diff --git a/Grade 9/Computer Science.md b/Grade 9/Computer Science.md
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+++ b/Grade 9/Computer Science.md
@@ -57,3 +57,343 @@
> |double|-1.7e+/-308 to 1.7+/-308|64 bits|stuff|double stuff;|
> |char|Single (unicode) characters|16 bits|letter|char letter;|
+> ## Operations
+
+>> |Operation|Notation|Equivalent|Result Type|
+>> |:--------|:-------|:---------|:----------|
+>> |equals|a == b||boolean|
+>> |addition|a + b||number|
+>> |subtraction|a - b||number|
+>> |multiplication|a * b||number|
+>> |division|a / b||number|
+>> |less|a < b||boolean|
+>> |less or equal|a > b|a <= b||boolean|
+>> |more|a >= b|not (a < b)|boolean|
+>> |more or equal|a >= b|not(a == b)|boolean|
+>> |negation|-b|0 - b|number|
+
+> ## Sample Code
+>> ``` java
+>> class AddTip{
+>> public static void main(String[]args){
+>> double amount; // variable declaration,
+>> // can store a double(real number)
+>> // no value assigned to it yet
+>> // variable name is identifier
+>> double pi = 3.1415926;
+>> amount = 19.95; // variable amount has been assigned value of 19.95
+>> amount = amount + 3.00; // takes current vaue of amount(19.95) and add 3.00 to it
+>> // then assigns the new value to amount(22.95)
+>> System.out.print("We will pay $");
+>> System.out.print(amount);
+>> System.out.print(" for the pizza delivery");
+>> // prints amount
+>> }
+>> }
+>> ```
+
+> ## Java Data Types
+>> ### Java primitive types
+>>> boolean
+
+>>> number:
+>>> - integer
+>>> - byte
+>>> - short
+>>> - int
+>>> - long
+>>> - char
+>>> - floating-point
+>>> - float
+>>> - double
+
+>>> **Remember** ```String``` is not a primitive type
+
+>> ### Boolean Expressions
+>>> can only have 2 possible value(true or false)
+>>> - compare must be compatible type
+
+>>> |Symbol|Meaning|
+>>> |:-----|:------|
+>>> |==|equal to|
+>>> |!=|not equal to|
+>>> |>|greater than|
+>>> |<|less than|
+>>> |>=|greater than or equal to|
+>>> |<=|less than or equal to|
+
+>> ```Modulus operator (%)```: returns the remainder of 2 numbers
+>> ```String```: stores characters / text
+>> ```String comparison```: different from primitive type comparison
+>> - cannot use ```==``` sign
+>> - use boolean expression ```object1.equals(object2)```
+
+> ## Reading Input (Scanner Class)
+
+>> ```import``` scanner class to use it
+>> use
+>> ```java
+>> import java.util.Scanner;
+>> ```
+>> to tell compiler to include scanner class in the .class file
+>> line at very top of file
+>> create scanner using
+>> ```java
+>> Scanner myScanner = new Scanner(System.in);
+>> ```
+
+>> |To read this...|...Make this method call|
+>> |:--------------|:-----------------------|
+>> |a number with no decimal point in it|nextInt()|
+>> |a number with a decimal point in it|nextDouble()|
+>> |a word(ending in a blank space, for example)|next()|
+>> |a line(or what remains of a line after you've already read some data from the line)|nextLine()|
+>> |a single character(such as a letter, a digit or a punctuation character|next.charAt(0)|
+
+
+> ## Selection statements
+>> allow java to make a decision
+>> depending on decision, program will function differently each time it is run
+
+>> ```java
+>> if(condition){
+>> //then perform some commands
+>> // outcome
+>> }
+>>```
+
+>> ### Example:
+
+>>> ```java
+>>> int age = 17;
+>>> if(age >= 16){
+>>> // execute if condition
+>>> System.out.println("You are " + age);
+>>> System.out.println("You can learn to drive");
+>>> } // skips if condition not met
+>>>```
+
+>> ### ```=``` vs ```==```
+>>> ```=```: assignment
+>>> ```==```: comparison
+
+>> ### if.... else if.... else
+
+>>> ```java
+>>> if(condition){
+>>> statement1;
+>>> } else{
+>>> statement2;
+>>> // -----------------------
+>>> }
+>>> if(num > 10){
+>>> System.out.println("Your number is greater than 10");
+>>> } else{
+>>> System.out.println("Your number is less than or equal to 10");
+>>> }
+>>> ```
+
+>>> - used when program needs to choose from a variety of situations
+>>> - use else if to specify another condition
+>>> - helps to control the flow of your program
+>>> - helps make decision when 3 or more conditions must be considered
+>>> - makes program more efficient
+
+>>> ```java
+>>> if(age >= 16){
+>>> System.out.println("You can drive");
+>>> }else if(age == 15){
+>>> System.out.println("You can drive next year");
+>>> }
+>>> ```
+>>> - else if statement is additonal part of if statement
+>>> - can be multiple else if statement in a single if construct
+>>> - one if statement can only run one outcome
+
+>> else - used to specify outcome that will be run if one other conditions are met
+>> debugging - the process of finding out and correcting erros in a program
