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[MUSIC]
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Programming languages use strings a lot,
but they use numbers even more often.
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Almost any language will let you do
addition, subtraction, multiplication,
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division and many other math prostrations,
and C# is no exception.
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Our next requirement for the Cat Food
Store is to calculate a total price for
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the order.
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Math operations will let us multiply
the quantity of cans by the price to get
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the total.
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Math operators take the values
to their left and right, and
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perform a math operation on them.
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The four most common operators, add,
subtract, multiply, or divide values.
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The addition and subtraction operators
look just like they do in most elementary
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school math textbooks.
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But most keyboards don't have a key for
the traditional multiplication or
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division symbols.
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So like most programming languages,
C# uses an asterisk for
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multiplication and
a forward slash for division.
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Here's a new program with all four of
the math operators we just showed you.
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Here, we add the numbers 1 and 2 together.
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Here, we subtract 4 from 12.5.
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Here, we multiply 5 by 8.
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And here, we divide 7 by 4.
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You might notice that on that last line,
the division operation, that even though
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we're working with whole numbers, we
included a decimal point in each of them.
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We'll explain why we did
that in a little bit.
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Let's try running this.
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Dotnet run.
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And you can see we get
the results we'd expect.
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1 plus 2 is 3.
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12.5 minus 4 is 8.5.
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5 times 8 is 40.
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And 7 divided by 4 is 1.75.
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Variables can be used in any
part of the math operation.
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So here, we take the integer 2 and
assign it to a variable named number.
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Oops, forgot the var keyword.
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So let me add that real quick.
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And let me make sure there is a semicolon
there at the end of the line.
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And now, let's try printing the results
of taking number and adding 3 to it.
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And let's also print
the results of taking 4 and
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multiplying it by the value in number.
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Save this.
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And try running it.
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And again,
the results aren't too surprising.
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Number contains 2, so
the result of adding 3 to 2 is 5.
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And the results of
multiplying 4 by 2 is 8.
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Using a variable in a math
operation leaves the value in that
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variable unchanged.
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So if we were to try to print
the value in number after doing theses
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various operations, and
then try running it,
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You can see that even though we add 3 to
number and then multiply 4 by that number,
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the value that number contains
after all that is still 2.
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If you wanna actually change the value on
a variable, you'll need to do your math
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operation on the variable, and then assign
the result back to that same variable.
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So we start by assigning the initial
value 2 to the variable number.
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And here,
we add 1 to the value in number and
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then assign it back to that same variable.
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Let's try printing that out.
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Let me save that and
try running it real quick.
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And you can see that we take
the value in number, which is 2,
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we add 1 to it, and
assign it back to number.
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And down here, when we print out
the value in number, we get 3.
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Let's try adding 1 to
the value in number again.
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And then print that updated value again.
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Number starts at 2, we update it to 3, and
then we update it again, and we get 4.
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Let's try a few more operations.
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I'll subtract 1 from number, and
then update the value it holds.
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Print the updated value.
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I'll multiply the number by 2,
and update the value it holds.
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And then print it again.
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And let's try running this.
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And our starting value is 2,
we add 1 to get 3.
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We add 1 again to get 4.
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We subtract 1 and wind up with 3 again.
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And then, we multiply the result by 2,
and now number contains the value 6.
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By the way,
all the math operations we've shown you so
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far work just like this in just about
every programming language out there.
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So you'll be able to apply what
you've seen in almost any programming
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language you want.
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The next C# feature we're going to show
you is something not every language has.
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Abbreviated assignment operators let
you take the value in a variable and
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add to it, subtract from it,
multiply it, or
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divide it, then reassign the result
back to the same variable.
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So we can rewrite the previous
statements like this.
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number equals number plus 1 can be
written as number plus equals 1.
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number equals number minus 1 can be
rewritten as number minus equals 1.
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And number equals number times 2 can
be rewritten as number times equals 2.
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There's also an abbreviated
assignment operation for division,
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which is written as slash equals.
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Let's trying running this.
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And we get the same results as before,
but with much shorter code.
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Now, let's go back and
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look at this line from when we
introduced the division operator.
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Notice that even though we're
working with whole numbers,
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we included a decimal
point in each of them.
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So 7.0 divided by 4.0
gives us a result of 1.75.
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What would happen if we
remove the decimal points?
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We get a whole number result of 1.
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There's a big difference between 1 and
1.75.
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If a supermarket cashier
owed you a $1.75 in change,
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you probably wouldn't be too
happy if they just handed you $1.
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So let's add those decimal points back in.
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If we rerun the program,
we'll see we have a result of 1.75 again.
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It also works fine if only the first
number has a decimal point.
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And it will work fine if only
the last number has a decimal point.
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So why the difference?
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When you include a decimal point in
a number, C# treats its type as double,
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a double-precision, floating-point number.
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When you leave the decimal point out,
C# treats its type as int and integer.
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When doing division, as long as
either the dividend or the divisor is
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a floating point number, the result
will be a floating point number.
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So if the point on either side of
the division operator is a floating point
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number, you'll be okay.
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But when doing division with integers,
the result is always an integer.
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C# turns the result into an integer by
throwing away the fractional portion.
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So even though the result of 7
divided by 4 would normally be 1.75,
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C# turns the result into an integer
by discarding the 0.75, leaving 1.
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This isn't just C# that does this.
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Many other programming
languages do the same.
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The whole reason integer data types exist
is to save space in computer memory.
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When you have a floating point number,
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you need to store every one of those
digits following the decimal point.
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If you have an integer, you can just
get rid of all those decimal places.
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It requires a lot less memory to store.
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There are some situations where
using an integer is appropriate.
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If you're counting people, or cars, or
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houses, you can safely assume you'll
only be working with whole numbers.
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In cases like that, you can save a lot
of memory by using an integer data type.
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When you use an integer
type in a computer program,
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you're basically saying to the compiler
that saving memory is more important to
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you than keeping track
of fractional numbers.
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So if you use integers in
a division operation and
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the result isn't a whole number,
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your program will simply throw those
pesky memory hogging decimal places away.
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But of course,
if you're even doing a division operation,
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you're probably not working
just with whole numbers.
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You might be counting dollars,
or pies, or hours, or
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something else where the fractional
portions matter a whole lot.
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Which is why it's important
to follow this rule.
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Unless you're certain you
know what you're doing,
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never ever use integer values
in a division operation.
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Even if they're whole numbers,
add a decimal point and
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a 0 so they'll be treated
as floating point numbers.
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That way, the result will also
be a floating point number.
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You won't lose those
important decimal places, and
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your users will be much happier.