Last Time

1
Last Time

• Unary operators
• Casting
• Boolean NOT
• Arithmetic operators
• Other Assignment operators
• Screen I/O (Console and simple GUI)
• Variable Scope
• if statements

2
Today

• Reminder Lab 1 and Assignment 1 are posted.
• Assn 1 is due one week from today at 7pm.
• Start out with a warm-up exercise!
• Math class
• Style and Documentation
• Loops
• More exercises!

3
Exercise An Easy One

1. Prompt the user for an integer.
2. Tell the user if the number is odd or even.

4
Exercise From Tuesday

1. Obtain an outdoor temperature (in degrees
   Centigrade) from the user.
2. If the temperature is less than -40, or greater
   than 40 tell him that the temperature is not
   legal and exit the program.
3. If the temperature is gt -40, but less than 0,
   display It is cold! Wear a parka..
4. If the temperature is gt 0, but less than 15,
   display It is cool. Wear a jacket..
5. If the temperature is gt 15 and less than 25,
   display It is nice! Wear shorts..
6. If the temperatuer is gt25 and less than 40,
   display It is hot! Seek the beach!.

5
The Math Class

• Javas standard mathematical methods are found in
  the Math class. They are invoked by naming the
  class name followed by the method name, separated
  by a period. For example, the sine of a value
  can be calculated as
• double y Math.sin(x)
• The Math class methods are static, (like the
  methods in the System.out class).
• This means you can call them directly (without
  instantiation).

6
The Math Class Cont.

• To get more information on all the methods, look
  in the API documentation.

7
Example Math.random() Method

• Provides a pseudorandom double value between 0
  and 1.0
• How to calculate a random int value between 1 and
  1,000, for example?

int randVal (int)(1000 Math.random()) 1
8
Programming Style Documentation

• Purpose is to make your code readable (and
  debuggable) by you or another programmer who is
  familiar with the Java language.
• Internal style elements are documentation
  (comments), spacing, and descriptive variable
  names.
• Select the conventions you want to use and be
  consistent.

9
Programming Style Documentation Cont.

• Comments
• Add a block comment to the top of the class and
  at the beginning of each method. Describe
  overall purpose of class/method, main algorithm
  used, author, date created, and any assumptions
  made and/or bugs found. Method comments should
  state what parameters are expected by the method
  and what the method returns.
• Comments for variable declarations, when the name
  of variable is not self-explanatory.
• Comments at the beginnings of logical blocks of
  code.
• In-line comments to indicate the closing brackets
  of blocks and what they close.

10
Programming Style Documentation Cont.

• Spacing (alignment)
• Class definition header starts at column 1, and
  closing bracket on column 1.
• Indent of about 3 or 4 spaces is adequate.
• Method headers and instance variable declarations
  indented once.
• Code inside any block, including method code
  indented once from alignment of method header, or
  outer block.

11
Programming Style Documentation Cont.

• Opening can be at the end of a statement
  line, or on the line below.
• Closing on same column as the column where
  the method header is declared, or the statement
  containing the opening . is usually by
  itself on a line.
• Add a comment after to indicate what is being
  closed.
• If you have an overlong line, it is OK to
  continue the line on the line below, but indent
  the continued part of the line. (Note do not
  try to continue a line in the middle of a String
  literal!)

12
Programming Style Documentation Cont.

• Spacing (white space)
• Add blank lines before and after methods and
  larger logical blocks.
• One statement per line. (Longer statements can
  be broken onto multiple lines.)
• Use a space before , ( and . Use a space
  after ) and (unless the next character is
  ).
• No code after or on same line.
• No space after ( or before ).
• Use space after , or in parameter lists or
  for loop arguments, but not before.
• Put a space on both sides of an binary operator.
• No space before .

13
Programming Style Documentation Cont.

• Variable Names
• Also applies to method and class names.
• Follow java restrictions on names
• Use only letters, numeric digits (0 to 9) and the
  _ character.
• Cannot start name with a number.
• Java is case sensitive!
• Variables and method names usually start with a
  lower case character. Class names start with an
  upper case character. Constants are all in upper
  case.
• Variables are usually nouns.
• Methods are verbs or verbs and nouns.

