1. Racket Introduction CSC270 Pepper
major portions credited to

2. What is Dr. Racket? Racket Dr. Racket
Full spectrum programming language with roots in functional programming which is a type of the declarative paradigm
Lisp / Scheme
Formerly called PLT Scheme
Objects, types, laziness, macros, new syntax builder
Dr. Racket
Integrated Development Environment

3. Declarative vs Imperative
What not how
Language can figure out how when you tell it what
No side effects –
No mutatable variables
Express data flow
Imperative
Commands manipulate state of system and variables.
Many side effects
Mutable variables
Control flow

4. Imperative vs. Functional (Declarative)
From :

5. Contrast: Imperative Types
Pure Imperative
SQL Data Manip Lang (DML : insert, update, delete)
Procedural
Exactly how algorithms
First do this and next do that C
Object Oriented
Manipulate objects through predefined methods
Classes
Send messages between objects
C++, Java

6. Another Functional Language: Excel
Formulas express data flow
Command sequence is not a consideration when coding Excel formulas
Cells hold one target value – changing inputs will change the value, but you never do anything with its value over time.

7. Declarative types Domain specific
Logic: Relationships defined in terms of inference rules
Prolog
Functional: Relationships defined in terms of functions
Haskell,
Subset of Racket
Subset of F#
Excel
Domain specific
Regular Expressions
SQL Select

8. Some Functional Advantages
Readability and Maintainability
One task and no reliance on an external state
Refactoring
Because the insides of the function do not effect anything outside itself, as long as the output is the same, refactoring is fine
Testing
Tested in isolation

9. Racket Strengths Language syntax builder
Continuations (capture future value of a computation and pass it around)
Dynamic typing
Manages its own memory
Function creation on the fly (lambda)
Function closure
Can pass it like an object
Remembers variable values of creation time scope

10. How to install Racket Panther will run Racket programs without IDE
racket programfile
Panther will run IDE with SSH X Forwarding to allow X Window System GUI
drracket
See a racket window good
See Gtk initialization failed for display – no x windows
Download on PC or Mac
IDE
choice : racket language with #lang racket
Context sensitive F1 help

11. Hello Racket Program #lang racket "Hello Racket" Save as hello.rkt
IDE: File / save definition
Running the program
IDE : run button
Panther without xterm: racket hello.rkt

12. Hello Racket with a defined variable
#lang racket
(define hellovar "Hello Racket again")
Hellovar
Notice that a variable is defined inside parentheses
All commands inside parentheses

13. Hello Racket With a Function
#lang racket
(define (sayhi ) "Hello from the function")
(sayhi)
Notice how the function call is in ()
Notice the function definition syntax used here: (define (function name ) (stuff function does))
Balanced parentheses

14. Comments Block comments: #| … |# Single comments: ; #lang racket
; define a function called sayhi
(define (sayhi ) "Hello from the function")
; and now call it
(sayhi)

15. Rules about literals String: " " (use \" to type a text quote)
number: 1, 1.3, 1/2, 1+2i, 6.003e+15, #x1A, #b10111, #o737,
will store a rational
true/false : #t for true, #f for false
logical: not, and, or : ex: (not #t) is false and (and 1 2) is false
Suppress expansion: just one leading ' : '(function a b) will be text not a function

16. Parentheses
Do not put a literal inside ()or Racket will evaluate it as a function
#lang racket
(define x 3) x
(x) ; racket hates this
(define (sayhi ) "Hello from the function")
(sayhi)
sayhi
; racket does not hate this, but wont run the sayhi function

17. Variable use ;Define for the program ;Define locally
(define x "outside")
;Define locally
(let ([x "inside"])
x) ; displays "inside"
x ; displays "outside"
; Define function argument
(define (myfunc num)
(+ num 3)) ;
(myfunc 4) ; displays 7
; Change a variable (let's avoid it)
(set! x 8)
x; displays 8

