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Scheme Programming 1

Arrow Image Into to scheme programming



Well, to start with, I’ll explain what scheme is. Scheme is a derivative of the language Lisp which stands for “list processing”. Scheme is a functional programming language as opposed to a procedural programming language such as java or c++. If you’re just learning the basic concepts of computer programming, scheme is a good place to start since there is very little syntax required. Also, most scheme interpreters allow you to program in real time (i.e. you can write your code in the terminal window).


COMBINATIONS:
If you want to perform a simple operation such as 1+2+3+4, you would type: (+ 1 2 3 4) and the interpreter would output 10. + is not an operator in scheme, it is a procedure that is generally predefined. The general form for combinations is:
(Procedure Parameter1 Parameter2 … ParameterN)
Procedures such as + and * and so on can take in an arbitrary number of parameters.


SPECIAL FORMS:
In scheme, the interpreter generally evaluates from left to right, but sometimes it’s necessary to not evaluate an expres<i></i>sion in normal order.
If you want to create your own abstraction (i.e. a procedure or value with a name), you must use the ‘define’ special form.
( define (procedureName Parameter1 Parameter2 … ParameterN)
expres<i></i>sion1
expres<i></i>sion2

expres<i></i>sionN
)
NOTE: scheme will only return the last expres<i></i>sion in a procedure, so in a procedure with 4 expres<i></i>sions, the first three would just be performed, but not outputted. Also, the ‘define’ special form is actually a shortcut created for the user, and is actually calling up the ‘lambda’ special form, but I won’t get into that.


CONDITIONALS: (note: true is represented as #t and false is #f)

-If (just a basic if-else)
This procedure is called up in this form:
(if Predicate Consequent Alternative(optional) )
So (if (this is true) (do this) (otherwise do this) )
Example: ( if #t (+ x x) )

-Cond (basically an else if)
(cond (predicate1 consequent1)
(predicate2 consequent2)
(…)
(predicateN consequentN)
(else consequent)
)

-Booleans (can take an arbitrary number of arguments)
(and a b) returns a true if both a and b are true
(or a b) returns a true if a or b is true
(not a) returns the opposite of a

-Relational Operators (> < >= <= =)
(note: there is no != operator in scheme, so instead you could use (not (= a b)) )
(> 5 3) will return a #t because 5 is > 3


PAIRS:
You can create pairs in scheme by typing (cons parameter1 parameter2) and if you define that pair as an abstraction, you can refer to it and get the individual pieces of it with (car pairName) which will return the left side of the pair or (cdr pairName) which will return the right side.
Example: (cons 1 2) will output (1 . 2)


LISTS:
Scheme is basically built around this concept (since lisp stands for list processing). The list is created using the pair.
- Example: (cons 1 (cons 2 (cons 3 () ))) gives you (1 2 3)
The list must be terminated with () as you can see above. This empty set of parenthesis is referred to as nil.
There’s a predefined function for you though: (list 1 2 3) will give you (1 2 3).


I will be making a Scheme Programming 2 article soon to continue. If you\'re interested in trying scheme, i recommend downloading the \"dr. scheme\" interpreter (it\'s free).

Comments

Malson October 28 2007 - 13:45:54
Nice, I never got into scheme myself. Do you program in lisp as well?
mr noobon October 28 2007 - 16:32:27
sounds... different. i like lisp, but this seems a little too weird. interesting read though, rated very good Smile
ynori7on October 28 2007 - 17:50:47
Thanks. i've never used lisp, so i dont know how similar they are, but scheme can be useful once you understand it. it's a good language for teaching programmers how things like recursion and lists are really working.
mr noobon October 28 2007 - 19:04:48
i prefer to use ASM for that but if you say so Smile
Malson October 28 2007 - 22:02:45
ynori7: You're missing out. Try common lisp.
richohealeyon November 15 2007 - 11:31:31
lisp > scheme. although i'm a python nut, so i'm not sure jsut how much talking i can really do..
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