1. 컴퓨팅사고

Elenaljh·2023년 6월 20일

CS50

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Lecture 0

Welcome!

This class is about more than computer programming!
Indeed, this class is about problem-solving in a way that is exceedingly empowering! You will likely take the problem solving that you learn here will likely be instantly applicable to your work beyond this course and even your career as a whole!
However, it will not be easy! You will be “drinking from the firehose” of knowledge during this course. You’ll be amazed at what you will be able to accomplish in the coming weeks.
This course is far more about you advancing “you” from “where you are today” than hitting some imagined standard.
The most important opening consideration in this course: Give the time you need to learn through this course. Everyone learns differently. If something does not work out well at the start, know that with time you will grow and grow in your skill.

What’s Ahead

You will be learning this week about Scratch, a visual programming language.

Then, in future weeks, you will learn about C. That will look something like this:

  #include <stdio.h>

  int main(void)
  {
      printf("hello, world\n");
  }

Further, as the weeks progress, you will learn about algorithms.
You will learn about memory.
You will learn about buggy code and what causes computer crashes.
You will learn about data structures such as a hash table.
Then, we will transition to a new, higher-level language called Python. Your code will look something like this:
```
  print("hello, world")
```
This class will give you a strong understanding of how recent programming languages developed from the earlier ones.
We will also be looking at how we can use databases and third-party frameworks to build web applications.

Community!

You are part of a community of those taking this course at Harvard College, Harvard Extension School, and via edX.org.
Puzzle Day and the CS50 Fair

Computational Thinking

Essentially, computer programming is about taking some input and creating some output - thus solving a problem. What happens in between the input and output, what we could call a black box, is the focus of this course.
For example, we may need to take attendance for a class. We could use a system called unary to count, one finger at a time. Computers today count using a system called binary. It’s from the term binary digit that we get a familiar term called bit. A bit is a zero or one.
Computers only speak in terms of zeros and ones. Zeros represent off. Ones represent on. Computers are millions, and perhaps billions, of transistors that are being turned on and off.
If you imagine using a light bulb, a single bulb can only count from zero to one.
However, if you were to have three light bulbs, there are more options open to you!
Using three light bulbs, the following could represent zero:
```
  0 0 0
```
Similarly, the following would represent one:
```
  0 0 1
```
By this logic, we could propose that the following equals two:
```
  0 1 0
```
Extending this logic further, the following represents three:
```
  0 1 1
```
Four would appear as:
```
  1 0 0
```
We could, in fact, using only three light bulbs count as high as seven!
```
  1 1 1
```
As a heuristic, we could imagine that the following values represent each possible place in our binary digit:
```
  4 2 1
```
Computers use ‘base-2’ to count. This can be pictured as follows:
```
  2^2  2^1  2^0
  4    2    1
```
Therefore, you could say that it would require three bits (the four’s place, the two’s place, and the one’s place) to represent a number as high as seven.
Computers generally use eight bits to represent a number. For example, 00000101 is the number 5 in binary.

Text

Just as numbers are binary patterns of ones and zeros, letters are represented using ones and zeros too!
Since there is an overlap between the ones and zeros that represent numbers and letters, the ASCII standard was created to map specific letters to specific numbers.
For example, the letter A was decided to map to the number 65.
If you received a text message, the binary under that message might represent the numbers 72, 73, and 33. Mapping these out to ASCII, your message would look as follows:
```
  H   I   !
  72  73  33
```
Thank goodness for standards like ASCII that allow us to agree upon these values!
Here is an expanded map of ASCII values:
If you wish, you can learn more about ASCII.

Emojis

As time has rolled on, there are more and more ways to communicate via text.
Since there were not enough digits in binary to represent all the various characters that could be represented by humans, the Unicode standard expanded the number of bits that can be transmitted and understood by computers.
There are emojis that you probably use every day. The following may look familiar to you:
Computer scientists faced a challenge when wanting to assign various skin tones to each emoji to allow the communication to be further personalized. In this case, the creators and contributors of emojis decided that the initial bits would be the structure of the emoji itself, followed by skin tone.
More and more features are being added to the Unicode standard to represent further characters and emojis.
If you wish, you can learn more about Unicode.
If you wish, you can learn more about emojis.

