CS1: Introduction to Computation

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CS1: Introduction to Computation

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Title: CS1: Introduction to Computation

1
CS1Introduction to Computation

Day 20 December 5, 2007
Java

2
The Hitchhiker's Guide to the Galaxy (1981)
3
Computer languages...

Are like a Tower of Babel
Many mutually-incomprehensible dialects
Our goal CS 1 should be a kind of "Babel fish"
enabling you to rapidly absorb new languages as
needed
We test this theory today

4
Reminisce

On day 1
we saw that we already knew many of the forms
and concepts needed to write Scheme code
We simply needed to
pick out the ones to use
be a bit more precise about how to specify a few
things

5
Throughout the term

Do arithmetic
Work with types
numbers, booleans, pairs, procedures
Perform conditional operations
if, cond
Write functions
Build compound data types
cons

Change values
set!, set-car!, set-cdr!
Reason about the meaning of programs
Build generic data types
Message passing
Manage program complexity
abstraction

6
Claim

Now that you know all these things
you know most of the concepts which arise in all
computer languages
? Largely a matter of concept-mapping and
transliteration to understand the others
Today transliteration to Java

7
Java

Language syntax roots in "C", "C"
We'll learn a bit of C, too.
Language features similar to Scheme
Mostly a subset!
Map analogies between features
A couple of new concepts
static typing
explicit framework for message-passing objects

8
Jumping Right In
9
Summing integers

public int sum (int n)
if (n 0)
return(0)
else
return(n sum(n - 1))

(define (sum-integers n)
(if ( n 0)
0
( n (sum-integers (- n 1)))))

Java
10
Summing integers

public int sum (int n)
if (n 0)
return(0)
else
return(n sum(n - 1))

(define (sum-integers n)
(if ( n 0)
0
( n (sum-integers (- n 1)))))

Scheme
11
Differences

public int sum (int n)
if (n 0)
return(0)
else
return(n sum(n - 1))

(define (sum-integers n)
(if ( n 0)
0
( n (sum-integers (- n 1)))))

12
Differences

public int sum (int n)
if (n 0)
return(0)
else
return(n sum(n - 1))

(define (sum-integers n)
(if ( n 0)
0
( n (sum-integers (- n 1)))))

Variables are explicitly typed in Java
13
Differences

public int sum (int n)
if (n 0)
return(0)
else
return(n sum(n - 1))

(define (sum-integers n)
(if ( n 0)
0
( n (sum-integers (- n 1)))))

Different symbols used for certain operators
14
Differences

public int sum (int n)
if (n 0)
return(0)
else
return(n sum(n - 1))

(define (sum-integers n)
(if ( n 0)
0
( n (sum-integers (- n 1)))))

infix instead of prefix notation for many
operations
15
Recall

prefix operator comes before operands
infix operator comes between operands
for some special two-operand functions
e.g. - / etc.

infix instead of prefix notation for many
operations
16
Differences

public int sum (int n)
if (n 0)
return(0)
else
return(n sum(n - 1))

(define (sum-integers n)
(if ( n 0)
0
( n (sum-integers (- n 1)))))

still use prefix notation (with parentheses in
new place) for function calls
17
Differences

public int sum (int n)
if (n 0)
return(0)
else
return(n sum(n - 1))

(define (sum-integers n)
(if ( n 0)
0
( n (sum-integers (- n 1)))))

return statement(s)
18
Differences

public int sum (int n)
if (n 0)
return(0)
else
return(n sum(n - 1))

(define (sum-integers n)
(if ( n 0)
0
( n (sum-integers (- n 1)))))

access modifier
19
Differences

public int sum (int n)
if (n 0)
return(0)
else
return(n sum(n - 1))

(define (sum-integers n)
(if ( n 0)
0
( n (sum-integers (- n 1)))))

curly braces ( ) and semicolons ( ) instead
of parentheses
20
Summing integers

public int sum (int n)
if (n 0)
return(0)
else
return(n sum(n - 1))

(define (sum-integers n)
(if ( n 0)
0
( n (sum-integers (- n 1)))))

The differences here are largely superficial!
21
Expressions in Java

Infix notation
Parentheses are often optional
? when omitted, language follows a defined set of
precedence rules.
Examples in Java and Scheme
2 3 ( 2 3)
axx bx c ( ( a x x) ( b x) c)
(ax b)x c ( ( ( ( a x) b) x) c)
((a0) (a a 0) (

22
Static Typing

In Java, we must always explicitly declare types
of values
of procedure return value
of procedure arguments
of local variables
Types have to match for code to execute
true in Scheme, too, but not enforced until
program actually runs
in Java, "type mismatches" are detected at
compile time
what's "compile time"?

