242-203 Comp. Eng. SW Lab II: FP with Scheme 1 Computer Eng. Software Lab II 242-203, Semester 2, 2014-2015 Who I am: Andrew Davison CoE, WiG Lab Office

242-203 Comp. Eng. SW Lab II: FP with Scheme 1

Computer Eng. Software Lab II242-203, Semester 2, 2014-2015

Who I am:Andrew DavisonCoE, WiG Lab Office [email protected]

Functional Programming with Scheme

Please askquestions


Overview

1. Why Scheme?

2. Constants (names)

3. Functions

4. Lists

5. No-name Functions: lambda

6. cond

7. Recursion

continued


8. Higher Order Functions

9. Using DrScheme

10. More Information


1. Why Scheme?

• Scheme is one of the simpler functional programming languages– a Scheme program is a collection of functions

• Scheme also has assignment for greater programming flexibility– but it is not needed in most programs

continued


• Scheme has plenty of advanced features– e.g. first-class functions

• function body code can be manipulated as data• data can be used as function body code

• Scheme is used in lots of Artficial Intelligence (AI) projects and textbooks.


2. Constants (names)

myPi is evaluated thenprinted

'myPi is not evaluated

More details on howto use DrScheme aregiven in Section 9.

myPi is defined here

'pi is pre-defined


3. Functions

• A function call is written as:(operator arg1 arg2 … argn)

• For example:(+ 2 3) means 2 + 3


Function Call Examples

The + operator(and other arithmetic)operators can takeany number ofarguments.

The same as: (2+3)*(1+2)


Test Functions (Predicates)

• Test functions return boolean values true or false.

• Their names end with a '?'– e.g. number? char?


Defining Functions (v.1)

Similar C code is: int sq(int x) { return x*x; }


Similar C code is: int myMax(int x,int y) { if (x > y) return x; else return y; }


Combining Functions

• Scheme functions can be easily combined together– make a function call be the argument of another

function

function callarguments


4. Lists

• List values are placed inside (list …):– (list ) // an empty list– (list 1 2 3) // a list of numbers– (list 'a 'b 'c) // a list of names– (list (list 1 2) (list 2 3 4)) // a list of

two lists

• If the language includes list abbreviations, then (list ...) can be written as '(...)– '(1 2 3) is the same as (list 1 2 3)


Basic List Operations• (null? x)

– returns true is x is an empty list; otherwise returns false

• (car x)– returns the first element (head) of a non-empty

list x• (cdr x)

– returns the tail of a non-empty list x• everything except the first element

continued


• (cadr x)– returns the head element of the tail of x

• (cons h l)– makes a new list where the head is h, the tail is l

• (list arg1 arg2 … argn)– places all of its arguments into a new list


List Operation Examples


plusList plusList pulls out the first two arguments of alist and adds them


Some Other Useful List Functions

• (length x)– returns the length of a list x

• (append x y)– makes a new list by appending list x onto the

front of list y• (reverse x)

– makes a list by reversing the list x

and many, many more...

continued


combining reverseand append


5. No-name Functions: lambda

Similar to the C-like code: int ??(int x) { return x *x; }

Similar to a C-likefunction call: ??(3)

The language 'level' must beincreased for lambda to be supported.


Defining Functions (v.2)

Similar to the C-like code: sq2 = int ??(int x) { return x * x; }


Why have lambda?

• For simple examples (like ours), there is no need for lambda.

• lambda becomes very useful in higher order programming, where data and functions need to be translated into each other– e.g. parsing, AI


6. cond: the multi-branch if

Similar to the C code: int sizer(int x) { if (x == 0) return 0; else if (x > 0) return 1; else return -1; }


7. Recursion

• Base case:– the function returns an answer and stops

• Recursion step:– the body of the function calls itself (with

smaller arguments)


length Defined

recursive step

base case


append Defined


member Defined mem? is a predicate(a test function)

mem is the wrongname


8. Higher Order Functions

• A higher order function has 1 or more arguments which are function names– higher order functions are also called

functionals

• Higher order functions are very important to advanced functional programming and AI.

An ‘Intermediate Student’ feature


apply

• (apply f x)– its first argument, f, is a function name– its second argument, x, is the input for f– same as executing (f x)



map

• (map f x)– returns a list by applying the function f to each

element of the list x


map and plusList

plusList is appliedto the 4 lists in theinput list.


9. Using DrScheme

• Download the installation program for DrScheme from:

http://fivedots.coe.psu.ac.th/Software.coe/LAB/

FuncProg/

• The filename:plt-4.1.4-bin-i386-win32.exe

• Installs on Windows XP or later– the installation is called PLT Scheme, v4.1.4


• Start DrScheme from the Windows Start menu item PLT Scheme:

definitions go here

execute functionshere


Some Notes

• Set the language mode to “Beginning Student with List Abbreviations”– under the Language >

Choose Language menu item

continued


• Press the "Run" button after adding a new function to the definitions window– this makes the new function visible down in the

execution window


Create a Scheme Programusing any text editor


Load myMax.scmuse the File/Open menu item

Press the "Run" button for the execution window to appear at the bottom.


Use the myMax Function


• Alternatively, you can type the function into DrScheme's definition window, and save it from there.– you get colour-coded syntax, indenting

(tabbing), and error checking as you type


10. More Information

• The "Help/ Help Desk" menu item:


PLT Scheme Racket

• PLT Scheme v.5 was renamed to be called Racket– two websites:

• http://plt-scheme.org/ (frozen in 2010)• http://racket-lang.org/ (active)

• At the beginner's level (i.e. for us), there's no difference between the two languages.

Documents

242-203 Comp. Eng. SW Lab II: FP with Scheme 1 Computer Eng. Software Lab II 242-203, Semester 2, 2014-2015 Who I am: Andrew Davison CoE, WiG Lab Office