RGB

Red, green, and blue (called RGB) is a combination of three numbers.
Taking our previously used 72, 73, and 33, which said HI! via text, would be interpreted by image readers as a light shade of yellow. The red value would be 72, the green value would be 73, and the blue would be 33.

Images, Video and Sound

Images are simply collections of RGB values.
Videos are sequences of many images that are stored together, just like a flipbook.
Music can be represented through MIDI data.

Algorithms

Problem-solving is central to computer science and computer programming.
Imagine the basic problem of trying to locate a single name in a phone book.
How might you go about this?
One approach could be to simply read from page one to the next to the next until reaching the last page.
Another approach could be to search two pages at a time.
A final and perhaps better approach could be to go to the middle of the phone book and ask, “Is the name I am looking for to the left or to the right?” Then, repeat this process, cutting the problem in half and half and half.
Each of these approaches could be called algorithms. The speed of each of these algorithms can be pictured as follows in what is called big-O notation:

Notice that the first algorithm, highlighted in red, has a big-O of n because if there are 100 names in the phone book, it could take up to 100 tries to find the correct name. The second algorithm, where two pages were searched at a time, has a big-O of ‘n/2’ because we searched twice as fast through the pages. The final algorithm has a big-O of log2n as doubling the problem would only result in one more step to solve the problem.

Pseudocode and the Basic Building Blocks of Programming

The ability to create pseudocode is central to one’s success in both this class and in computer programming.

Pseudocode is a human-readable version of your code. For example, considering the third algorithm above, we could compose pseudocode as follows:

  1  Pick up phone book
  2  Open to middle of phone book
  3  Look at page
  4  If person is on page
  5      Call person
  6  Else if person is earlier in book
  7      Open to middle of left half of book
  8      Go back to line 3
  9  Else if person is later in book
  10     Open to middle of right half of book
  11     Go back to line 3
  12 Else
  13     Quit

Pseudocoding is such an important skill for at least two reasons. First, when you pseudocode before you create formal code, it allows you to think through the logic of your problem in advance. Second, when you pseudocode, you can later provide this information to others that are seeking to understand your coding decisions and how your code works.
Notice that the language within our pseudocode has some unique features. First, some of these lines begin with verbs like pick up, open, look at. Later, we will call these functions.
Second, notice that some lines include statements like if or else if. These are called conditionals.
Third, notice how there are expressions that can be stated as true or false, such as “person is earlier in the book.” We call these boolean expressions.
Finally, notice how these statements like “go back to line 3.” We call these loops.
In the context of Scratch, which is discussed below, we will use each of the above basic building blocks of programming.

Scratch

Scratch is a visual programming language developed by MIT.
Scratch utilizes the same essential coding building blocks that we covered earlier in this lecture.
Scatch is a great way to get into computer programming because it allows you to play with these building blocks in a visual manner, not having to be concerned about the syntax of curly braces, semicolons, parentheses, and the like.
Scatch IDE (integrated development environment) looks like the following:

Notice that on the left, there are building blocks that you can use in your programming. To the immediate right of the building blocks, there is the area to which you can drag blocks to build a program. To the right of that, you see the stage where a cat stands. The stage is where your programming comes to life.

Scratch operates on a coordinate system as follows:

![scratch coordinate system](https://cs50.harvard.edu/x/2023/notes/0/cs50Week0Slide167.png "scratch coordinate system")  Notice that the center of the stage is at coordinate (0,0). Right now, the cat’s position is at that same position.

수업시간에 Sesame Street Game, Ivy's Hardest Game을 스크래치로 만들었음. 궁금하면 위에 Scratch 제목 클릭해서 강의 노트 홈피로 ㄱㄱ

Summing Up

In this lesson, you learned how this course sits in the wide world of computer science and programming. You learned…

Few students come to this class with prior coding experience!
You are not alone! You are part of a community.
Problem solving is the essence of the work of computer scientists.
This course is not simply about programming – this course will introduce you to a new way of learning that you can apply to almost every area of life.
How numbers, text, images, music, and video are understood by computers.
The fundamental programming skill of pseudocoding.
How abstraction will play a role in your future work in this course.
The basic building blocks of programming, including functions, conditionals, loops, and variables.
How to build a project in Scratch.

See you next time!

Elenaljh

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