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VIPUL SHEKHAWAT: Hi.

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In this video I'll be introducing you to one of most

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fundamental concepts in logic and

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programming, the Boolean value.

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If you're curious about the name, Boolean values and

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conditions are named after George Boole, a 19th century

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mathematician who pioneered what is now called Boolean

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logic, which is based by grouping and

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comparing Boolean values.

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>> So what is a Boolean value?

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A Boolean value is a variable that just has two possible

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conditions, true and false.

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You can think of it as a light switch.

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It can be either on or off, true or false.

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Similarly, binary numbers can be either one or zero, which

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is analogous to the same thing, true or false.

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Simple, right?

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The concept of a Boolean variable is easy to

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understand, but the ways in which you can manipulate and

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combine them allows for much of greater complexity.

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In addition to the two fundamental Boolean values,

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there are many Boolean operators that can combine two

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Boolean values into a single one.

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>> Two of the most basic, but most important

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operators, are AND and OR.

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The AND operator results in a value of true only if both of

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the values it takes are true, so false AND true is false.

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Likewise, false AND false is false.

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Only true AND true equals true.

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The OR operator results in the value of true if either of the

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values it takes are true.

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So false OR false is false, but true OR false is true, and

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true OR true is also true.

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The NOT operator simply takes a Boolean variable and gives

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you the opposite of it.

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So true becomes false and false becomes true.

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If you put the whole thing together, variables with

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operators, you create a Boolean expression.

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>> Now let's look at an example of nesting these Boolean

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expressions.

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Do you remember the order of operations?

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As with numbers, Boolean expressions can be grouped by

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using parentheses.

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So there are basically three expressions here, NOT z, y OR

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NOT z, and x AND y OR NOT z.

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We can figure out the values of these by looking at the

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inside and working our way out.

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So let's suppose x is true, y is true, and z is also true.

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What would NOT z evaluate to?

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Since we start with true, NOT z would simply be false.

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So now we have false OR y.

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If you look on top, you can see that y is true, y OR false

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would still just be true.

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Lastly we, have x AND true.

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So what's x AND true?

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x is true and true is also true, so this whole thing

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evaluates to true.

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>> Next, let's look at how these Boolean expressions can

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actually be used in a programming language.

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In C, the syntax for Boolean operations is a bit different

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from the words and, or, and not.

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Let's cover the syntax.

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To use the AND operator, we write a double ampersand.

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The OR operator is a double pipe line character.

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This is the straight vertical line, which you can probably

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find above the Enter or Return keys on your keyboard.

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And the NOT operator is simply an exclamation mark.

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So to rewrite the expression we had before, we would just

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write this x && y || !z.

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That's just taking exactly what we had before and turning

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it into C syntax.

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>> Now that we've translated our Boolean expression into code,

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how do we actually use it?

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Let's say we have some code that should only execute if a

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certain expression is true.

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For this purpose, pretty much all programming languages

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support the if condition.

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Let's say we have a Boolean variable, x, and we want some

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code to execute only if x is true.

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We would simply write the word if, put parentheses, and put

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the Boolean expression within those parentheses.

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After that, we wrap the code we want to

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execute in curly braces.

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What if there's some code you'd like to

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execute if x is not true?

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Simply write the word else after the if statement, wrap

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the other code in curly braces, and then that code will

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execute if x is not true.

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>> Another useful language construct is else if.

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Suppose there are two Booleans you would like to consider,

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let's call them x and y.

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We declare these variables to be true and false.

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If x and y are true, you execute the first block of

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code within those curly braces.

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Else if x or y are true, you execute the next block of

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code, and else you execute the last block of code.

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Working with Boolean values like this is useful, but

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you're really only limited to a few conditions.

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Booleans can become much more powerful when you introduce

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comparisons.

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These are ways to compare values that are

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not originally Boolean.

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To see if two values are the same, you can use equals

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equals, which is true if they're equal and false if

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they are not.

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Other common comparisons are less than, greater than, less

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than or equal to, and greater than or equal to.

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>> Everything I've covered so far has been pretty abstract, so

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let's introduce these comparisons in one last

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concrete example.

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Suppose there are two variables,

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temperature and isHungry.

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Temperature is a floating point number, so it can have

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decimal places.

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You're programming a very simple application which tells

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someone what to eat depending on the temperature.

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If you're hungry, AND AND the temperature is greater than or

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equal to 100, you can print eat ice cream.

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Else if you're hungry AND AND the temperature is less than

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or equal to zero, you can printf("eat spicy food").

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Lastly, if you're not hungry at all, you can print "don't

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eat anything."

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>> I am Vipul Shekhawat, and this is CS50.