Visualizing Assignment Statements

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来自 University of Toronto 的课程

学习编程：基础

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University of Toronto

学习编程：基础

2435 个评分

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.

从本节课中

Python, Variables, and Functions

This module gives an overview of the course, the editor we will use to write programs, and an introduction to fundamental concepts in Python including variables, mathematical expressions, and functions.

Visualizing Assignment Statements2:45

Built-in Functions5:00

Defining Functions6:41

与讲师见面

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

You've seen variables in assignment statements.

What we're going to do now is explore a tool called the visualizer, that

will allow you to see the state of computer memory, as a program executes.

In the text box are three assignment statements.

The first assignment statement, assigns the value one to x.

The second assignment statement evaluates the expression x plus 2.

And assigns to y the result.

The third statement evaluates 7, and assigns it to variable x again.

So x will change on line 3.

Let's watch that happen.

This indicates the line that is about to be executed.

As you can see, we haven't done any execution

yet, and so there are no variables created yet.

When I click forward, we're going to take the one and assign it to x.

And as

we saw, in the variables lecture, we're going

to end up with the value one inside an

object, in computer memory, and variable x is going

to contain the memory address of the, that value.

So when I hover over this box there, we see that

there's an arrow that points to this chunk of computer memory.

So now x refers to value 1.

I'm about to execute line 2.

And that is going to evaluate what x plus 2 is.

Well we know that 1 plus 2 is 3, so we should see the value 3.

Get created and stored somewhere in computer memory.

And after we execute line two we should see

a variable y containing the memory address of that value.

So I'll click Forward here and indeed we see that happen.

When I hover over y,

I see that there is a link between y and the value 3.

We also told you that the

assignment statement was quite different from equality.

We're not saying that y is always equal to x plus 2.

Remember the two steps.

Evaluate the right-hand side of the assignment statement.

That gives you back a memory address.

Take that memory address and store it in variable y.

We're about to demonstrate that here, because we're

going to assign to x the value 7.

This is going to change this box right here and not y's box.

So when I click Forward, we see that y still refers to value 3.

Even though x now refers to value 7.

So the assignment statement changes the value of the variable.

It's not stating that y is always equal to x plus

2, it's only doing it for this particular snapshot in time.

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