Code Ninjas: Introduction to Computer Science. Macomb Science Olympiad Presented by Swati Dharia Shane Storks

Transcription

1 Code Ninjas: Introduction to Computer Science Macomb Science Olympiad Presented by Swati Dharia Shane Storks

2 Goals of the Math Topics Learn basic math concepts used in computer science Introduce basic computer programming definitions and functions for elementary school level

3 Supplies to Bring to the Test Pencils 8.5 x 11 sheet of notes (both sides, handwritten and/or typed) Nonprogrammable scientific calculator

4 Math Symbols

5 Order of Operations

6 Order of Operations

7 Computer-Based Units Units will be based on the Microsoft Windows operating system (JEDEC 100B.01) Know what bits, bytes, kilobytes (KB), megabytes (MB), and gigabytes (GB) are 1 bit = one binary digit (0 or 1) 1 byte = 8 bits 1 KB = 1,024 bytes 1 MB = 1,024 KB 1 GB = 1,024 MB Know how to convert between these units (e.g., how many bytes are in 1 MB?) Answers to questions like these will be able to be found with a scientific calculator

8 Computer-Based Units

9 Collections of Numbers

10 Collections of Numbers Know the meaning of these collections of numbers, and how to recognize them: Arithmetic series A series of numbers that comes from adding the same number over and over again 1, 4, 7, 10, 13, 15, 3, 13, 23, 33, 43, 53, Geometric series A series of numbers that comes from multiplying by the same number over and over again 1, 2, 4, 8, 16, 32, 64, 2, 6, 18, 54,

11 Boolean Algebra Boolean algebra is algebra involving only 0 and 1, which we think of as false and true respectively Important for computer scientists because: It s a standard way to represent logic, which drives every computer program Almost everything a computer does boils down to 0s and 1s, or the binary number system (see next slides) We can visualize Boolean algebra expressions by drawing them as Venn diagrams Not required on main part of the test, but could show up in tie breakers The Boolean algebra expressions we ll study will always simplify down to false or true

12 Boolean Algebra If A and B are each true or false, then the basic operations are AND (, or && in code) A B means A and B A B is only true if both A and B are true OR (, or in code) A B means A or B A B is true if either A or B are true, or if both are true NOT (, or! in code) A means not A or the opposite of A A is true if A is false

13 Boolean Algebra Secondary operations are more complex operations that are made by combining several basic operations together Material implication ( ) A B is true unless A is true and B is false Think of A as whether you make a promise to a friend, B as whether you keep the promise, and A B as whether your friend will be happy If A is true, you made a promise If B is false, you broke the promise, so your friend will not be happy (A B is false) If B is true, you kept the promise, so your friend will be happy (A B is true) If A is false, you didn t make a promise If you didn t make a promise, there s no promise to keep, so your friend can t be upset with you (A B is true whether B is true or false) Exclusive OR, XOR ( ) A B is only true if either A or B is true, but not both are true Equivalence (, or == in code) A B is true if A and B have the same value

14 Boolean Algebra Make sure you know: The symbols and meanings of the three basic operations and how to translate between them and descriptions of the logic they represent The symbols and meanings of the three secondary operations, and how to translate between them and descriptions of the logic they represent There will only be one operation per expression on the main portion of the test, but more complicated expressions could show up in tie breakers Won t be expected to convert between basic and secondary operations, but remember that secondary operations are made up of basic operations

15 Boolean Algebra Example 1: I will go outside if I finish my homework, and it is sunny outside. If A represents whether I finish my homework, and B represents whether it is sunny outside, then what basic operation represents whether I will go outside? Example 2: If A represents whether I got all As on my report card, and B represents whether I got a B or lower on my report card, is A B true or false? A B? A B? Example 3: If A represents whether I have a cat, and B represents whether I have a dog, what does A B represent? (answers on next slide)

16 Boolean Algebra Answer 1: A B Answer 2: False; true; true Answer 3: Whether I have both a cat and a dog Applications and history of Boolean algebra Has some extra info we don t need to worry about, but may help understanding Basic and secondary operations explained with Venn diagrams Be aware that this article uses different symbols for some of the operations

17 Number Systems The number system we re all used to: decimal (base-10) In the decimal number system, we use 10 different digits to represent numbers: 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9 We count from 0 to 9, then jump to 10, then count from 10 to 19, then jump to 20, and so on First humans probably started to count this way because we have 10 fingers But there are other number systems that are useful in computer science Binary (base-2) Use only 0 and 1 to represent numbers Count from 0 to 1, then jump to 10, then count from 10 to 11, then jump to 100, and so on Computer hardware reads instructions and stores information in binary We count on ten fingers, but computers only count on two

