cmsc 131 discussion 09-07-2011

CMSC 131Object-Oriented Programming I

Instructor: Larry Herman

Section: 0302

Room: CSI 2118

People to Know

• Instructor• Larry Herman• Email: larry@cs.umd.edu

•Teaching Assistant• Philip Dasler• Email: daslerpc@cs.umd.edu

Things I Lied About / Need to Clarify

• You must use Eclipse as provided on the class website.

• You can’t use laptops in lecture.

• Closer to 80 students in lectures.

• Recommended books are just that. Also, they are all the same.

• Office hours will be in A.V. Williams 1112

More Administrivia

• Check for access to the grades server• grades.cs.umd.edu• Also, a link can be found on the class web page

• If you are officially registered for the course, you should have access.

• Grades server allows • View scores• View statistics• Report an absence

Let’s Play a Game

What was covered in lecture?

Let’s Play a Game

What was covered in lecture?• Parts of a computer• Bits, bytes, and words (0s and 1s)• N bits can represent 2N values• SI Prefixes (kilo, mega, kibibyte, mebi, etc.) ASCII

• Programming language types• machine code• assembly

An Aside

Prefixes

1027 ??

1024 yotta1021 zetta1018 exa1015 peta1012 tera109 giga 106 mega103 kilo102 hecto101 deka

An Aside

Prefixes

1027 hella

1024 yotta1021 zetta1018 exa1015 peta1012 tera109 giga 106 mega103 kilo102 hecto101 deka

BASES

Relearning How to Count

What number is this?

32,768

Relearning How to Count

But how do you know?

32,768

Relearning How to Count

32,768

Ones

Tens

Hundreds

Thousands

Ten Thousands

Positional Notation

Relearning How to Count

8

60

700

2,000

+ 30,000

32,768

Relearning How to Count

8 8 x 1

60 6 x 10

700 7 x 100

2,000 2 x 1000

+ 30,000 3 x 10000

32,768

Relearning How to Count

8 8 x 100

60 6 x 101

700 7 x 102

2,000 2 x 103

+ 30,000 3 x 104

32,768

Relearning How to Count

8 8 x 100

60 6 x 101

700 7 x 102

2,000 2 x 103

+ 30,000 3 x 104

32,768 Base 10(or radix 10)

Other Bases

• Can be done with any base number.

• Numbers in Base N are made of the digits in the range 0 to N-1• Base 10 numbers are made up of 0, 1, 2, . . . 8, 9

• Examples• Base 3 numbers use 0, 1, and 2• Base 6 numbers use 0, 1, 2, 3, 4, and 5• Base 8 numbers use 0, 1, 2, 3, 4, 5, 6, and 7

Other Bases - Example

• Notation: Xn means some number X in Base n

• Example:

Convert 13924 from Base 4 to Base 10.

Other Bases - Example

2 2 x 40

36 9 x 41

48 3 x 42

+ 64 1 x 43

15010 Base 4

Other Bases

• What is 2536 in Base 10?

Other Bases

• What is 2536 in Base 10?

• 10510

Other Bases

• What is 2536 in Base 10?

• 10510

• What is 1367 in Base 10?

Other Bases

• What is 2536 in Base 10?

• 10510

• What is 1367 in Base 10?

• 7610

Other Bases

• What is 2536 in Base 10?

• 10510

• What is 1367 in Base 10?

• 7610

• What is 4235 in Base 10?

Other Bases

• What is 2536 in Base 10?

• 10510

• What is 1367 in Base 10?

• 7610

• What is 4235 in Base 10?

• 11310

Going The Other Way

1. Do integer division by the base

2. Record the remainder as the next smallest place

3. Repeat on the result

Example: convert 7610 into Base 7

• 7610 divided by 7 gives 10 the remainder is 6 __6

• 1010 divided by 7 gives 1 the remainder is 3 _36

• 110 divided by 7 gives 0 the remainder is 1 136

7610 = 1367

Going The Other Way

1. Do integer division by the base

2. Record the remainder as the next smallest place

3. Repeat on the result

Example: convert 11310 into Base 5

• 11310 divided by 5 gives 22 the remainder is 3 __3

• 2210 divided by 5 gives 4 the remainder is 2 _23

• 410 divided by 5 gives 0 the remainder is 4 423

11310 = 4235

Reminder!

= ?

Reminder!

= 0

0 with a remainder of 4

Bringing It Back

So what does this have to do with computers?

Computers store everything as a series of 0s and 1s.

This is actually just regular numbers in Base 2!

What is 101010 in Base 10?

Some Common Terms

•Base 2, Binary•Base 8, Octal•Base 10,Decimal•Base 16,Hexadecimal or Hex

These are the most common bases used in computing.

Wait, Base 16?

But there aren’t enough numbers!

Wait, Base 16?

But there aren’t enough numbers!

Hex uses the following digits:•0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, and F

•Thus, 4210 = 2A16

Easy Conversions

We use binary, octal, and hex often because they are all powers of two.

This allows for easy conversions by just translating digits.

Easy Conversions

Octal is Base 8

Binary is Base 2

8 is just 23

Thus, for every three digits in binary we can directly translate to a single octal digit.

Easy Conversions

Example:Convert 758 to binary.

78 is 111 in binary.

58 is 101 in binary.

Thus, 758 = 1111012

Easy Conversions

Works just as well for hex or backwards.

Convert 1011111101112 to hex.

10112 is B in hex.

11112 is F in hex.

01112 is 7 in hex.

Thus, 1011111101112 = BF716

Easy Conversions

Decimal Binary Hex

0 0000 0

1 0001 1

2 0010 2

3 0011 3

4 0100 4

5 0101 5

6 0110 6

7 0111 7

Decimal Binary Hex

8 1000 8

9 1001 9

10 1010 A

11 1011 B

12 1100 C

13 1101 D

14 1110 E

15 1111 F

Why Use Bases?• Computers store information in two states: “charged” or

“uncharged”

• These can easily be represented by 0s and 1s, making binary a convenient method for mathematical manipulation

• Hex is more compact and easily converts to and from binary

• 111111101110110111111010110011102 = FEEDFACE16

QUESTIONS?

Fun Stuff

1000111

1001111

1010100

1000101

1010010

1010000

1010011

Hint: In ASCII, the letter ‘A’ is represented by the number 65.

Individual digits

Hours Minutes Seconds

20

21

22

23