+
+> ## Types of Errors
+
+>> ```Compile-Time-Errors``` - Complier checks syntax
+>> - syntax error - how you write your statements(your grammar)
+>> - Spelling mistakes
+>> - Punctuation errors
+>> - Missing brackets, curly brackets, key words
+>> - Incompatible types of data
+>> - .class file not created
+
+>> ```Logical Error``` - Hardest to detect/solve
+>> - compiles and runs correctly but incorrect results
+>> - wrong formula
+>> - wrong approach taken
+
+>> ```Run-time Error``` - occur during program execution
+>> - causes program to terminate abnnormally
+>> - try dividing by zero
+
+
+> ## Logical Statements
+>> True/False
+>> 3 basic logical operator
+>> ```OR/AND/NOT```
+
+>> **```AND```**: when all values are true
+>> **```OR```**: when at least one value is true
+>> **```NOT```**: flips the value of a statement
+
+>> ### Truth Table
+>>> Used to study logic by computer scientists
+
+>>> | X | Y |X OR Y|X AND Y|
+>>> |:--|:--|:-----|:------|
+>>> |T|T|T|T|
+>>> |T|F|T|F|
+>>> |F|T|T|F|
+>>> |F|F|F|F|
+
+>> operator precendence - evaluate left to right
+>> 1. AND( & & ) - checks first statement, if it is true, check the second one
+>> 2. OR( | | ) - checks the first statement and second statement
+>> 3. NOT( ! ) - reverses the statement
+
+> ## Loops
+
+>> Allows coders to repeat code
+>> Exit condition in loop
+
+>> ```java
+>> while(condition){
+>> // do something
+>> }
+>> ```
+
+>> ### While Loops
+>>> consists of a condition statement that are executed while the condition is true
+>>> types of conditions used in a while statement are the same as in the ```if statement```
+>>> while the condition is true, repeat the statements in the loop
+
+>>> ```java
+>>> while(num <= 10){
+>>> System.out.println("Enter a number higher than 10.");
+>>> num = scan.nextInt();
+>>> }
+>>> ```
+
+>>> the program continues repeating until the num is greater than 10
+>>> there may be times when you may want to specify more than one condition
+>>> no limit to the number of conditions, but should limit to 2 or 3
+>>> can combine conditions(logical statements) using ```&&``` and ```||``` conditional operators
+
+>>> ```java
+>>> while(num >= 1 && num <= 10){ // runs until num is between 1 and 10
+>>> num = myScanner.nextInt();
+>>> }
+>>> ```
+
+>> Incrementing By One
+>> ``` java
+>> mynumber = mynumber + 1;
+>> // is the same as:
+>> mynumber += 1;
+>> // is the same as:
+>> mynumber++;
+>>
+>> ```
+
+>> Why counter: To keep track of how many times a loop has executed
+
+>> ```java
+>> int counter = 0;
+>> int guess = 0;
+>> while(guess != 7){
+>> System.out.println("Guess a number between 1 and 10");
+>> guess = scan.nextInt();
+>> counter++;
+>> }
+>> System.out.println("It took you " + counter + " tries!");
+>> ```
+
+
+>> ### For Loops
+>>> include three distinct statements separated by semicolon
+>>> first statement declares the counter variable
+>>> second statement specifies exit condition
+>>> third statement increments the counter variable after each iteration of the loop
+>>> counter variable only exists within loop
+
+>>> ```while``` and ```for``` loop: - may not need to run if condition is false
+
+>>> ```java
+>>> for(int i = 0; i < 10; i++){ // this loop runs 10 times
+>>> // do stuff here
+>>> }
+>>> ```
+
+>> ### Accumulator
+>>> variables that add any value to a sum or total
+>>> realize the need for the variable
+>>> declare the variable
+>>> initialize it (give it a start value, usually 0)
+>>> accumulate the value in a loop (add the value to the variable)
+
+>> ### Do while Loop
+
+>>> They must runu minimum once
+>>> After running once, it checks the continue condition at the end of the iteration
+
+>>> ```java
+>>> do{
+>>> // do stuff here
+>>> }while(condition);
+>>> ```
+
+>>> Example
+
+>>> ```java
+>>> public static void main(String[]args){
+>>> Scanner sc = new Scanner(System.in);
+>>> int total = 0, value;
+>>> do {
+>>> System.out.println("Enter values to sum and 0 to quit");
+>>> value = input.nextInt();
+>>> total = total + value;
+>>> }while(value != 0);
+>>> System.out.println("Your total is: " + total);
+>>> }
+>>> ```
+
+> ### Random Number Generation
+>> ### Method 1
+
+>> ```java
+>> import java.util.Random;
+>> public class {
+>> public static void main(String[]args){
+>> Random randGen = new Random();
+>> int randNumber = randGen.nextInt(100) + 1;
+>> }
+>> }
+>> ```
+
+>> ### Method 2
+
+>> using Math class
+
+>> ```java
+>> int myRandNumber = (int) (Math.random() * 100) + 1;
+>> ```
+
+
+# Unit 2 Arrays
+
+> ```Arrays```: a data structure that allow you to hold multiple pieces of data, in a single object
+> - hold a predetermined number of elements
+> - must be same type
+
+
+
+
+
+
+