Scheme for Python Programmers CSE Valeria Montero

Scheme  LISP origins  AI (MIT): High level features reprogram Slower  Teaching  Scheme and Python: Lambda, deep lexicals

Scheme vs. Python  Synthax: Prefix vs. main-stream  Lexical Scope: Parentheses vs. Whitespace  Control Structures: Functional (tail-recursion-optimizing) vs... mainstream (while, for loops)  Support: Standard (R5RS, IEEE) vs. not  OOP: indirect support vs. Standard object system  Orientation: Mathematical vs. Processor

Scheme vs. Python  Implementation: Many vs. 2 synchronized. (Portable)  Garbage collection: Strong (cycles) vs. weak  Built in: No standard library vs. many data types, functions. (regular expressions, internet connectivity)  Macros vs. not  Scheme great for scripting/extension

Getting Started Define ;Create new objects: Global Variable or Function (define ) Operators ( …) Common Operators: and, or, not, -, +, *, %, /

Scheme Python Pre-Operator Order ( #t(+ 3 5) => 8(and #t #f) => #f (Blocks)

Quick Sort

Conditional (cond ( ) ( ) ( )) Absolute Value: (define (abs x) (cond ((< x 0) (- x)) (x))) Python: If Elif Else

CAR and CDR (car list) => first element (car '(a b c)) => a (cdr list) => list excluding car (cdr '(a b c)) => (b c)

Play Around with Nested Lists: Reference maximum four car/cdr (cadar '((1 2 3) 3)) => 2 (cadr '(1 2 3)) => 2 (cadar '((1 2 3) 4 5 6))=> 2 (cddddr '( ))=>(5 6)

APPEND and CONS Append: Concatenates two lists Cons: Constructs list with input: car, cdr > (append '(1 2) '(3 4)) ( ) > (cons '(1 2) '(3 4)) ((1 2) 3 4)

Let Variables are bounded locally within Let body. (let ((variable 1 value 1 )...) expression 1 expression 2...) (let ((a (* 4 4))) (+ a a)) => 32 (let ((+ *)) (+ 2 3)) => 6 Absolute Value: (define (abs n) (let ((a n) (b (-n))) (max a b)))

Let vs. Let* Let evaluates the variables in parallel. Let* evaluates the variables sequentially. (let ((a 5) (b (* a 10))) b) => reference to undefined identifier: a (let* ((a 5) (b (* a 10))) b) => 50 Python Parallel assignments a, b = b, a

Scheme Functional Programming: Lambda Calculus COND Absolute Value: (define (abs x) (cond ((< x 0) (- x)) (x))) LET Absolute Value: (define (abs n) (let ((a n) (b (-n))) (max a b))) Lambda (λ) ((lambda (variable 1...) expression 1 expression 2...) value 1...) λ Absolute Value: (define abs (λ (n) (if (>= n 0) n (- n))))

List Recursion Style Style Style …

Permutations

For Comparison

Do: Iterative vs. Recursive (do ((variable 1 value 1 ) (variable 2 value 2 ) …) ((exitCondition) (exitFunction)) Body)

Fun Debugging in DrScheme (car '()) > car: expects argument of type ; given () UESTIONS?