Class 16 Mutation M C Escher Day and

Class 16: Mutation M. C. Escher, Day and Night CS 200: Computer Science David Evans University of Virginia 24 February 2003 CS 200 Spring 2003 http: //www. cs. virginia. edu/evans Computer Science

Menu • Mutation Primitives • PS 5 24 February 2003 CS 200 Spring 2003 2

From Lecture 3: Evaluation Rule 2: Names If the expression is a name, it evaluates to the value associated with that name. > (define two 2) > two 2 24 February 2003 CS 200 Spring 2003 3

Names and Places • A name is not just a value, it is a place for storing a value. • define creates a new place, associates a name with that place, and stores a value in that place (define x 3) 24 February 2003 CS 200 Spring 2003 x: 3 4

Bang! set! (“set bang”) changes the value associated with a place > > 3 > > 7 (define x 3) x x: 7 3 (set! x 7) x 24 February 2003 CS 200 Spring 2003 5

set! should make you nervous > (define x > (nextx) 3 > (nextx) 4 >x 4 24 February 2003 2) Before set! all procedures were functions (except for some with side-effects). The value of (f) was the same every time you evaluate it. Now it might be different! CS 200 Spring 2003 6

Defining nextx (define (nextx) (set! x (+ x 1)) x) syntactic sugar for 24 February 2003 (define nextx (lambda () (begin (set! x (+ x 1)) x)))) CS 200 Spring 2003 7

Evaluation Rules > (define x 3) > (+ (nextx) x) 7 Dr. Scheme evaluates or 8 application sub> (+ x (nextx)) 9 or 10 24 February 2003 CS 200 Spring 2003 expression left to right, but Scheme evaluation rules allow any order. 8

set-car! and set-cdr! (set-car! p v) Replaces the car of the cons p with v. (set-cdr! p v) Replaces the cdr of the cons p with v. These should scare you even more then set!! 24 February 2003 CS 200 Spring 2003 9

> (define pair (cons 1 2)) > pair (1. 2) pair: 1 24 February 2003 CS 200 Spring 2003 2 10

> (define pair (cons 1 2)) > pair (1. 2) pair: > (set-car! pair 0) > (car pair) 0 1 21 0 > (cdr pair) 2 > (set-cdr! pair 1) > pair (0. 1) Any reason to be afraid yet? 24 February 2003 CS 200 Spring 2003 11

> pair (0. 1) > (set-cdr! pair) > (car pair) pair: 0 > (car (cdr pair))) 0 > pair #0=(0. #0#) 24 February 2003 CS 200 Spring 2003 2 1 1 pair 0 12

Functional Programming • Programming without mutation – Side-effects like printing and drawing on the screen are really mutations (of the display, printer, bell, etc. ) • If an expression has a value, it is always the same – order of evaluation doesn’t matter • Substitution mode of evaluation works fine 24 February 2003 CS 200 Spring 2003 13

Imperative Programming • Programming with mutation (assignment) • Value of an expression might be different depending on when it is evaluated • Substitution model of evaluation doesn’t work anymore! 24 February 2003 CS 200 Spring 2003 14

Why Substitution Fails? (define (nextx) (set! x (+ x 1)) x) > (define x 0) > ((lambda (x) (+ x x)) (nextx)) 2 Substitution model: (+ (+ 0 (nextx)) (begin (set! x (+ x 1)) x)) (begin (set! 0 (+ 0 1)) 0)) 0 0) 24 February 2003 CS 200 Spring 2003 15

Why would a programming language allow mutation? • Does it allow us to express computations we couldn’t express without it? No! We can express all computations without mutation. (We’ll see why before the end of the course…) • Mutation allows us to express some computations more naturally and efficiently – But it also makes everything else more complicated 24 February 2003 CS 200 Spring 2003 16

Adelphiaize • Define a procedure that takes a list of numbers and replaces each positive number with 0. > (adelphiaize (list 23 -3 5 6 0 -12)) (0 -3 0 0 0 -12) 24 February 2003 CS 200 Spring 2003 17

Functional Solution • Build a completely new list (define (adelphiaize lst) (if (null? lst) lst (cons (min 0 (car lst)) (adelphiaize (cdr lst))))) 24 February 2003 CS 200 Spring 2003 18

Imperative Solution • Mutate the input list (define (adelphiaize! lst) (if (null? lst) (void) ; nothing to return (begin (if (< (car lst) 0) (set-car! lst 0)) (adelphiaize! (cdr lst))))) 24 February 2003 CS 200 Spring 2003 19

Programming with Mutation > (adelphiaize! (list 23 -5 12 -2 0)) > (define profits (list 23 -5 12 -2 0)) > (adelphiaize! profits) > profits (0 -5 0 -2 0) Imperative > (define profits (list 23 -5 12 -2 0)) > (adelphize profits) (0 -5 0 -2 0) > profits (23 -5 12 -2 0) Functional 24 February 2003 CS 200 Spring 2003 20

PS 5: 24 February 2003 CS 200 Spring 2003 21

Databases • Database is tables of fields containing values • Commands for inserting, selecting and mutating entries in the tables number lastname firstname college 1 Washington George none 2 Adams John Harvard 3 Jefferson Thomas William and Mary 24 February 2003 CS 200 Spring 2003 22

Representing Tables • A table is just a cons pair: – fields (list of fields) – list of entries • Each entry is a list of values number lastname firstname college 1 Washington George none 2 Adams John Harvard 3 Jefferson Thomas William and Mary (define presidents (make-table (list ‘number ‘lastname ‘firstname ‘college))) (table-insert! presidents (list 1 “Washington” “George” “none”)) 24 February 2003 CS 200 Spring 2003 23

Selecting Entries number lastname firstname college 1 Washington George none 2 Adams John Harvard 3 Jefferson Thomas William and Mary > (table-select presidents ‘college (lambda (college) (not (string=? pitem “Harvard”)))) ((‘number ‘lastname ‘firstname ‘college). ((“Washington” “George” “none”) (“Jefferson” “Thomas” “William and Mary”)) 24 February 2003 CS 200 Spring 2003 24

PS 5 • You will implement database commands for selecting and deleting entries from a table, and use your database to implement an auction service • The database commands similar to SQL used by commercial database – In PS 8, you will use SQL to interact with a provided database 24 February 2003 CS 200 Spring 2003 25

Charge • PS 5 – You know everything you need to do it after today, so start early! – More open ended than previous problem sets • Wednesday – How does mutation effect our evaluation rules? • Friday: no class 24 February 2003 CS 200 Spring 2003 26