23
Java code life cycle

Write code in text editor
Save to text file e.g. somefile.java
Compile at command line
javac somefile.java
some kinds of errors detected reported here
Successful compilation produces "machine-level"
code in somefile.class
Compiled files can be executed at command-line
java somefile

24
Primitive types in Java

int
finite-value, 32-bit storage
1 bit used for sign
so max magnitude 231
long
just like int, but 64 bits

25
Primitive types in Java

float
IEEE floating-point number, 32 bits
1 bit for sign 8 bits for exponent 23 bits
for "fraction" part (AKA "mantissa")
e.g. 6.02E23
double
IEEE "double-precision" float, 64 bits
1 sign bit 11 bits for exponent 52 mantissa
bits
e.g. 6.02E1332

26
Primitive types in Java

boolean
similar to Scheme
t in Scheme ? true in Java
f in Scheme ? false in Java
char
"character"
16 bits, no sign bit
(lots of international characters!)

In Java, mutation is the norm
int x 2
? (define x 2)
x 5
? (set! x 5)
x 2 x
? (set! x ( 2 x))
int y 4 foo(x)
? (define y ( 4 (foo x)))

is not a comparison operator!
Since is "taken" for assignment operation, what
to use for comparison?
e.g. boolean b (x y)
Common source of confusion

29
Java Statements

In addition to infix operations, Java has
"statements," many of which exist entirely for
their side effects
any return value intentionally tossed aside and
ignored
All statements end with a semi-colon
a b c (set! a ( b
c))
d sum(n) (set! d (sum
n))
return (n sum(n - 1)) ( n (sum (- n 1))

30
Procedure definitions

Procedure in Scheme
(define (foo arg1 arg2) ...)
Procedure in Java
foo( arg1, arg2)
...
? comma-delimited arguments inside parens
types of arguments included in list
procedure name outside and in front of parens
return type before procedure name

31
Procedures in Java

Can be used in expressions
f (x bar(q r s)) z
compare (set! f ( ( x (bar ( q ( r s)))) z))
Can be used as statements for their side-effects
bar(43)
compare (bar 43)

32
Procedure Definition (in detail)

return-type procname(type1 arg1,
type2 arg2,
.)
type var initial-value
type var2 initial-value // etc.
statement1
statement2 // etc.

33
Sample Procedure Definition

int aquad(int a, int b, int c, int x)
int res 0
res resc
res resbx
res resaxx
return(res)

(define (aquad a b c x) (let ((res 0))
(set! res ( res c)) (set! res ( res ( b
x))) (set! res ( res ( a x x)))
res)))
34
Comments, 1

Two ways to make comments.
/ this is a Java comment /
/ comments can span
multiple lines /
Open with /
Close with /
Can be almost anywhere
a b / will you date me? / c
Often called "block comments"

35
Comments, 2

Another comment style similar to Scheme
Open with //
Comment extends to the end of the line
// this is a comment
a b c // sure, why not?

36
Statement if

if is a built-in statement in Java
not an expression like in Scheme!
Does not return/evaluate to a value
So, anything interesting better happen via
side-effects
assign to (mutate) a value
print something to the screen
write to a file
else is an explicit keyword

37
Statement if

Form
if (test-expression)
if-clause
else
else-clause
else part is optional

38
if Example

int max (int a, int b)
int res
if (a b)
res a
else
res b
return (res)

(define (max a b) (let ((res 0)) (if
( a b) (set! res a)
(set! res b)) res) )
39
if Example

int max (int a, int b)
int res
if (a b)
res a
else
res b
return (res)

(define (max a b) (let ((res 0)) (if
( a b) (set! res a)
(set! res b)) res) )
40
Block statements

In Java, anywhere you can put a single statement,
you may put a block statement instead
Block statement an arbitrary number of
statements inside a pair of curly braces
variable declarations
statement1
statement2
Similar to Scheme begin expression

41
Block example

int tmp1 max(a, b)
int tmp2 min(a, b)
c c - tmp2
d d - tmp2
e c / (tmp1 - tmp2)
f d / (tmp1 - tmp2)

(let ((tmp1 (max a b))
(tmp2 (min a b)))
(set! c (- c tmp2))
(set! d (- d tmp2))
(set! e (/ c (- tmp1 tmp2)))
(set! f (/ d (- tmp1 tmp2))))

42
Blocks often convenient with if

if (a b)
max a
min b
else
max b
min a

(if ( a b) (begin (set! max a)
(set! min b)) (begin (set! max
b) (set! min a)))
43
return Statement

return can occur anywhere in a procedure
Explicit mechanism for exiting a procedure
immediately
Since procedures have a defined return type, the
value passed to return must have that type

44
Example

int scalebc(int a, int b, int c)
int LARGE 1000000000 // for instance...
if (b0) return(LARGE) // avoid division
by 0
if (c0) return(LARGE) // similarly
return (a / (b c)) // return an int

45
Iteration for loop

Vaguely similar to Scheme do loop
Has a "continue-test" instead of an exit-test
for ( )
look familiar? ?

46
for example

Start with i set to 0,
while i
execute res res i
execute i i 1
go back to 2.

int sum(n)
int res 0
int i
for (i 0 i
res res i
return(res)

47
for with block statement

int sum 0
int zeros 0
for (int i0 ii i 1
sum f(i) // shortcut for sum sum f(n)
if (f(i) 0)
zeros

48
Interlude
49
Useful reference
50
(ok, not really)
51
Admin Interlude
52
Admin

Course Feedback Questionnaires
Pick up, fill out, turn in now.
More thoughts? Email!
Lectures, interludes
Ideas?
Final Ombuds lunch tomorrow noon
mmmmm... foood...

