Computer Science 15-111 (Sections A & B), Spring 2007

Class Notes: 17-Jan-2007

Course Web Site, Syllabus, etc:

These should be online by tomorrow. We will review them in recitation, where you will also receive the programming portion of Homework #2, which is due Monday.

My contact info:

Email me at koz@cmu.edu. My office is Doherty 4301-D (most easily found from Wean 8^th floor passage into Doherty). Office hours are right after each class (including today).

Textbook:

For the first 1/3^rd or so of the course, we will use an online textbook:

Java by Dissection, 2^nd Edition, by Ira Pohl and Charlie McDowell

Download the pdf for $5 (you may also buy the printed book for about $20) at:

http://www.lulu.com/content/267149

You can find all the example code online at:

http://www.soe.ucsc.edu/~charlie/java/jbd/

Reading:

On Monday, we covered Chapter 1 (Introduction). This lecture, we are covering:

Chapter 2: Program Fundamentals

Appendix A.1: Integer Binary Representation

Appendix B.1: Operator Precedence Table (portions thereof)

Quiz:

Our first quiz, covering Chapters 1-3 and Appendices A-B, is on Wednesday 24-Jan (note: we are moving quickly as this is mostly material you should have seen already).

Topic Outline:

1. Lexical Elements

White space, comments, keywords, identifiers, literals, operators, punctuation

literal	Value
32	decimal 32
032 (octal)	decimal 26 (3*8 + 2)
0x32 (hexadecimal)	decimal 50 (3*16 + 2)
132l	132 as a long
1.32f	1.32 as a float
1.32d	1.32 as a double
1.32	1.32 as a double (default)
2.17e-32	2.17 x 10^-32
‘a’	The character ‘a’ (ASCII 97)
“a”	The String “a”

2. Primitive Data Types (8)

byte, short, int, long, float, double, char, boolean

Type	Bits	Min	Max
byte	8	-128	127
short	16	-32768	32767
char	16	0	65536
Int	32	-2147483648	2147483647
long	64	About -9.2 x 10^18	About 9.2 x 10^18

Type	Bits	Approximate Range	Approximate Precision
float	32	+-10^-45 to +-10^+38	7 decimal digits
double	64	+-10^-324 to +-10^+308	15 decimal digits

3. User Input (Section 2.6)

Scanner (see: http://java.sun.com/j2se/1.5.0/docs/api/java/util/Scanner.html)
import, assert and AssertionException
Problems with sample code: no comments, throws exceptions, terrible UI

4. printf

%c character

%d decimal integer

%e floating-point number in scientific notation

%f floating-point number

%g the shorter of e-format or f-format

%s string

Code	Output
System.out.printf("<%6.2f>",13.579);	< 13.58>
System.out.printf("%c",65);	A
System.out.printf(“%d”,3.2)	Runtime exception!
System.out.printf("%f",67);	Runtime exception!

5. ASCII codes you need to know:

0 null (in some OS’s signifies end-of-file)

7 alert/bell (it really rings!)

9 tab

10 linefeed

13 carriage return
32 ‘ ‘ (space)
48 ‘0’ … 57 ‘9’
65 ‘A’ … 90 ‘Z’
97 ‘a’ … 122 ‘z’

6. Mixed mode arithmetic

a. Java arithmetic uses only int, long, float, double (not char, byte, short)

b. Java uses numeric promotion (a kind of widening conversion) to make both arguments the same type

c. Widening conversions are rarely lossy, but can be

eg: System.out.println((int)(float)123456789); à 123456792

d. Narrowing conversions only by explicit casting. These often are lossy.

7. Arithmetic Errors

a. Overflow is unchecked! (results in gibberish)

b. Underflow is unchecked! (results in 0)

c. Floating point has no errors, just +Inf, -Inf, and NaN

d. Floating point is approximate, so don’t use ==

Code	Output
System.out.println(0/0);	Runtime exception!
System.out.println(0f/0);	NaN
System.out.println((1.3 * (1.8 / 1.3)) == 1.8);	true
System.out.println(((1.3 * 1.8) / 1.3) == 1.8);	false

8. Division operator (/)

Important: Integer division truncates, it does not round! (eg, 7 / 4 == 1, not 2)

9. Modulus operator (%)

a. Integers: remainder (eg, 23 % 7 == 2)

b. The textbook is wrong about floating-point modulus!!! Don’t worry, though, since nobody uses it (and neither should you, at least not without a really wonderful comment in your code!!!).

10. Operator precedence and associativity

Operator	Associativity
( ) ++ (postfix) -- (postfix)	left-to-right
+ (unary) - (unary) ++ (prefix) -- (prefix)	right-to-left
new	right-to-left
* / %	left-to-right
+ -	left-to-right
= += -= *= /= …	right-to-left

Hint: When in doubt, use parentheses!!!

11. Two’s Complement

a. To negate: flip bits and add 1

b. So subtraction is addition (less silicon!), and no +-0.

12. Base conversions (time permitting)

You should be comfortable converting from and to binary, octal, decimal, and

hexadecimal, and doing so without a calculator.