Intro to Scheme – The Greatest Language That Ever Was – Lists
Intro to Scheme – The Greatest Language That Ever Was – Lists
0 0
scheme-presentation
On Github keeyon2 / scheme-presentation
Intro to Scheme
The Greatest Language That Ever Was
Created by Keeyon Ebrahimi
What is Scheme
- Functional Programming Language
- One of the two dialects of Lisp
- Created to teach Functional Programming
Scheme Basics
Every expression is either an atom or a list You evaluate a list by applying the first element of the list to the rest of the arguments. Iteration (looping) in Scheme is accomplished via recursion Parentheses contain expressionsLists
> (list 1 2 3 4) (1 2 3 4) > '(1 2 3 4) (1 2 3 4) > (quote (1 2 3 4)) (1 2 3 4)
Lists can be nested
> '(1 2 (3 4) 5 6) (1 2 (3 4) 5 6)
Defining Variables
> (define x 5) > x 5
Defining Function
> (define (f x y)
(+ x y))
> (f 5 3)
8
Conditionals
> (define x 5)
5
> (if (= x 3) 'yes 'no)
no
> (cond ((= x 3) "x is three")
((= x (- 6 1)) 5)
(else 'I_DUNNO))
5
List Operations
Car Cdr ConsCar
Accesses the first element of a list
(car '(3 4 5 6)) 3
Cdr
Returns the rest of the the list after Car
(cdr '(1 2 3 4)) (2 3 4)
Example using Car and Cdr
Here is a function that that accesses the nth element of a list
> (define (nth n L)
(cond ((= n 1) (car L))
(else (nth (- n 1) (cdr L)))))
> (nth 4 '(2 4 6 8 10 12))
8
Cons
(cons x L) creates a new list whose first element is x and whose subsequent elements are the elements of the list L
> (cons 3 '(4 5 6)) (3 4 5 6)
Append
Combines 2 lists into a single list
(append '(1 2 3) '(4 5 6)) (1 2 3 4 5 6)
Now Lets write our own version
(define (app L1 L2)
(cond ((null? L1) L2)
(else (cons (car L1) (app (cdr L1) L2)))
))
> (app '(1 2 3) '(4 5 6))
(1 2 3 4 5 6)
Map
MAP applies a function to every element of a list, returning a list of the results
> (define (f x) (+ x 3)) > (map f '(2 4 6 8 10)) (5 7 9 11 13)
Lambda
(lambda (inputs) (operations))
> map (lambda (x) (* x 3)) '(1 2 3 4)) (3 6 9 12)
Let
Let Introduces local Variables
(define (foo)
(let ((x 3)
(y (+ 7 4)))
(* x y)))
> (foo)
33
Also have LET* and LETREC