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來自 University of Toronto 的課程

学习编程：基础

2164 個評分

University of Toronto

学习编程：基础

2164 個評分

Behind every mouse click and touch-screen tap, there is a computer program that makes things happen. This course introduces the fundamental building blocks of programming and teaches you how to write fun and useful programs using the Python language.

從本節課中

Booleans, Import, Namespaces, and if Statements

This module introduces Booleans (logical values True and False), how to convert between types, how to use Boolean expressions in if statements to selectively run code, and the concept of a Python module.

If Statements9:11

No if Required4:18

Structuring if Statements6:08

與講師見面

Jennifer Campbell
Associate Professor, Teaching Stream
Department of Computer Science
Paul Gries
Associate Professor, Teaching Stream
Department of Computer Science

You've seen Boolean's functions end of statements.

We're going to investigate a style issue now that can happen when you combine all

three. We're going to start by writing a function

that returns true if the parameter is even, and false otherwise.

As an example, if a call is even with value four and hoping that I get back

true. And if a call is even 77, then hopefully

I'll get back false. We can tell that we're sending it back an

int and that we hope to get back a Bool. Our header is we're going to call the

function as even and let's call the parameter num because that's a number.

And, What we're going to do here is return

other numb is even. Now, that I know the, the details of my

function, it's time to figure out some code that will let me know whether a

number is even or odd. I'm going to investigate this in the

Python shell. And, notice that if I divide four by two, there is no remainder. But,

if I divide three or five or seven by two, there is a remainder.

So, we're going to try out mod for mod two is zero,

77 mod two is one. And so, any number I pick that is odd is going to have a

remainder of one, when I divide it by two. Continuing this thought, four mod two =

zero is true. But, something like 77 mod two = zero is

false. So, this expression, the number mod two

equals zero is going to give me the result that I want.

We might decide to write something like, if num mod two is zero, then return true

otherwise return false. This will certainly work.

Let's explore that. I will run this so we can have it there.

Now, I can access the is even int function and say, hey, there's even four?

Yes, it is. How about three?

No, that's false. So our function works.

However, The function body here is three lines too

long. I don't actually need to use an if

statement. The issue here is that, num mod two equals

zero is already a Boolean expression. As we saw over here, four mod two = zero

produced the value true. I don't need to say if something is true,

then return true, Otherwise, if it wasn't true, return

false. I can actually come into this out and

replace it with a single line. I want to return a Boolean.

Num mod two equals zero produces a Boolean, so I can just do return num mod

two equals zero. Let's try this new version out in the

shell. I still get true, I still get false.

So, my new version works just as well as the old one.

Its shorter, and after a while, you are going to find it preferable to the longer

version of the if and the else. Working with Boolean expressions is new to

many of you, and so this may make you a little uncomfortable, it might seem a

little fancy or even confusing. Play with it for awhile, see what you can

come up with in the shell when you need a Boolean expression, and see if you can

simplify some of your return statements. If you ever find yourself doing if

something where to return true, otherwise, return false.

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