highschool/Grade 9/Computer Science.md

Computer Science Review Sheet

Exam Layout

Part	Description	Marks
A	True / False	10
B	Multiple Choice - All java code learnt, selection, loops, methods, Arrays, methods, strings	38
C	Short & Long Answers - Compare and contrast short code, methods, problem solving	20
Total		68

Intro To Programming

Comments: are used to explain and clarify program ocde for human reader

Operator	Uses	Description
+	a + b	Addition
-	a- b	Subtraction
*	a * b	Multiplication
/	a / b	Division
%	a % b	Mod, the Remainder

BEDMAS rules still apply

Strings

String: means a group of many characters
String Concatenation: means that two strings are combined into one using the “r” sign

Varaibles

• Holder for data
  
• We can use words instead of just a single letter
• can store more than just numbers
  
• a place in memory (RAM: random access memory) where it can be stored or referred to
  
• Name, type, value

To declare a variable 1. Name 2. Data Type

A variable is the name of a reserved memory location
A varaible name is called an identifier
Reserved words cannot be used as an identifier

Java Primitives

Variables can be created for any of the data types listed
The chart shows examples of how to create a new variable for each primitive

Type	Range	Size	Variable	Declaration
byte	-128 to 127	8 bits	bits_8	byte bits_8;
short	-32768 to 32767	16 bits	TALL	short TALL;
int	-2 billion to 2 billion	32-bits	sum	int sum;
long	-9 quintillion to 9 quintillion(huge)	64 bits	mile	long mile;
float	-3.4e+/-38 to 3.4e+/-38	32 bits	pi	float pi;
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
more	a >= b	not (a < b)	boolean
more or equal	a >= b	not(a == b)	boolean
negation	-b	0 - b	number

Sample Code

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

String - object data type equals(String)

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

import java.util.Scanner;  

to tell compiler to include scanner class in the .class file
line at very top of file create scanner using

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

if(condition){
   //then perform some commands
   // outcome
}

Example:

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

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

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

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

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

while(num >= 1 && num <= 10){ // runs until num is between 1 and 10
    num = myScanner.nextInt();
}

Incrementing By One

  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

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

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 run minimum once After running once, it checks the continue condition at the end of the iteration

do {
  // do stuff here
}while(condition);

Example

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

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

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 java int, String, char, boolean, double, complex types - [] - square brackets means index - arrays always start at 0 index - must be assigned a fixed size - use counted loop with array to do things

Example:

 // declare an array of 5 integer
 int [] numArray = new int[5];
 // set the 0 index of numArray to 123
 numArray[0] = 123;
 // get the array size
 int arraySize = numArray.length;

Mid - program initialization of arrays

• can declare the array at the beginning ant set size
• All variable and arrays must be declared at the beginning of the program for proper conventions > java > // declare variables and array > String[] names; > int numValues; > System.out.println("How many names?"); > numValues = scan.nextInt(); > // initialize array > names = new String[numValues]; >

Unit 3: Sorting

• sorting: the process of arranging a list of items into a well-defined order
• final list rearrangement of the original list

Bubble Sort

• when x numbers in array, loop x - 1 times
• The bubble sort algorithm works by swapping adjacent pairs in the list until all adjacent pairs are sorted in order, at which the entire list is sorted
• by making passes through the array, each pass moves from left to right
• the first pass compares element 1 and element 2 and swaps them if they are out of order, then compares element 2 and element 3 and swaps them if they are out of order, and so on
• the largest element is moved to the right
• for a length of n, there are n - 1 passes

Pass 1	Pass 2	Pass 3	Pass 4
3 6 5 4 2	3 5 4 2 6	3 4 2 5 6	3 2 4 5 6
3 6 5 4 2	3 5 4 2 6	3 4 2 5 6	2 3 4 5 6
3 5 6 4 2	3 4 5 2 6	3 2 4 5 6
3 5 4 6 2	3 4 2 5 6
3 5 4 2 6

Number of Elements	Number of Passes	Number of Comparisons
4	3	6
5	4	10
6	5	15
7	6	21
8	7	28
9	8	36
10	9	45
n	n - 1	n * (n - 1) / 2

Algorithm

repeat the following n -1 times
  for each element in the array, starting at the first element
      compare items in current positions with the element in the next position
      if item in current position is greater than item in next position then
         swap the 2 items using the following steps
         temp = current list item
         current list item = next list item
         next list item = temp

Code

// array called list
for (int i = 0; i < list.length - 1; i++){
    for(int j = 0; j < list.length - 1 - i; j++){
        if(list[j]  > list[j + 1]){
            int temp = list[j];
            list[j] = list[j + 1];
            list[j + 1] = temp;
        }
    }
}

CompareTo(String) - result si a negative integer if this String object lexicographically(alphabetically) precedes the argument string - result is a positive integer if this string object - result is zero if the string are equal