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Data

Data. Tonga Institute of Higher Education. Variables. Programs need to remember values. Example: A program that keeps track of sales needs to remember data: A loaf of bread was sold to Sione Latu on 14/02/04 for T$1.00. Customer Name: Sione Latu Date Sold: 02/14/04

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Data

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  1. Data Tonga Institute of Higher Education

  2. Variables • Programs need to remember values. • Example: A program that keeps track of sales needs to remember data: • A loaf of bread was sold to Sione Latu on 14/02/04 for T$1.00. • Customer Name: Sione Latu • Date Sold: 02/14/04 • Item Sold: Loaf of Bread • Item Cost: T$1.00 • Variable – Place in the computer’s memory where information is stored.

  3. Value Types vs. Reference Types • A variable stores a value or object. • Value Types only store values • int only stores: • 1 • 2 • 5 • boolean only stores: • True • False • Reference Types only store references to an object • Object - A self-contained entity that contains data and procedures to manipulate the data. • A reference is a location in memory • The location is the starting point of an object

  4. Primitive Data Types • Primitives - The basic types supported by the programming language. • Use lowercase for primitive data types in code

  5. Primitive Data Type Selection Practice • How to choose the right primitive variable type. • Byte, short, int and long store whole numbers. • Float and double store fractions. Double is more accurate, so if accuracy is important, use double. • If you pick something that is too big, you’re wasting memory space. Wasting memory space slows things down. • If you pick something that is too small, then you’re going to crash. • If you need a character, use char. • If you need a true/false value, use boolean. • What is a good data type for: • Someone’s age? • A customer’s identification number for a video rental store in Nuku’alofa? • A very large number with decimals? • The price of an item?

  6. Using Primitive Variables • 2 Steps to using variables • Declare the variable • Initialize the variable

  7. Declaring Primitive Variables • Declare the variable – Tell the computer to reserve a space in memory for the variable. • You need to tell the computer 2 things: • Name of the variable • Type of the variable (What kind of variable you have) • Primitive types • byte • short • int • long • float • double • char • boolean Name Type

  8. Declaring Primitive Variables Guidelines • Use a name that is easy to remember. • Do not use x, y, z • Variable names must start with a letter, underscore or dollar sign. • Begin variables with a lowercase character. Then, use a capital letter for each word. • Examples • firstName • customerID

  9. Initializing Primitive Variables • Initialize the variable – Assign an initial value to a variable. • Char values must be enclosed in single quotes. • String values must be enclosed in double quotes. • Boolean value should be True or False Initial Value

  10. Declaring and Initializing Primitive Variables in 1 line • You can declare and initialize a variable in 1 line.

  11. Demonstration Declaring and Initializing Variables

  12. Converting Data TypesSmaller to Larger • It is possible to automatically move a value from a smaller data type to larger data type • Example Maximum Short Value Maximum Long Value Is 9.2 x 1015

  13. Converting Data TypesLarger to Smaller • If a value from a larger data type is moved into a smaller data type, the value may be too big for the new data type • This will cause an error • Using this: • Results in: • Do not do this! Maximum Short Value is 32767

  14. Converting Data Types • Force conversion of data types by casting it. • To cast, use this format: (<Desired data type>)<Variable name> • If the Cast function isn’t able to convert the value, you may not get the answer you expect. int is bigger than a byte Forcing a cast from a larger data type to a smaller data type is not recommended! Without this, we would get an error But the final value Is -24, which is strange!

  15. Demonstration Converting Data Types

  16. Arithmetic Operators • Declare and Initialize x, y and z • Get values from x and y • Adds x and y together • Assigns the sum of x an y to z

  17. String Concatenation • Addition • You can add strings. Adding strings is called concatenation. • This converts all non strings into strings. Make sure you add your numbers before converting them to strings.

  18. Demonstration String Concatenation

  19. Division • Tricky because result may not be an integer. In this case the number gets cut off. NOT rounded! • Even having a result of float doesn’t work. Because Java has rules for dealing with data types. The result of a division between two integers is always an integer. So the float just added a .0 to it. • To get around this, we need to divide two floats!

  20. Demonstration Division

  21. Multiplication and Modulus • Multiplication • Use asterisk (*) instead of x • Modulus • The remainder of a division • Can be used to determine whether a number is divisible by another number • Can also determine if a number is even or odd

  22. Order of Operations • When you have a lot of operations, they are performed in a certain order. • Parentheses () • Multiplication or Division operations from left to right • Addition or Subtraction operations from left to right. • Examples: • 3 + 6 + 9 / 3 = 3 + 6 + 3 = 12 • (3 + 6 + 9) / 3 = 18 / 3 = 6

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