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>> [MUSIC PLAYING]

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>> DOUG LLOYD: Hi, so let's
talk about operators in C.

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So, we've already seen one, in fact,
equals the assignment operator.

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It allows us to just put
a value into a variable.

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That's the assignment
operator, single equal sign.

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In order to manipulate and work
with values and variables in C,

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we have a number of operators
at our disposal that we can use.

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Let's take a look at
some of the common ones

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starting with arithmetic operators.

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As you might expect, we can do
pretty basic math operations in C.

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We can add, subtract, multiply, and
divide numbers using plus, minus, star,

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and slash, respectively.

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Here's a couple of lines of
code in which we do that.

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So, we have int x equals y plus 1.

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Let's assume that somewhere
up above this line of code

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we had said int y equals 10.

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What's the value of x after I
execute this first line of code?

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Did you say 11?

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You'd right.

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Why is that?

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Well, y was 10.

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Some I'm saying int x equals 10 plus 1.

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10 plus 1 is 11.

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So, the value 11 gets
stored in the variable x.

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Not too bad, right?

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>> How about this next line of
code? x equals x times 5.

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Well, before we executed
this line of code, x was 11.

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So, what is the value of
x after this line of code?

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Take a second.

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So, x equals x times 5.

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x was 11.

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So, x equals 11 times 5.

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Or 55.

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So if you said 55, you'd be right.

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>> Now, it may be a little confusing, but
with the way that assignment works in C

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is the value on the right gets
assigned to the value on the left.

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So, first we evaluate x times 5.

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So, 11 times 5 is 55.

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And then we store that value in x.

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The 11 that was there
before is now overwritten.

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So x's value is now 55.

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Hopefully that's fairly straightforward.

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>> There's another operator that you've
probably not necessarily heard

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called this, but you've
certainly worked with in the past

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if you remember your days of long
division way back in grade school.

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It's called the modulus operator.

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What modulus does is it
gives you the remainder

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when you divide two numbers together.

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So, if I say 13 divided by
4, what's the remainder?

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And that value would be calculated
by the modulus operator.

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>> So, I have a line of code
here, int m equals 13 mod 4.

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And I say here in a comment
that m's value is now 1.

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Why do I say that?

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Well, do the long division out in your
head, if you bear with me for a second.

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So, I have 4 divided by 13.

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4 goes into 13 three times
with a remainder of 1.

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So, basically, all the
modulus operator does

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is it tells you when you
divide, you get the remainder.

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You might think that's actually
not a terribly useful thing,

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but you'd be surprised, actually,
by how frequently that modulus

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operator can come in handy.

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>> There's a couple of problems
we'll do CS50 that deal with it.

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It's also good for doing
things like random number.

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So, for example if you've ever
heard of a random number generator,

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that's going to give you a number
from 0 to some huge number.

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But maybe you only really
need a number from 0 to 20.

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If you use the modulus operator
on that giant number that

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gets generated by the
random number generator,

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you're going to take whatever
huge value it is, divide it by 20,

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and get the remainder.

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The remainder can only
be a value from 0 to 19.

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So, you use modulus operator
to take this huge number

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and whittle it down into something
a little more meaningful.

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I'm pretty sure you'll be
able to use both of those

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at some point in the future in CS50.

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>> So, C also gives us a way
to apply an arithmetic

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operator to a single variable
in a little more shorthand way.

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So, in the previous slide,
we saw x equals x times 5.

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That worked.

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x times 5 then gets stored back in x.

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There's a shorter way to do it, thought,
and it's the syntax x times equals 5.

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It's the same exact thing as
saying x equals x times 5.

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It's just a slightly
shorter way to do it.

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And when you see some
distribution code or you

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see some sample code that
does things like this,

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just be familiar with
what the syntax means.

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You certainly don't have
to use it, but if you do,

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it might make your code
look a little slicker.

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And know that you can also use any of
the different operators we've already

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seen before instead of times.

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You could say x plus equals 5, minus
equals 5, times, divide, and mod.

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All of those work.

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>> There's also something
that's so common in C

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that we've decided to
refine that even further.

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Incrementing a variable by 1
or decrementing a variable by 1

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is such a common thing-- especially
when we talk about loops a little later

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on-- that we've decided instead of
saying something like x plus equals 1,

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or x equals x plus 1, we've even
short handed that to x plus plus.

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So, x equals x plus 1, x plus equals 1,
and x plus plus all do the same thing.

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They all increment x by 1.

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But that incrementing
and decrementing by 1

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is so common that we have
plus plus and minus minus

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that allow us to shorthand
that even further.

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>> So, let's switch gears for second
and talk about Boolean expressions.

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All which are also kind of fall into
the overall category of operators.

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But Boolean expressions,
unlike arithmetic operators,

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are used for comparing values.

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So, again, all Boolean expressions in C
evaluate to one of two possible values,

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recall.

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True or false.

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That's the only two values that
Boolean variable can take on.

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We can use the results
of a Boolean expression

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in a lot of ways in programming.

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In fact, you'll be
doing this quite a lot.

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>> For example, we might decide,
well, if some condition is true,

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maybe I'll take this
branch down my code.

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A conditional, so to speak.

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We'll learn about those soon too.

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Or maybe, as long as
this is true, I want

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to keep doing this
over and over and over.

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A loop.

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In both cases, know that we're using
a Boolean expression, a true or false,

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to decide whether or not
to take a particular path.

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Sometimes when we're working
with Boolean expressions,

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we will use variables of type Bool.

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You might have declared
a Bool typed variable,

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and you'll use in your
Boolean expression.

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But you don't always have to do.

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As it turns out, in C, every non-0
value is the same as saying true.

