CS50x 01. Scratch

This is the summary of CS50x course.

Computer Science and Problem Solving

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• computer programming is solving a problem
• input -> black box -> output
• We could use a system called unary (also called base-1) to count one finger at a time.
• Computers today count using a system called binary from the term binary digit that we get a familiar term called bit (zero or one: on or off).
• Computers use base-2 to count. This can be pictured as follows:

  2^2  2^1  2^0
  4    2    1
  

• Computers generally use eight bits (also known as a byte) to represent a number. For example, 00000101 is the number 5 in binary. 11111111 represents the number 255. (128/64/32/16/8/4/2/1)

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ASCII

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• American Standard Code for Information Interchange
• Letters are represented using ones and zeros, just as numbers.
• The ASCII standard was created to map specific letters to specific numbers.

  H  I  !
  72 73 33
  

• More about ASCII
• Since binary can only count up to 255 we are limited to the number of characters represented by ASCII

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Unicode

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• The Unicode standard expanded the number of bits that can be transmitted and understood by computers. Unicode includes not only special characters, but emoji as well. 🤣😒
• While the pattern of zeros and one is standardized within Unicode, each device manufacturer may display each emoji slightly differently than another manufacturer.
• More and more features are being added to the Unicode standard to represent further characters and emoji.
• More about Unicode, emoji.

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RGB

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• Red, Green and Blue (called RGB) is a combination of three numbers.
• Yellow = R:72, G:73, B:33
• Three three bytes required to represent various colors of red, blue, and green make up each pixel (or dot) of color in any digital image.
• Images are simply combinations of RGB values.

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Algorithms

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• An algorithm is a step-by-step set of instructions to solve a problem.
• Programmers translate text-based, human instructions into code.

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Pesudocode

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• This process of converting instructions into code is called pseudocode.
• Pseudocode is a human-readable version of your code.
• Pseudocode is such an important skill for at least two reasons.
  • When you pseudocode before you create formal code, it allows you to think through the logic of your problem in advance.
  • When you pseudocode, you can later provide this information to others that are seeking to understand your coding decisions and how your code works.

    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
    

• Unique features
  • Some of these lines begin with verbs like pick up, open, look at. Later, we will call these functions.
  • Some lines include statements like if or else if. These are called conditionals.
  • How there are expressions that can be stated as ture or false. We call these boolean expressions.
  • How there are statements like “go back to line 3.” We call these loops.

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Artificial Intelligence

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• Look at the following pseudocode:

  If student says hello
    Say hello
  Else if student says goodbye
    Say goodbye 
  Else if student asks how you are
    Say well
  Else if student asks why 111 in binary is 7 in decimal
  ...
  

• Rather than programming conversational AI like the above, AI programmers train large language models (LLMs) on large datasets.