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# Computer Science Review Sheet
## Exam Layout
|Part|Description|Marks|
|:---|:----------|:----|
|```A```|**True / False**|10|
|```B```|**Multiple Choice**<br>- All java code learnt, selection, loops, methods, Arrays, methods, strings|38|
|```C```|**Short & Long Answers**<br>- Compare and contrast short code, methods, problem solving|20|
|```Total```||68|
# Unit 1: 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
## Variables
- 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<sup>+/-38</sup> to 3.4e<sup>+/-38</sup>|32 bits|pi|float pi;|
|double|-1.7e<sup>+/-308</sup> to 1.7<sup>+/-308</sup>|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
- ```equals(String)``` - Method compares the current string to another string and returns true if the two contain the same value, false if different
- ```substring(int, int)``` - takes two values - an int for the first character and an int for the last character (exclusive)
- substring(1, 10) returns the substring from index 1 to index ```9```
- returns that part of the string
### Strings
- String - a set of individual characters
- string start at zero - zero indexing
- an array of characters
- Char(acter) - hold the same type of information as a string - but only one character
- use single quotes rather than double quotes
- many different methods that can be used with char
- uses less memory than a string
```java
String name = "Ms Andrighetti";
String firstThree, lastEleven;
firstThree = name.substring(0, 3);
lastEleven = name.substring(4, 15);
System.out.println(firstThree);
System.out.println(lastEleven);
```
- first character is index 0 because zero-indexed
- substring specifies the first characters you want and the last character you don't want
- if you try to access a point beyond the end of the String, you will get an error
- ```length()``` - tells you how many character are in a string
- length gives you the length of string not last index of the string
```java
String word = "Hello";
int length = word.length();
System.out.println(length);
```
### 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)```|
## Output
- using standard output system:
```java
System.out.println()
```
- this outputs to the console
## 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 run 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;
```
## Reading From Files
### Why read from files?
- speed
- less errors
- easier to read, individual cases
- get data from sources other than the keyboard
- changing info is easy (only on the file)
- more efficient (less time to test)
- large data can be entered quickly (less room for error)
- at the ```testing stage```, data can be carefully chosen to test the program
### Intializing
```java
Scanner fsc = new Scanner("myfile.txt");
```
### Example Program
```java
import java.io.File;
import java.util.Scanner;
public class ReadData {
public static void main(String[]args) throws IOException{
File file = new File("summerschool.txt");
Scanner input = new Scanner(file);
while(input.hasNext()) {
String firstName = input.next();
double avg = input.nextDouble();
System.out.println(firstName + " " + avg);
}
input.close();
}
}
```
### Why write from files? (store to disk)
- you can save it for future use
- print hard copy results and distribute it
- write to a retrievable medium
- store the info
- make it more available to others
- large data can be collected, stored, formatted acoording to needs
```java
import java.io.PrintWriter; // can be used to create a file and write
// for example
output.println("hello world!"); output.printf("hello world!\n"); output.print("hello world!");
```
### Intializing
```java
PrintWriter output = new PrintWriter("mytextfile.txt");
```
### Example Program
``` java
import java.io.File;
import java.io.PrintWriter;
public class PrintWriterExample {
public static void main(String[]args) throws IOException {
File myFile = new File("secretmessage.txt");
PrintWriter output = new PrintWriter(myFile);
output.println("hello");
output.println("world!");
out.close();
}
}
```
# 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:
```java
// 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];
```
## Display
- Simply iterate throughout the array and just print out all the elements
- A ```for``` loop or ```while``` loop can be used
- Simple display code
```java
System.out.print("The elements in the array are: ");
for(int i = 0; i < array.length; i++){
System.out.print(array[i] + " ");
}
System.out.println();
```
## Search
- To iterate through in array, or to search an element
- Just loop through the array until you find the element
- Use ```length``` to iterate through the entire array
- Use appropriate comparators like ```==``` and ```.equals()```
- Simple search code
```java
int element = 12;
for(int i = 0; i < array.length; i++){
if(array[i] == element){
System.out.println("Element found at index: " + i);
break;
}
}
```
## Finding The Average
- Sum all the elements of the list and divide by the number of elements
- Simple average finding code
```java
int sum = 0;
for(int i = 0; i < array.length; i++){
sum += array[i];
}
