1. Last Time Unary operators: Other Assignment operators
Casting
Boolean “NOT”
Arithmetic operators
Other Assignment operators
Screen I/O (Console and simple GUI)
Variable Scope
if statements
Spring 2006
CISC101 - Prof. McLeod

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!
Spring 2006
CISC101 - Prof. McLeod

3. Exercise – An Easy One Prompt the user for an integer.
Tell the user if the number is odd or even.
Spring 2006
CISC101 - Prof. McLeod

4. Exercise – From Tuesday
Obtain an outdoor temperature (in degrees Centigrade) from the user.
If the temperature is less than -40, or greater than +40 tell him that the temperature is not legal and exit the program.
If the temperature is >= -40, but less than 0, display “It is cold! Wear a parka.”.
If the temperature is >= 0, but less than 15, display “It is cool. Wear a jacket.”.
If the temperature is >= 15 and less than 25, display “It is nice! Wear shorts.”.
If the temperatuer is >=25 and less than 40, display “It is hot! Seek the beach!”.
Spring 2006
CISC101 - Prof. McLeod

5. The Math Class
Java’s 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).
Spring 2006
CISC101 - Prof. McLeod

6. The Math Class – Cont.
To get more information on all the methods, look in the API documentation.
Spring 2006
CISC101 - Prof. McLeod

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;
Spring 2006
CISC101 - Prof. McLeod

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.
Spring 2006
CISC101 - Prof. McLeod

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.
Spring 2006
CISC101 - Prof. McLeod

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.
Spring 2006
CISC101 - Prof. McLeod

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!)
Spring 2006
CISC101 - Prof. McLeod

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 “;”.
Spring 2006
CISC101 - Prof. McLeod

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.
Spring 2006
CISC101 - Prof. McLeod

14. Programming Style & Documentation – Cont.
Be descriptive, but not excessive!
Examples:
numStudents
setPassingGrade ( parameter_list )
Somewhat too long…:
flagThatIsSetToTrueIfAProblemArisesWhenThereIsAFullMoonOverMyHouseInTheWinterWhileMyProgramIsRunning
It is OK to use single letter variable names such as i, j, k for counters in loops.
Spring 2006
CISC101 - Prof. McLeod

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.
Spring 2006
CISC101 - Prof. McLeod

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
Spring 2006
CISC101 - Prof. McLeod

17. Don’t! public class StyleDemo{ public static int s(int l,int l1){
int S=l+l1; return S; }}
Spring 2006
CISC101 - Prof. McLeod

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”
Spring 2006
CISC101 - Prof. McLeod

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.
Spring 2006
CISC101 - Prof. McLeod

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?
Spring 2006
CISC101 - Prof. McLeod

21. Repetition - Cont.
A simple example - suppose we wanted a loop to execute only 20 times:
i = 1
if true
i < 21
if false
i = i+1
etc.
Spring 2006
CISC101 - Prof. McLeod

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”.
Spring 2006
CISC101 - Prof. McLeod

23. Repetition - Cont.
Suppose, in the previous example, i was decremented by one instead. What would happen?
i = 1
if true
i < 21
if false
i = i - 1
etc.
Spring 2006
CISC101 - Prof. McLeod

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.
Spring 2006
CISC101 - Prof. McLeod

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.
Spring 2006
CISC101 - Prof. McLeod

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 < 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 “{}”.)
Spring 2006
CISC101 - Prof. McLeod

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.
Spring 2006
CISC101 - Prof. McLeod

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 ); {
Spring 2006
CISC101 - Prof. McLeod

29. “while” loop - Cont.
The boolean expression tested in a while loop could be false to start with:
int i = 40;
while (i < 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:
Spring 2006
CISC101 - Prof. McLeod

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.
Spring 2006
CISC101 - Prof. McLeod

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 < 1 || aVal > 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.
Spring 2006
CISC101 - Prof. McLeod

32. “for” loop The kind of while loop shown above:
int i = 1;
while (i < 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 < 21; i = i + 1) {
Spring 2006
CISC101 - Prof. McLeod

33. “for” loop - Cont. Or, as written with an increment operator: Syntax:
for (int i = 1; i < 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.
Spring 2006
CISC101 - Prof. McLeod

34. “for” loop - Cont.
You don’t 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.
Spring 2006
CISC101 - Prof. McLeod

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.
Spring 2006
CISC101 - Prof. McLeod

36. “for each” Loop in Java 5.0, Cont.
(We don’t 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; }
Spring 2006
CISC101 - Prof. McLeod

37. “for each” Loop in Java 5.0, Cont.
Equivalent normal “for” loop:
double sum = 0;
for (int i = 0; i < data.length; i++) {
sum = sum + data[i]; //or sum += data[i]; }
The “for each” loop is a bit easier with arrays, but is even better suited for other kinds of collections.
Spring 2006
CISC101 - Prof. McLeod

38. Loops - Misc.
Don’t 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 <= 100; i++)
for (j = 1; j <= 10; j++)
sum++;
sum would be 1000.
Spring 2006
CISC101 - Prof. McLeod

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.
Spring 2006
CISC101 - Prof. McLeod

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.
Spring 2006
CISC101 - Prof. McLeod

41. Use of “continue” For example, Would print: Spring 2006
CISC101 - Prof. McLeod

42. Use of “break” For example, Would print: 16 September 2002
Fall 2002 CISC121 - Prof. McLeod

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 can’t you put that condition in the loop test?
Overuse of break statements can lead to “spaghetti” code - just like the use of “goto” statements!
Spring 2006
CISC101 - Prof. McLeod

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. 20 40
Spring 2006
CISC101 - Prof. McLeod

45. Loop Exercise 2
Obtain a positive integer number, > 1 from the user. If the number supplied is illegal, continue to prompt him until it is.
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
Spring 2006
CISC101 - Prof. McLeod

46. Loop Exercise 3 Prompt the user for a positive number of numbers.
If this number is > 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
Spring 2006
CISC101 - Prof. McLeod