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Duke University의 강좌에서

Programming Foundations with JavaScript, HTML and CSS

5027개의 평가

Duke University

Programming Foundations with JavaScript, HTML and CSS

5027개의 평가

Specialization Java Programming and Software Engineering Fundamentals에서 5의 강좌 1

Learn foundational programming concepts (e.g., functions, for loops, conditional statements) and how to solve problems like a programmer. In addition, learn basic web development as you build web pages using HTML, CSS, JavaScript. By the end of the course, will create a web page where others can upload their images and apply image filters that you create. After completing this course, you will be able to: 1. Think critically about how to solve a problem using programming; 2. Write JavaScript programs using functions, for loops, and conditional statements; 3. Use HTML to construct a web page with paragraphs, divs, images, links, and lists; 4. Add styles to a web page with CSS IDs and classes; and 5. Make a web page interactive with JavaScript commands like alert, onClick, onChange, adding input features like an image canvas, button, and slider.

수업에서

Algorithms and Programming Concepts

DukeLearnToProgram Environment10:17

Conditional Execution7:12

강사 만나기

Susan H. Rodger
Professor of the Practice
Computer Science
Robert Duvall
Lecturer
Computer Science
Owen Astrachan
Professor of the Practice
Computer Science
Andrew D. Hilton
Associate Professor of the Practice
Electrical and Computer Engineering

Welcome back.

Now you've seen a better code that works with images and

numbers, two different types of data.

But what if we do something that does not make sense?

Like try to do w.getWidth?

w is the number for 80 which doesn't have a getWidth method

In this particular case, the code will crash when execution reaches this line.

Why is this call to get width fine, but this one crashes the program?

If we go back to our diagram that depicts the state of the program,

you will see that we were working with two different types of data.

fgImage refers to a simple image.

Which has a getWidth method.

But w is a number, a type of data that does not have a getWidth method.

So what exactly is a type?

Well, remember that everything is a number.

But we have lots of different things we want those numbers to represent.

Letters, images, sounds, and just plain numbers.

So the type tells us what those numbers mean.

That is, what kind of thing the number represents.

This is important because the type specifies not only how

the program should interpret the number, that is what the numeric value means.

But also how to operate on it.

The same operation may have different behavior on different types.

This may sound confusing so let's look at an example.

Here we have some code which has two plus operations in it.

As you are about to see, these two pluses will do different things since

they will operate on different types of data.

Let's step through the code.

n1 = 26, n2 = 16, s1 = "a".

Here, s1 is a string, a sequence of letters.

This particular sequence has just one letter, A.

s2 = "b".

Now, we're about to do the first plus operation.

Here we are doing N1 which is 26 plus N2 which is 16.

Both operands are numbers, so we are just doing numeric addition.

26 plus 16 is 42.

Now we are about to do the second plus operation.

However, here we are doing plus on two strings, a and b.

What does it mean to do plus on strings.

For strings, plus doesn't mean numeric addition it means concatenation.

Concatenation is a fancy word for taking two strings and

sticking them together one after the other.

So you get s3 equals the string ab.

Notice how plus meant two different things based on the type of the data it

was operating on.

So how does JavaScript keep track of the type of the data.

It keeps information about the type,

explicitly along with each value when it stores it in the computer's memory.

Then whenever it needs to work with a value, it reads the type and

value out of the memory to figure out what to do.

Because JavaScript keeps the types of all values in memory,

meaning that it tracks and works with the types while it runs the program,

It's called a dynamically typed language.

If you stay for course two and

beyond in which you will learn Java types will work differently.

Java is a statically typed language which means that ever variable and

every expression has exactly one type.

Which must be known before the program can even be run.

For this to work in Java,

you, the programmer, write down the types of your variables explicitly in your code.

Then your code can be checked for certain kinds of mistakes,

like trying to call getWidth on a number before it is even run,

having this kind o mistake found before the program runs is really helpful.

You'll learn more about types and Java in course 2.

For now however, we have a bit more JavaScript to do so

that you can implement the green screen algorithm.

Thank you.

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