53
Labs

No rework on lab9
and no min grading
Solutions for all labs posted on weekend
(Don't consult if reworking, obviously)

54
Final

Final exam cover sheet available online soon.
Read it in advance.
cs1man submit final final.txt
turn in hard copy of diagrams to box in lab
Thursday/Friday recitations ? open QA
Final out Saturday, due Friday 4am
We'll grade Friday afternoon, so don't be late!

55
Final review

Final review
Thursday (tomorrow) 9pm JRG 074
go over exam-related material
can ask questions, work through examples, etc.

56
Final Content

List Processing
Tagged Data and Message-Passing
Side-effects and Mutation
Environment model
Draw environment diagrams
Reason about sameness/equality
Remember stuff from first half
Recursion, lambda, complexity, evaluation, .
Read/understand code
Decompose/design operations

57
Movie clip

Futurama math lecture
Play Spot-the-nerdTM

58
(No Transcript)
59
(No Transcript)
60
return

Back to Java

61
Message-passing

We called message-passing "the root of
Object-Oriented (OO) programming."
Java is an OO language.
Contains explicit, built-in constructs for
designing, building, implementing "objects"
i.e. amalgamations of code and data

62
Classes

class Complex
private int realVal 0
private int imagVal 0
public Complex(int real, int imag)
realVal real
imagVal imag
public int getReal()
return(realVal)
public int getImag()
return(imagVal)

63
Classes

class Complex
private int realVal 0
private int imagVal 0
public Complex(int real, int imag)
realVal real
imagVal imag
public int getReal()
return(realVal)
public int getImag()
return(imagVal)

data members
64
Classes

class Complex
private int realVal 0
private int imagVal 0
public Complex(int real, int imag)
realVal real
imagVal imag
public int getReal()
return(realVal)
public int getImag()
return(imagVal)

procedures
65
Classes

class Complex
private int realVal 0
private int imagVal 0
public Complex(int real, int imag)
realVal real
imagVal imag
public int getReal()
return(realVal)
public int getImag()
return(imagVal)

access modifiers
66
Classes
Usage Complex foo new Complex(1, 3) int wow
foo.getReal() int yay foo.getImag()

class Complex
private int realVal 0
private int imagVal 0
public Complex(int real, int imag)
realVal real
imagVal imag
public int getReal()
return(realVal)
public int getImag()
return(imagVal)

67
Classes
Usage Complex foo new Complex(1, 3) int wow
foo.getReal() int yay foo.getImag() foo.realVa
l // compile error!

class Complex
private int realVal 0
private int imagVal 0
public Complex(int real, int imag)
realVal real
imagVal imag
public int getReal()
return(realVal)
public int getImag()
return(imagVal)

68
Objects

Classes act as definition of "message-passing"
object.
Messages ? regular procedures!
We call those procedures "methods" in OO-speak
Methods are executed on an instance of a class
using dot (.) operator
foo.someMethod()
foo.someOtherMethod(55.2334, true)

69
Objects

"Access modifier"
in front of each data member
in front of each method
public
accessible to outside user
private
inaccessible to outsiders
accessible to internal code within object
Very similar to Scheme
except enforced by keyword(s) and Java compiler
rather than by a more implicit set of
environment rules

70
Complex

Defining class Complex creates a new type in
Java.
Objects of type Complex can now be manipulated
just like int, float, char
Complex a new Complex(1, 3)
Complex b new Complex(15, -2)
Complex result someFunction(a)

71
Complex addition

Could now write standalone add
public Complex add(Complex a, Complex b)
return (new Complex(a.getReal() b.getReal(),
a.getImag() b.getImag()))

72
Complex addition

Usage
Complex a new Complex(1, 3)
Complex b new Complex(15, -2)
Complex c add(a, b)

73
Complex addition

Alternative
add add as a "method" (message) instead?
Put inside class definition block...

74
add method

public Complex add(Complex other)
return(new Complex
(realVal other.getReal()),
(imagVal other.getImag()))

75
add method

Usage
Complex a new Complex(1, 4)
Complex b new Complex(-5, 25)
Complex c a.add(b)

76
Whats Missing?

Java does not have lambda
Procedures can't be passed in as arguments
Procedures do not have their own persistent
environments (their own state)
Procedures cannot return new procedures

77
What did we miss?

Java brings us arrays
Contiguous blobs of homogeneous data
Very fast look-up
Expensive to resize
Note Scheme has a kind of array called a
"vector" too
not homogeneous (can contain arbitrary types of
data)

78
What did we miss?

Around objects, Java adds
a system to create hierarchies of objects
additional access modifiers to baffle the mind
and
occasionally control inter-object access
privileges in useful ways

79
What did we miss?

Java's compiled "machine-level" code is actually
an intermediate "bytecode"
Any architecture for which a suitable interpreter
of that bytecode exists can execute the exact
same compiled program
Available for Linux, Windows, OS/X, etc.
Some Schemes work like this too

80
What did we miss?