14
Programming Style Documentation Cont.

• Be descriptive, but not excessive!
• Examples
• numStudents
• setPassingGrade ( parameter_list )
• Somewhat too long
• flagThatIsSetToTrueIfAProblemArisesWhenThereIsAFul
  lMoonOverMyHouseInTheWinterWhileMyProgramIsRunning
  
• It is OK to use single letter variable names such
  as i, j, k for counters in loops.

15
Programming Style Documentation Cont.

• The java compiler ignores all white space
  including space characters, tabs and carriage
  return/line feed characters.
• Note that most java keywords are in lower case.
• You will get an error message if you attempt to
  use a keyword as a variable name.

16
Do

• public class StyleDemo
• public static int someSum (int num1, int
  num2)
• int sum num1 num2
• return sum
• // end someSum method
• // end StyleDemo class

17
Dont!

• public class StyleDemo
• public static int s(int l,int l1)
• int Sll1 return S

18
Repetition or Using Loops

• We will discuss
• while
• do/while
• for
• The for each loop in Java 5.0
• Use of break and continue

19
Repetition or Loops

• Suppose we combine a boolean test with some kind
  of structure that allows us to branch back up to
  an earlier piece of code

if true
if false
etc.
20
Repetition - Cont.

• The boolean test determines when to stop the
  repetition - as long as the condition is true,
  the loop keeps going.
• Something inside the looping part must affect
  what is tested in the condition - right? What if
  it did not - what would happen?

21
Repetition - Cont.

• A simple example - suppose we wanted a loop to
  execute only 20 times

i 1
if true
if false
i lt 21
i i1
etc.
22
Repetition - Cont.

• The number of repetitions is controlled by
  changing the limit value for the loop counter -
  i in the example on the previous slide.
• That example had i increasing by one each time.
  The loop counter was being incremented by one.
• It could have been incremented by some other
  value, 2, 3, or whatever.
• You could use something like i i 2 to
  increment the counter.
• If the counter is decreased in value each time,
  it is being decremented.

23
Repetition - Cont.

• Suppose, in the previous example, i was
  decremented by one instead. What would happen?

i 1
if true
if false
i lt 21
i i - 1
etc.
24
Repetition - Cont.

• The dreaded infinite loop!
• The java compiler will not prevent you from
  coding a loop like the one shown - it will
  compile, and it will run!
• And run, and run, and run, and run, and run, and
  run, and run, and run, and run, and run
• As a programmer, you must be on guard for such
  logic errors in your code.

25
Reminder - Increment and Decrement Operators

• In loops, expressions like
• j j 1 and
• k k - 1
• are used so often, it is typical to see the
  postincrement operator
• j is the same as j j 1
• k-- is the same as k k - 1
• You can use either notation.

26
while loop

• A java while loop can be used to code the
  structure shown in the flowchart above (the
  increment one on slide 27)
• int i 1
• while (i lt 21)
• // other statements
• i i 1 // or you could use i
• // end while
• The brackets enclose the statements that
  are repeated.
• (A single statement to be repeated in the loop
  does not require the .)

27
while loop - Cont.

• Note that java (thank goodness!!!) does not have
  anything equivalent to a goto statement.
• (And if it did, I would not tell you about it,
  anyways!!)
• So, you cannot construct a loop with an if
  statement and a goto.
• An if statement cannot give you repetition, it
  only allows you to decide on a single pass
  through a branch of code.

28
while loop - Cont.

• while loop syntax
• while ( boolean_expression )
• block_of_code
• As long as boolean_expression evaluates to true
  the statements in the block_of_code continue to
  execute.
• By mistake, you might write the following - what
  would happen?
• while ( boolean_expression )
• block_of_code

29
while loop - Cont.

• The boolean expression tested in a while loop
  could be false to start with
• int i 40
• while (i lt 21)
• // other statements
• i i 1
• In this case, the loop would not execute at all.
• Use a do/while loop if you need a loop that
  will always run at least once

30
do/while loop

• Syntax
• do
• block_of_code
• while ( boolean_expression )
• Note the at the end of the while statement.
• Since the conditional test is at the end of the
  loop, it will always execute the loop at least
  once.

