CS 111A Lecture Notes - Calculations - Chapter 2 part 2

Primitive Datatypes

float - floating point number, uses 4 bytes (real number approximation) in the range of +/-3.4e-38 to +/-3.4e38
double - floating point number, uses 8 bytes in the range of +/-1.7e-308 to +/-1.7.4e308
byte - integer number, uses 1 byte (range -128 to 127)
short - integer number, uses 2 bytes
int - integer number, uses 4 bytes (range approx. -2.1 billion to 2.1 billion)
long - integer number, uses 8 bytes
char - stores one character (one keyboard key) using 2 bytes (unicode character set). Character literals are enclosed with apostrophes, e.g. 'w'
boolean - stores a Boolean value, e.g. true or false
The Scanner class has methods to input directly into an int: scan.nextInt(), and also to input directly into a double: scan.nextDouble()

Calculations, Assignments

Arithmetic operators:
   *    for multiplication
   +    for addition
   -    for subtraction
   /    for division (beware of integer division, as explained below)
   %    for modulus (the remainder from integer division)
   =    for assignment (stroring the result of operation at right in variable at left; think of it like an arrow to the left)
Integer division produces an integer result. To get a floating point (real number) result, one or both operands must be a floating point value.
Semi-colons terminate most lines of code.

Shortcut Operators

+= adds a value to a variable. Similarly, to subtract from or multiply or divide by a value, or take the modulus of a value, use: -= or *= or /= or %=
++ increments a variable. So x++; is equivalent to x += 1; and x = x + 1;
-- decrements a variable

Formatting Output using printf

printf is a function that is used in the C language to format output, and its popularity has led it to be used in Java and other languages. With printf, you specify a string to output, with special codes in the string.
Those codes are called format specifiers. They are placeholders for variables which are to be output, and also specify how those variables should be formatted when they are output. Format specifiers begin with the percent-sign (%) and end with the format type.
Here is a list of the most common format types:
- d - decimal integer - as distinguished from binary, octal, or hexidecimal integers.
- f - fixed floating point - displayed without scientific notation
- s - string
Between the percent-sign and format type, you can insert many optional items, such as width, optionally followed by a decimal point and the number of digits to display after that decimal point. For example, %10.2f would show a floating point number right-aligned in a field of 10 characters, with 2 digits after the decimal point.
Between the percent-sign and the width, you can also insert a comma to indicate that groups of 3 digits (thousands) are to be separated by commas, and/or a minus-sign to make output left-aligned instead of the default right-alignment. For example, %-,15d would show an integer number left-aligned in a field of 15 characters, with commas separating groups of 3 digits.
Because printf uses string codes to denote formatting, these codes are not checked at compile time. Make sure to test your output thoroughly.
After your output string is complete, you close the quote for it, and have a comma-seprated list of the variables or exressions you want to insert into the string: one for each format specifier.
Example: CalcFormatDemo.java
In-class exercise: Write a program that has the user enter 3 numeric homework grades, and outputs their average, formatted to show just 1 digit after the decimal point. Make sure to use a floating point data type for the grades.
GradeAverage.java: Solution to above in-class exercise averaging grades

Mathematical Functions (Methods)

method or function - like in algebra, a function can take input (arguments) and produce output (return value). It performs a calculation or executes other code. We'll write our own functions later, but for now we are going to use pre-defined functions. A "method" is a function that's defined inside a class. Since all functions are defined inside classes in Java, the words "method" and "function" are interchangeable in Java.
function call - a line of code that executes a function
arguments - the data passed into a function when it is called. Also called actual parameters
return value - the "answer" produced by the function, returned from it to the function call.
The class Math is defined for you, with methods for common mathematical operations.
Math.pow function - pre-defined mathematical function to calculate an exponent. For example, to store in the variable x the result of 2 to the 3rd power:
x = Math.pow(2, 3);
Another useful mathematical function is sqrt, which returns the square root of a number.

Type Casting

Type Cast - an expression to make a value look like it has a different datatype. Type casting never changes the datatype of a variable -- that's impossible. Instead, a type cast is like a function that returns the same value in a different datatype.
The notation for type casts uses the type name only. E.g. to cast the variable x to type double, you write: (double)x

Example use of casting: avoiding truncation in integer division. Remember that the result of a division operation will be truncated to an integer if both the operands were integers. So to get an accurate floating point result, cast one of the operands to a floating-point type. So for example when finding the average word length in a document:

  int num_chars, num_words;
  double avg_length;
  String input;

  input = JOptionPane.showInputDialog("How many characters are in the document?");
  num_chars = Integer.parseInt(input);
  input = JOptionPane.showInputDialog("and how many words are there?");
  num_words = Integer.parseInt(input);

  avg_length = (double)num_chars / num_words;

  JOptionPane.showMessageDialog(null, "The average word length is " +  avg_length + " characters.");

  /* Sample Output:

  How many characters are in the document? 100
  and how many words are there? 28
  The average word length is 3.57143 characters.

  */
  // without the type cast, the average word length calculated would have been 3

Constants

A constant is a named location in memory that has a datatype (just like a variable) except that its value cannot be changed after it is initialized. Use the keyword final before the datatype to declare a constant.

In-class exercise: You're planning a wedding, and you need to transport all your wedding guests from the ceremony to the reception. A bus can carry 40 passengers. Your program should ask the user how many guests are expected, and it should output how many busses are needed, and the number of extra people you could carry with those busses.

Busses.java: Solution to above in-class exercise

Reference Variables, Memory Allocation, and Variable Scope

A reference variable stores the memory address of an object or array. It's similar to the C++ concept of a pointer. (It's the closest thing Java has to pointers.) Unlike C++ pointers, reference variables do not need to be de-referenced.
new - a keyword used to dynamically allocate memory for an object. Returns a reference to that object. Used with all constructor calls in Java, except it's optional with the String class.
null - keyword indicating an empty reference. I.e. a reference variable whose value is null is one that doesn't refer to an object.
A null reference is often returned to indicate no value can be returned. E.g. when we click "Cancel" on an InputDialog, the showInputDialog method returns null.
In Java, strings are stored in the standard String class, so strings are stored as objects, and we use reference variables to refer to them.

String methods

charAt(i) - returns the character at position "i" in the string (starting from 0)
length() - returns the integer length of the string (number of characters in it)
toLowerCase() - returns the same string with all letters in lower case.
toUpperCase() - returns the same string with all letters in upper case.
To make it easier to work with Strings and get the expected behavior, the String class is designed so it is immutable, meaning strings are never modified after they're created.
For example, the String method toUpperCase does not modify the string it is called upon; it returns a reference to a new String object which is the upper case version of the original.
indexOf(s) - returns the 0-based index where the string or character "s" is found in the string. (Location of first occurrence of s.)
substring(i) - returns the string from character at position "i" to the end.
substring(i, j) - returns the string from character at position "i" to character at position "j" (not including the character at "j").
equals(s) - returns true if the string is equal to the string "s" or false if they are not the same (case in-sensitive version is equalsIgnoreCase) Do not use == to compare strings!
Example: StringDemo.java: Shows various ways to manipulate strings.

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