry

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and the function won't accept anything
until the user gives them an integer.

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>> So how do we use get_int?

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If you open your workspace and
create a file named integer.c let's

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type the following.

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Hashtag include cs50.h.

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This is necessary because get_int
is a cs50 library function,

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so we must hashtag include
its declaration in order

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to use the function.

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And then in my main
function I'm just going

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to simply call the function get_int.

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>> So let's run this and see how it works.

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I've already compiled it, so let's
just go ahead and run this program,

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./integer.

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Here I have a prompt and
here's where I input the value.

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Say I put in just some integer,
50, the program excepts and quits

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and that's that.

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>> But say I run it again and
I input something else.

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Perhaps, hello world.

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That's not an integer so the
program will prompt me to retry.

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Let's try again with
perhaps a decimal this time.

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0.5, again, not an integer so
the program won't accept it

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and it'll tell me to retry.

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So let's give it another number.

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The program accepts it,
quits, and we're done.

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>> So now we have a function
that allows us and ensures

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that the user inputs an
integer, but how can we

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actually keep track of the
integer inputted by the user?

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Well, all we'll do is store
this value in a variable, say n.

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So if I declare an integer n,
and I set that value to get_int,

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then n will then store whatever
value the user inputted.

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>> All right, so now we've ensured that
the user will give us an integer

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and we know how to keep track
of the integer, but remember,

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integers are both positive and negative.

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So it doesn't really make sense
in the context of this problem

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that the user takes a shower
of say negative 12 minutes.

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So we need to make sure that the user
actually gives us a positive integer.

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Now we don't have just a
single function for that,

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so we're going to have
to create that ourselves.

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>> So we want to continually
prompt the user

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until they give us a positive integer.

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If I'm doing something continuously then
that's akin to a loop, a repetition.

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So one of the constructs that we use
in c to implement repetition and loops

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is a while loop.

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So a while loop, as
shown here, will execute

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whatever is in the body of the loop as
long as the given condition evaluates

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to true.

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As soon as that condition
evaluates to false, then

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the program will proceed to whatever
comes after the body of the loop.

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>> So while loops are going to
be really useful in CS50.

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But in this particular
case, we know that we're

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going to prompt the user at least
once and then only loop if necessary.

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So here we come to a
special construct and that's

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very similar to the while
loop called a do-while loop.

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>> So a do-while loop executes the
body of the loop at least once

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and then it checks to see
whether it should execute,

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as opposed to a while loop,
which will check the condition

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and then execute the body.

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So in a do-while loop what we might
do is prompt the user for an integer,

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and then check if it's invalid or not.

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If it's invalid, then we'll
repeat the process asking the user

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to give us another integer, and then
only when that integer is valid,

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we'll continue to
whatever comes afterwards.

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>> Now you'll notice that the
declaration of the integer n

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is slightly different to what we
did before in the previous example

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and this is because of scope.

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If we had declared the integer n inside
of the body of the do-while loop,

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then we wouldn't be able to access
that value of n outside of those curly

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braces indicating the body of the loop.

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But we do in fact want to access the
value of n later on in our program.

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>> OK, so now let's talk about
what this condition should be.

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We want to only reprompt the
user as long as n is invalid.

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So think to yourself what an invalid
integer value would look like

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and then create a Boolean
expression to express that.

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>> We're almost finished with our subtask
of prompting and validating user input.

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So let's make this a little
bit more user friendly

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and give the user a little
bit more information

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of what we're prompting them for.

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So let's prompt the user, per the
spec, with the string minutes.

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So using your printf statements, make
sure that you match this exactly.

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>> All right.

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So now we have a valid user
input, a positive integer

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value for how many minutes
they spent in the shower.

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So what's next is to calculate
the equivalent number of bottles.

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What we're going to do here might
be very obvious to you at first,

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and that's OK.

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What we want to do is we
want to start practicing

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the idea of detecting patterns and
developing formulae for the problem.

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>> So we're told per the spec
that one minute in the shower

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is equivalent to about
12 bottles of water.

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So then two minutes would
be equivalent to 24,

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and five minutes would
be equivalent to 60.

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So now if you think
you've got it, then let's

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see if you can come up
with a pattern or formula

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to express if we have say, n
minutes, how many bottles of water

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would that be expressed in terms of n?

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>> Again, even though this
might be simple at first,

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later on when we get to
more complicated problems

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we're going to want to get into
the practice of identifying

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patterns and developing formulae
to figure these things out.

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>> In c you have access to
standard arithmetic operations,

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addition, subtraction,
multiplication, and division.

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So I'll leave it to
you now to figure out

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how to express the
equivalent number of bottles

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that the user consumed
during their shower.

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>> All right, we're almost done.

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We've prompted the user for their input,
we've ensured that it's valid input,

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and then we've figured out how
to calculate the equivalent

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number of bottles that they consumed.

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So the last thing to do is to output
the equivalent number of bottles

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and hopefully encourage
them to conserve some water.

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>> Outputting the value will
be a printf statement.

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If I wanted to tell you that I
have three pets, for instance,

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I might use a printf statement
that looks something like this.

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I have three pets, and a new
line for nice formatting.

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>> Now we don't want to
simply hard code things.

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Say the number of my
pets changes over time.

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Then I'm going to make use of
placeholders in my printf statement.

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So here my number is an integer.

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So I'm going to make a placeholder
for an integer using %i.

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I'm going to write the string,
and then after the string

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I write a comma and then the
variable that I want to print.

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So that value will be printed
in lieu of that placeholder, %i.

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>> You can use placeholders, as
well, for floats with a %f.

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You can also have multiple
placeholders in a string.

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For instance, if I have some number
of dogs and some number of cats,

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I put two placeholders here and then
my two variables in respective order.

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>> So now that we know how to print
the values stored in variables,

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the very last thing to
do is to ensure that we

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print the value in the exact format
indicated in the specification.

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With that, we've prompted the
user and validated their input.

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We've then calculated the equivalent
number of bottles of water

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that they consume during their shower,
and we've outputted that value to them.

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And so, we've completed water.

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>> My name is [? Zamila, ?]
and this is CS50.

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