18 Number Systems But there are other number systems that are useful in computer science Hexadecimal (base-16) Use 16 different digits to represent numbers: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, and F Count from 0 to F, then jump to 10, then count from 10 to 1F, then jump to 20, and so on Memory locations and colors are often represented in binary Make sure you know: Digits used by binary, decimal, and hexadecimal number systems Uses of binary, decimal, and hexadecimal number systems How to count from 0 to 10 in binary, and from 0 to 16 in hexadecimal

19 Number Systems

20 Number Systems

21 Postfix Notation

22 Postfix Notation May be expected to evaluate simple expressions that are given in postfix notations, or convert simple expressions to postfix notation Example 1: Evaluate the postfix expression 9 3 / Answer: 9 / 3 = 3 Example 2: Convert 2 * 3 to postfix notation Answer: 2 3 * More complicated expressions could appear in tie breaker questions May want to review this more detailed guide

23 Encryption/Decryption Computers encrypt private information before sending it over the Internet to other computers Use an algorithm to scramble up the private data before sending it, so a hacker who tries to steal the data along the way won t be able to understand it When the other computer receives the data, it must know how to decrypt it, or unscramble the data to be able to understand it Many different ways to encrypt and decrypt data Understand what encryption and decryption are, but don t worry about all of the ways Cipher: a secret method of coding or disguising a message

24 Encryption/Decryption

25 Encryption/Decryption Answer 1: science rules Answer 2: pcjg czh hasg Spaces will stay spaces unless specified otherwise Messages will typically be short, but longer messages could appear in tie breaker questions

26 Data types- we will focus on basic concepts and terminology Almost all programming languages explicitly include the notion of data type, though different languages may use different terminology. Common data types include: integers booleans characters floating-point numbers alphanumeric strings

27 Java Terminology- basics Class: a class is an encapsulation of the abstract object. All of your code in Java will be contained in a class. Method: These are functions within a class that perform a user defined function and return to where they were called from. It can either return a value or not. This must be specified in the method header. Variable: A symbolic represebtation in code to represent a value or data structure. Names are user defined and using meaningful variable names is the biggest pitfall of a lot of new and self taught programmers. Object: This is a hard one for new programmers to wrap their head around. An object is a data abstraction or blue print for a data structure. For example a employee object class had defined variables for the name, ID and payrate as well as methods to calculate pay by passing in hours worked. There's no specific information in this object but when an instance of this object is created it is passed all of the information it needs for each employee object created. Instance: When the blue print is used to actually create an object giving it meaningful values and creating a reference in memory to where this object is located. This is done with the new keyword.

28 Java Terminology- basics (cont d) Constructor: this is the part of the object that is written as a method but with an identical name to the class. This is the default method that gets called when an instance is created. Typiclly you have multiple constructors depending on the use, but keep in mind they have to have different parameters or else Java doesn't know which one to use. Parameters/attributes: These are what is being passed to a method or constructor or whats being passed in. For example in your main method you might call printsum(x); and your method header looks like public void printsum(int x){} were x is the attribute being passed in the method call and int x is the parameter of the method. You are basically handing the value to the method when you call it. Main method: this is where your program starts. The header for your main method is public static void main(string args[ ]){ }. You can have many different classes within a program but only one main method. The class with the main method is known as your main class. As a beginning programmer most of your code is likely to be written in the main method but as you get more experience you'll find the main method only has a few calls to get the program started. static/nonstatic(dynamic): these are how variables and methods are allocated in memory. If you declare something as static it means that it is in a fixed place in memory and any reference to that method or variable will all go to that same place. You must specify static attributes with the keyword static otherwise they are dynamic by default. Non static variables cannot be accessed from static context. A non static or dynamic attribute means that each instance of that attribute is seperated from all others. ---There's a ton more and I'm sure other people will give you lots of other terms to learn because there are a lot.

29 Cryptology is the study of hidden writings, or writings with hidden meanings. It is the use of any means to send a secret message. This includes cryptography and steganography Cipher- A cipher is an algorithm which somehow changes the letters within a message. Decrypt/Decipher- To decrypt (or decipher) a message is to use revert the ciphertext back into the plaintext using the cipher and key. Digraph- A digraph is a pair of letters or characters. It is useful to look at common digraphs in frequency analysis, and they appear regularly in ciphers that use a fractionation. Encrypt/Encipher- To encrypt (or encipher) a message is to take the original message and perform the necessary changes to the text to create the ciphertext according to the cipher and key chosen.

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