31
do/while loop - Cont.

• For example, suppose we must obtain a value
  between 1 and 100, inclusive, from the user
• int aVal 0 // The compiler will force us to
• // initialize aVal
• do
• System.out.print(Enter value between 1 and
  100)
• // code to obtain a value from the user
• while (aVal lt 1 aVal gt 100)
• As long as the user does not do what he is told,
  the loop will continue to re-prompt him for the
  correct value.

32
for loop

• The kind of while loop shown above
• int i 1
• while (i lt 21)
• // other statements
• i i 1
• is used so often, that Java has provided another
  looping structure that does all that is shown
  above, but needs only one line
• for (int i 1 i lt 21 i i 1)
• // other statements

33
for loop - Cont.

• Or, as written with an increment operator
• for (int i 1 i lt 21 i)
• // other statements
• Syntax
• for (initialization boolean_expression update)
  
• block_of_code
• for loops are used when you know, in advance, the
  number of repetitions desired.

34
for loop - Cont.

• You dont have to declare the counter inside the
  for loop, if you have declared it earlier in your
  program.
• But if you do declare it in the for statement
  then the scope of that variable will only be
  inside the loop block.

35
for each Loop in Java 5.0

• Often, you will want to visit every element in
  a collection, not just a part.
• Syntax of the for each loop
• for (type_variable collection)
• // statements
• These loops are only used with collections.

36
for each Loop in Java 5.0, Cont.

• (We dont know what arrays are yet, but just for
  now)
• For example, suppose we have an array called
  data, containing a collection of double type
  numbers, and you want to add them all up
• double sum 0
• for (double e data)
• sum sum e // or sum e

37
for each Loop in Java 5.0, Cont.

• Equivalent normal for loop
• double sum 0
• for (int i 0 i lt data.length i)
• sum sum datai //or sum datai
• The for each loop is a bit easier with arrays,
  but is even better suited for other kinds of
  collections.

38
Loops - Misc.

• Dont declare variables inside loops, as the
  repeated declaration process uses up time and
  memory unnecessarily.
• Loops are often nested - to usually not more than
  three levels. For example
• int i, j
• int sum 0
• for (i 1 i lt 100 i)
• for (j 1 j lt 10 j)
• sum
• sum would be 1000.

39
Loops - Misc. - Cont.

• There is no limit in Java to how many levels you
  can nest loops.
• It is customary, but not necessary, to use the
  variables i, j, k as loop counters.
• Loops really demonstrate the strength of
  computers as they allow the machine to complete
  mind-numbingly boring tasks with perfect
  accuracy!
• Loops will always be used with any file I/O and
  array operations.

40
Other Java Keywords Used With Loops

• break and continue
• The continue statement interrupts the execution
  of a loop, and returns control to the top of the
  loop.
• The break statement interrupts the execution of a
  loop, and transfers control to the first
  statement after the loop.

41
Use of continue

• For example,
• Would print

42
Use of break

• For example,
• Would print

43
Use of break continue

• Only use these keywords when it makes your code
  easier to read.
• Avoid the use of more than one break or continue
  inside a loop.
• If you use a condition to issue a break
  statement, then why cant you put that condition
  in the loop test?
• Overuse of break statements can lead to
  spaghetti code - just like the use of goto
  statements!

44
Loop Exercise 1

• Obtain from the user a positive upper limit.
• Also obtain an increment value.
• Print out all the numbers from 0 to the number
  before the upper limit using the increment, one
  number per line. For example, if the user
  supplies 50 and 20, the output would be
• Supply an error message if the input values are
  not legal.

0 20 40
45
Loop Exercise 2

1. Obtain a positive integer number, gt 1 from the
   user. If the number supplied is illegal,
   continue to prompt him until it is.
2. Print out all the numbers that divide this number
   evenly, including the number itself, starting
   from 2. For example, if the user supplies 10,
   the output would be

2 5 10
46
Loop Exercise 3

1. Prompt the user for a positive number of numbers.
2. If this number is gt 0, display to the screen this
   many prime numbers, starting from 2. For
   example, if the user supplies 5, the output would
   be

2 3 5 7 11