Lecture #15

November 5, 1999

Bob Pitha

bpitha@cs.ucsd.edu

http://www-cse.ucsd.edu/classes/fa99/cse5a

let’s talk about the quiz

• Stats:

– average score: 15.5

– each question got answered correctly by someone

question 1

• Remember the rules for identifiers:

– Letters, digits, and underscore

– Can’t start with a digit

let’s talk a little more about identifiers

• Identifiers are just names you can use in your program

– they’re kind of a general-purpose set of rules for naming things

• variables

• functions

• new data types

•

We haven’t covered these yet

data structures

• symbolic constants

valid identifiers

• There’s a difference between a legal identifier and one that can be used in a particular context

– legal means it just passes the simple rules for identifiers

– useable means the compiler will let you use it

• The main reason a legal identifier may not be used is that it’s already used in a way that would conflict

conflicting names

• For example, main is a perfectly valid identifier

– it’s all letters

– it doesn’t begin with a digit

• But I can’t name a variable main, because it would conflict with the function named main.

• Similarly, when your book says you shouldn’t name variables starting with an underscore, it’s not because it’s not a legal identifier

– it’s just that sometimes you’ll find that the name has already been used

so back to question 1…

so back to question 1…

• It is perfectly legal to mix upper and lower case

– case-sensitive means that 2 identifiers are considered different even if they only differ by the case of one or more letters (i.e., i vs. I)

so back to question 1…

so back to question 1…

• There is a data type named double, which means it is a keyword that cannot be used as a variable

– but it’s still a valid identifier

– anyway, this is Double, not double.

so back to question 1…

• This is a legal identifier

– just as your book says, it’s a good idea to avoid identifiers that begin with a single underscore

question 2

• There are some special-purpose computers that have other byte sizes, but these are rare. All common modern computers use an 8-bit byte.

question 3

question 4

decimal	binary	hexadecimal
10	1010
	1111
		2c

question 4

decimal	binary	hexadecimal
10	1010
	1111
		2c

• Let’s look at these numbers that are already filled in…

– so first, let’s review what we know about binary

question 4 – binary review

• So 1010 in binary is:

1*2³ + 0*2² + 1*2¹ + 0*2⁰

8 + 2

10

question 4

decimal	binary	hexadecimal
10	1010
15	1111
		2c

• Similarly, 1111 in binary is:

1*2³ + 1*2² + 1*2¹ + 1*2⁰

8 + 4 + 2 + 1

15

question 4

decimal	binary	hexadecimal
10	1010	a
15	1111
		2c

• Now for some hex:

– remember the hex digits are

1 2 3 4 5 6 7 8 9 a b c d e f

• where a-f represent 10-15

– so this first one is easy: 10 in hex is a

question 4

decimal	binary	hexadecimal
10	1010	a
15	1111	f
		2c

• And 15 is just f

question 4

decimal	binary	hexadecimal
10	1010	a
15	1111	f
44		2c

• Now going from hex to decimal:

– 2c is:

2*16 + c

32 + 12

44

question 4

decimal	binary	hexadecimal
10	1010	a
15	1111	f
44		2c

• Now the hard one: convert 44 to binary

– this essentially means expressing 44 as a sum of powers of 2

• so what powers of 2 can we sum up to make 44?

– What’s the largest power of 2 that is less than 44?

• 32 (2⁵)

– in binary, therefore, 32 is 100000

question 4

decimal	binary	hexadecimal
10	1010	a
15	1111	f
44		2c

• So 44 is 2⁵ + 12

– so we need to express 12 as a sum of powers of 2

– what’s the largest power of 2 that is less than 12?

• 8 (2³)

• So 44 is 2⁵ + 2³ + 4

question 4

decimal	binary	hexadecimal
10	1010	a
15	1111	f
44	101100	2c

• So 44 is 2⁵ + 2³ + 4

– so we need to express 4 as a sum of powers of 2

^– 4 is just 2²

^• So 44 is 2⁵ + 2³ + 2²

– which means in binary, 44 is 101100

^• that is, 1*2⁵ + 0*2⁴ + 1*2³ + 1*2² + 0*2¹ + 0*2⁰

a comment about conversions

• What’s the point of this?

– these were simple numbers.

– it helps to know simple hex/binary/decimal numbers

extra credit

• It turns out that when the order of one base is a power of the order of another (i.e., 16 is 2⁴) it is very convenient to convert between them

– you only have to look at each group of 4 bits

• 4 bits represents a number from 0 to 15

• 1 hex digit represents a number from 0 to 15

question 5

• This is a perfectly legal expression

– ‘a’ and ‘b’ are character constants

– remember that characters are stored as numbers

– so ‘a’ and ‘b’ are simply integers, and they can be added together

homework #3

• Should be on the web page sometime today

• This is the first real program you get to write

– a program to print out a Fibonacci series

• 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, …

– Your program should prompt the user for the number of elements to print

• I’ve given you the code to use to read the number in from the keyboard