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If you had declared a
variable of type Boolean,

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and assigned it the value true, that's
the same as declaring an integer

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and assigning it the value
1, 2, 3, or really any value

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whatsoever other than 0.

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Because in C, every non-0 value is true.

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0, on the other hand, is false.

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This might come in
handy later on to know,

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but just something to keep in mind.

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We don't always have to use
Boolean type variables when we

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are working with Boolean expressions.

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>> There are two main types of Boolean
expressions that we'll work with.

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Logical operators and
relational operators.

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The language there is
not terribly important.

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It's really just how I'm grouping them.

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And you'll certainly, I think, quickly
realize what a relational operator is,

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based on what they are when we
talk about them in a second.

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But don't worry about necessarily
memorizing the term logical operator

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or relational operator.

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I'm just using it to group
them in a logical way.

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>> So, let's take a look at
the three logical operators

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that we'll see quite a
bit in programming in CS50

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and in programming more generally.

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Logical AND is true, if and
only if both operands are true.

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Otherwise false.

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Where does that mean?

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So, let's say that I am at a
point in my code where I have

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two variables, x and y.

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And I want to decide whether
to do something in my code

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based on if x is true and y is true.

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I only want to do it if
both of them are true,

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otherwise I don't want to go down that
path because it's not going to help me.

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What I can say is if x & & y.

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That will be a logical Boolean
expression comparing x and y

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and taking a certain path
based on what their values are.

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So, if x is true and y is true
based on this truth table here,

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only then will we go down that path.

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If x, & & y.

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It's only true-- the and is only
true if x is true and y is true.

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If either one is false,
as we see the truth table,

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then both x and y are not true.

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And so, x & & y is false.

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>> Logical OR is true if and only
if at least one operand is true.

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Otherwise false.

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So logical AND required
both x and y to be true.

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Logical OR requires x to be true or y
to be true or both x and y to be true.

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So, again, we kind of find
ourselves in a situation

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where we're going to our code,
and we reached a fork in the road.

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And we want to go down a
particular path if x is true

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or y is true, but not
necessarily if both are true.

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But possibly if both are true.

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So if x is true and y is
true, we'll go down that path.

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x is true.

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One of them is true, right?

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If x is true and y is true.

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If x is true, and y is false,
one of them is still true.

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So, x or y is still true.

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If x is false, and y is true,
one of them is still true, right?

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y is true, in this case.

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So, it's true that x or y is true.

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Only if x is false and y is false
do we not go down that path,

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because neither x nor y is true.

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Now, if you're looking
at the screen right now

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and wondering what that
symbol is for logical OR,

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it's called the vertical bar.

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And if you looking at your keyboard
for a minute, as I'm doing now,

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it's usually just above the
Enter key, on most keyboards,

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on the same key as the backslash.

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It's also usually right
next to the square brackets.

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So, it might be a key that you
haven't typed very much in the past.

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But, if you're ever doing
logical comparisons,

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as we'll be doing a
lot in the course, it's

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going to be useful to
find that key and use it.

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So, it's usually on the same key
as backslash just above Enter.

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>> The final logical operator is NOT.

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And NOT's pretty straightforward.

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It inverts the value of its operand.

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If x is true, then not x is false.

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If x is false, then not x is true.

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Sometimes you'll hear this symbol
pronounced as bang or exclamation

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or not.

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It's pretty much all the same thing.

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In case you hear that spoken and
you're not sure what that means,

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it's just the exclamation
point, but sometimes it's

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called a couple different things.

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All right, so that takes
care of logical operators.

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So, let's talk about
relational operators.

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Again, if you're familiar with this
arithmetic back in grade school,

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you're probably familiar
with how these work already.

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These behave exactly as you'd expect.

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So less than it's true, in this
example, if x is less than y.

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So, if x is 4 and y is
6, x is less than y.

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That's true.

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Less than or equal to
works pretty similarly.

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If x is 4, and y is 4, then
x is less than or equal to y.

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Greater than. x is greater than y.

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And greater than or equal to, x
is greater than or equal to y.

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If it's true, then you'll
pass that expression,

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and you'll go down
that path on the road.

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If you have an if x is greater than y,
and x is, in fact, is greater than y,

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you'll do whatever is
subject to that condition.

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>> Notice that we don't have a
single character for less than

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or equal to, as you might be
familiar with from math textbooks.

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So, we have the less than symbol,
followed by an equal sign.

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That's how we represent
less than or equal to.

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And similarly, do we do that
for greater than or equal to.

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>> The final two relational
operators that are important

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are testing for equality and inequality.

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So, if x equals equals y, is true
if x and y's value is the same.

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If x is 10, and y is 10, then
x equals equals y is true.

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If x is 10 and y is 11, x
equals equals y is not true.

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We can also test for inequality using
exclamation point or bang or NOT,

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again.

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If x is not equal to y, if
that's the test we're using here,

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we'd be good to go.

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So, if x is not equal to
y, we'll go down that path.

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>> Be really careful here.

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It's a really common mistake-- and
one I certainly made quite a lot when

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I was getting started--
to accidentally mistake

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the assignment operator, single equals,
for the equality comparison operator,

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double equals.

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It'll lead to some weird
behavior in your code,

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and usually the compiler will
warn you about it when you try

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and compile your code, but sometimes
you might be able to sneak it by.

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It's not necessarily a good thing
that you sneak it by, though.

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Just so if you are doing
an inequality test,

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if you're checking whether two
different variables have the same value

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inside of them, make sure to use
equals equals, and not single equals.

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And that way your program will
have the behavior you intend.

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I'm Doug Lloyd and this is CS50.

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