18. Pictures Variable can contain a picture (require picturing-programs)
(define dog )
(define cat )
( above dog1 cat1)
(above (flip-vertical dog1)
(above dog1 cat1))

19. Variables Rules Summary
definition: (define varname value)
example: (define x 3)
use: just use the name; example: x
define locally inside a let expression: (let ([varname value]) expression )
use let * if you want to use the first set of variables to define another set
use a variable: just the name - do not put a variable inside () or Racket will evaluate it as a function
change a variable – let's avoid it: (set! varname value) example: (set! n (add1 n))

20. Arithmetic: All arithmetic is a function
syntax: ( operator operand#1 operand#2)
operators: +,-,/,*,expt,quotient, remainder,
special operators: exact->inexact (from rational to real), gcd, lcm
(+ 1 2)
(/ 5 2) ; not integer division!
(expt 2 3) ; 2 to the 3rd power
(remainder 11 3) ;

21. Functions Already defined functions with parms
Return is value of last expression
(define (add8 num)
"hello"
(+ num 8)
"hello again")
(add8 3)
Resolves to "hello again"

22. Simulate Excel Define 2 cells, one for income and one for deductions
Define another cell that represents your gross income (income – deduction)
Define another cell that represents your taxes at 30%

23. Booleans #t is true; #f is false = or eq? are functions
Use = for numbers only
(= 3 3.0) will be #t
(eq? 3 3.0) will be #f
(eq? "abc" "abc") will be #t
(not (eq? "abc" "def")) will be #t
<, > , <=, >=,

24. Decision - Cond (cond [ (= 1 x) (add1 x) ] [ (= 2 x) (+ x 4) ]
[ else (+ x 6 ) ] )
2 when x = 1;
6 when x = 2
13 when x = 7

25. Random
(random 6) ; gives 0 to 5 (+ (random 6 ) 1 ) gives a dice value Create a throw dice function that rolls 2 dice and returns the total. What are your inputs? What is your output? What is your function name? No need to display the individual dice

26. Dice Roll
(define (roll ) ( + (+ (random 6 ) 1) (+ (random 6 ) 1) )) (roll)

27. Repetition - Recursion
add from 1 to a max value
(define (addnum max)
(cond [ ( = max 0) 0 ]
[ else ( + max
(addnum (- max 1))) ] ))
(addnum 5) ; gives 15

28. Recursion Thought Process
1) What is true about the problem? (truth statements will end up in your code)
2) What are the base cases? (small simple truths - adding up 0 numbers yields 0)
3) What are you taking in and what is being returned ? ( give a max and get a total)
4) Make some samples:
Addnum(0) should give 0
Addnum(1) should give 1
Addnum(2) should give 3
Addnum(3) should give 6
Addnum(4) should give 10

29. Test Cases before coding
(require test-engine/racket-tests)
;; addnum function adds from 1 to a max argument
;; input max number
;; output total of 1 to argument
(define (addnum 0) 0)
(check-expect (addnum 0 ) 0)
(check-expect (addnum 1 ) 1)
(check-expect (addnum 3 ) 6)
(check-expect (addnum 4 ) 10)
(check-expect (addnum 10 ) 55)
(check-expect (addnum -1 ) 0)
(test)

30. Coding the recursion
Define the function without a body giving names to input arguments
(define (addnum num ) )
Fill in the body with a cond
(cond [ ( ) ]
[ else ])
Put the base case into the first condition
(cond [ ( num <= 0 ) ]

31. Coding the Recursive Call
Consider how to handle one pass of the repetition;
think about one of the later calls as a sample (addnum 4)
Write what is available to you
Your input arguments
Good return values from your function (see your tests)
Define the rest of the information when one part is removed
Call that part recursively
(cond [ (<= num 0 ) ]
[ else num + addnum(num-1) ])

32. Summary Define Functional Programming
Declarative vs Imperative paradigms
How to enter literals
Create and use variables
Create and use functions
Decisions
Recursive functions
Parentheses, Parentheses, Parentheses