Website snapshot

content/ta/spring2018/cpsc220/jan18.md

@@ -0,0 +1,188 @@
+# Lecture for January 18
+
+## Variables and Assignment
+
+Think about variables as buckets that hold information. Once the bucket is created, only one type of item can go in the bucket.
+
+```java
+sand bucket1;
+```
+
+We can say that bucket1 is of type `sand`, that means the only thing that can go in the bucket is sand.
+
+```java
+int bucket1;
+double bucket2;
+```
+
+From the two lines above, we have *declared* the variable.
+
+Variables store state, they are a name for a location in memory. 
+
+Always remember to initialize your variables. Otherwise there's nothing in the bucket!
+
+```java
+bucket1 = 5;
+```
+
+ You can combine both the declaration and initialization
+
+```java
+int count = 15;
+```
+
+Remember when dealing with variables to stay true with the type, don't mix a bucket of water with a bucket of sand.
+
+We can update `count` to contain a true value
+
+```java
+count = 55;
+```
+
+`count` no longer has the value of `15` in it. There's no record of it! It has been overwritten with the value `55`
+
+### Primitive Types
+
+There are 8 primitive types in Java
+
+- boolean
+- char
+- byte
+- short
+- int
+- long
+- float
+- double
+
+byte through double are all *numeric* types
+
+#### Boolean
+
+`boolean` can only be equal to `true` or `false`
+
+```java
+boolean student = true;
+```
+
+#### Char
+
+Stores a single character from the Unicode set
+
+There are 16 bits per character which adds up to 65,536 characters
+
+ASCII is the US subset of the characters. You can look this up online when needing to deal with ASCII values
+
+```java
+char firstLetter = 'A';
+```
+
+### Numeric types
+
+The different numeric types determine the precision of your number. Since numbers are not represented the same in the computer as they are in real life, there are some approximations.
+
+The default type you can use your code is `int` for integers and `double` for numbers with a decimal point
+
+There are certain types of operations you can perform on numeric type
+
+| Symbol | Meaning         | Example    | Value |
+| ------ | --------------- | ---------- | ----- |
+| +      | addition        | 43 + 8     | 51    |
+| -      | subtraction     | 43.0-8.0   | 35.0  |
+| *      | multiplication  | 43 * 8     | 344   |
+| /      | division        | 43.0 / 8.0 | 5.375 |
+| %      | remainder / mod | 43 % 8     | 3     |
+| -      | unary minus     | -43        | -43   |
+
+#### Increment/ Decrement
+
+There are two types of in/decrementers postfix and prefix
+
+Postfix:
+
+```java
+int x = 0;
+int y = 7;
+x++; // Shortcut for x = x + 1
+y--; // Shortcut for y = y - 1
+```
+
+Prefix
+
+```java
+int x = 0, y = 7, z;
+z = y * x++; // Equivalent to (y * x) + 1 = 7 * 0
+z = y * ++x; // Equivalent to y * (x + 1) = 7 * 1
+```
+
+### Data Conversion
+
+There are two types of data conversion, implicit and explicit
+
+The compiler can perform implicit data conversion automatically.
+
+Performing an explicit data conversion requires additional work on the programmer's part
+
+A conversion is implicit if you do **not** lose any information in it
+
+```java
+double price = 6.99;
+int sale = 3;
+double total = price - sale;
+```
+
+A *cast* is an explicit data conversion. This is requested by a programmer, this can lead to loss of information
+
+```java
+int nextChar = 'b';
+Character.isAlphabetic( (char) nextChar); // Let's you print the actual letter 'b' instead of the number corresponding to it
+
+float price = 6.99;
+int cost = (int) price; // cost is now 6
+```
+
+### Printing variables
+
+You can print the values of variables using `System.out.println` and `System.out.print`
+
+The difference is that `System.out.println` adds a new line at the end. Meaning the next print out will be on the next line.
+
+```java
+int cost = 5;
+double sale = .30;
+System.out.print(cost);
+System.out.print(sale);
+// Prints out '5.30`
+System.out.println(cost);
+System.out.println(sale);
+// Prints out '5'
+// Prints out '0.30'
+```
+
+To add a space between two variables in a print, add `" "` to the expression in between the two variables
+
+```java
+System.out.println("The total cost is " + 5 " dollars and" + " " + 93 + " cents");
+// The total cost is 5 dollars and 94 cents
+```
+
+### Input from User
+
+You can get import from the user, we can do this using the `Scanner` class.
+
+First import it  at the top of your file
+
+```java
+import java.util.Scanner;
+```
+
+All you can do with `Scanner` is outlined in the Java API at this link https://docs.oracle.com/javase/8/docs/api/index.html?java/util/Scanner.html
+
+Create a Scanner object
+
+```java
+Scanner input = new Scanner(System.in);
+System.out.print("Please enter an integer: ");
+price = input.nextInt(); // The integer that the user inputs is now stored in price
+System.out.println("Your input: " + price);
+```
+