double average = sum / array.length;
System.out.println("The average is: " + average);
```
## Finding Min / Max
### Finding the Min
- Algorithm
```
set the min to the first element, we assume its the minimum
go through each element in the array, if we find an element that is smaller than the current min
change the current min to it, we can use either Math.min() or the < operator
after iterating through the entire list, the min will be the smallest number
```
- Code
```java
int min = array[0];
for(int i = 0; i < array.length; i++){
if(array[i] < min){
min = array[i];
}
}
System.out.println("Smallest value is: " + min);
```
### Finding the Max
- Algortihm
```
set the max to the first element, we assume its the maximum
go through each element in the array, if we find an element that is bigger than the current max
change the current max to it, we can use either 'Math.max()' or the '>' operator
after iterating through the entire list, the min will be the biggest number
```
- Code
```java
int max = array[0];
for(int i = 0; i < array.length; i++){
if(array[i] > max){
max = array[i];
}
}
System.out.println("Biggest value is: " + max);
```
# 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
```java
// 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 is 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
# Unit 4: Methods
- ```methods``` - subprograms in java
- a group of programming statements that are given a name
- two types of methods
- ```function-type methods``` - calculates and returns a value
- ```procedure-type methods``` - executes some commands and has a void return type
## Why Methods
- allows for code to be reused throughout a program
- more efficient and neatly organized programs
- allow for easy modification later on
### Examples:
```java
public static int square(int number){
return number * number;
}
public static void printHelloWorld(){
System.out.println("Hello World!");
}
```
- int - return type
- square - method name
- int number - formal parameter
## Built in methods
|Method|Description|
|:-----|:----------|
|```Math.abs(x)```| returns the absolute value of the paramter x|
|```Math.random()```|returns a pseudorandom value uniformally distributed between 0 and 1|
|```Math.round(x)```|returns the value of x rounded according to the usual arithmetic rules|
|```Math.ceil(x)```|returns the value of x rounded up to the nearest integer|
|```Math.floor(x)```|returns the value of x rounded down to the nearest integer|
|```Math.max(x, y)```|returns the greatest of values x and y|
|```Math.min(x, y)```|returns the smallest of values x and y|
|```Math.sqrt(x)```|returns the value of the square root of x|
|```Math.pow(x, y)```|returns the value if x raised to the power of y|
|```Math.PI```|returns the double value of the mathemetical π|
|```.charAt(x)```|returns the character at index x|
|```.substring(st, ed)```|returns the string beginning at st and ending at ed(exclusive)|
|```.toLower()```|returns the lower-cased version of the string|
|```.toUpper()```|returns the upper-cased version of the string|
|```.split(regex)```|splits the string at index(s) where the ```regex``` appears and returns an array of strings|
|```isCharacter(x)```|checks if x is an character|
## More on Methods
- ```signature``` - method name and parameter
- if method is to be used outside the class in which it is defined, it must be declared as a public in the class
- call or invoke a method to use it
- ```formal parameter``` - the names of the parameter accepted in the signature
- ```actual parameter``` - the values passed into a method
- if no parameters, empty set of parentheses are used
- ```java
c.readInt();
```
## Return Type Methods
- public static ```return type``` ```method name```(parameters)
- Example:
```java
public static int square(int num1){
// do stuff here
}
```
## Void Type Methods
- public static void ```method name```(parameters)
### Example:
```java
public static void drawPicture(){
// do stuff here
}
public static void draw4Circles(int x1, int y1, int x2, int y2){
// do stuff here
}
```
### Java language - Strongly typed
- meaning you are not allowed to assign a value to a variable that is consistent with its declare type
- **```Scope of variable```** - the part of the program over which the variable can be accessed or referenced
- referes to the ```accessibility``` of a variable
- variables cannot be accessed before they are declared
- Variables can be declared in several different places
- class bodies (referred to as global or class level variables)
- as parameters to methods(in method signature)
- in a method body
- in a statement block (like a loop or a while loop)
```java
public class VariableExample{
static Scanner myScanner = new Scanner(System.in);
public static void main(String[]args){
// main method
}
}
```
- Description of Example
- variable myScanner is declared outside of the main method in the class level
- the variable is considered to be a global variable that can be accessed anywhere in the class
- for now, global variables should have the keyword ```static``` preceding the declaration
- In methods
- a method may declare local variable in the body of the method for use onlyin that method
- variable may also be declared in the parameter list - only can be accessed in the method
```java
public static int thirdPower(double number){
int cube; // local variable
cube = number * number * number;
return cube;
}
public static double thirdPower(double number){
double cube;
cube = number * number * number;
return cube;
}
```
- variable cube in thirdpower is local to that method
- local variables cannot be accessed from outside of the method
- In blocks of code
- variables defined in a block are only accessible from within the block
- the scope of the variable is the block in which it is defined
```java
for(int x = 0; x < 5; x++){
System.out.println(x);
}
```
- the variable x can only be accessed in the for loop
## Method void return type
- return type void means that a method will not return a value
- the method can still have parameter when the return type is void
## Naming conventions
- method names should indicate an action
- verbs make good methods names
- methods names should begin with a lowercase letter and then an uppercase letter should begin with each word within the name
- method names may not contain spaces
## Pass By Value
```java
public static void drawBar(int length){
for(int i = 0; i < length; i++){
System.out.print("* ");
}
System.out.println();
}
```
- means that when a method is called, a ```copy``` of the value of each argument is passed to the method
- this copy can be changed inside the method, however such a change will have no effect on the actual argument
- copies of the actual parameter values from main are sent to the methods, where they become ```foramal parameters```. When the method is finished, the copies are discarded. The actual ```paramter``` values remain unchanged. Notice that nothing is returned in the above method and how values ```a``` and ```b``` are not changed in the main method when passed in the method below
```java
public static void main(String[]args){
int a = 0, b = 10;
System.out.println("The starting value of a and b are: " + a + " and " + b);
change(a, b);
System.out.println("The values of a and b are: " a + " and " + b);
}
public static void change(int a, int b){
a = 999;
b = 21;
}
```
## Pass by reference
- occurs when an object is passed to a method, its memory address location (its ```reference```) is used
- ```Arrays``` behave like objects, their memory location is passed to the method
- that means that when an array is manipulated in the method that we are actually ```changing``` the array
- be cautious when sending an array in the method as it will change the data in the original array
```java
public static void main(String[]args){
int array[] = new int[4];
a[0] = 1000;
a[1] = 2000;
a[2] = 3000;
a[3] = 4000;
System.out.print("The values of the array are: ");
for(int i = 0; i < a.length; i++){
System.out.print(a[i] + " ");
}
System.out.println();
System.out.println();
change(array);
System.out.print("The values of the array are: ");
for(int i = 0; i < a.lengh; i++){
System.out.print(a[i] + " ");
}
System.out.println();
}
public static void change(int array[]){
array[0] = 1;
array[1] = 2;
array[2] = 3;
array[3] = 4;
}
```
# Unit 5: Software Design Process
## 5 Steps
|Step #|Description|
|:-----|:----------|
|Analysis|Also called ```problem solving```, its the process of ```breaking problems down into smaller more manageable parts```|
|Design|In this stage, an ```algorithm``` which is a set of instructions defined to solve the problem|
|Implementaion|Also referred to as ```coding```. Where you express the algorithm in a ```programming language```|
|Testing|Invlovles the running of the program will a full range of data. Valid data: ```the data the user should be inputting.``` Invalid data: ```incorrect or unexpected data```|
|Maintenance|```Maintaining a program over time so that any changes are needed are incorporated```|
- **```valid data```**: the data the user should be inputting
- **```invalid data```**: ```incorrect``` or ```unexpected``` data
- **```algorithm```**: a set of ```insturction``` defined to ```solve``` the problem
- **```Well designed program```**: a program that ```reliably``` solves the problem it was created to solve
- **```Waterfall Model```**: A flow chart, each phase must be completed before the next one
## Programmers
- Programmers have the abilites to:
1. Solve Problems
2. Organize information
3. Design Solutions
4. Express instructions in a logical sequence
5. Input them into a computer
- My Points for abilities and characteristics:
1. Communicate, collabration
2. Present their solution
3. Work as a team
# Unit 6: Problem Solving
## Code Correction
- ```;``` semicolons
- Accessing outside of specified range of a ```string``` or ```array```
- Missing or extra brackets ```{}```, ```()```, ```[]```
- Undeclared variables, scope of an variable
- Wrong method calls
- incorrect syntax
## Problem solving
- **Debugging**
- Printing out code
- Rubber ducking your code
# Credits
- Creator, Editor: --> magicalsoup (ME)!
- Helper, Information Provider(s): --> Matthew Wong, Samantha Ouyang
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