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Administrivia

Administrivia. Mid-semester survey this week (Thur/Fri) you NEED to do this. Interviews should be starting in a few weeks. I apologize for not getting the midterm solutions and last week's lecture on-line: check tomorrow. > (find-evens '(2 3 4 5 6)). sent = ( 2 3 4 5 6 ). (se 2.

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Administrivia

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  1. Administrivia • Mid-semester survey this week (Thur/Fri) • you NEED to do this. • Interviews should be starting in a few weeks. • I apologize for not getting the midterm solutions and last week's lecture on-line: check tomorrow.

  2. > (find-evens '(2 3 4 5 6)) sent = ( 2 3 4 5 6 ) (se 2 sent = ( 3 4 5 6 ) sent = ( 4 5 6 ) (se 4 sent = ( 5 6 ) sent = ( 6 ) (se 6 sent = ( ) () • (se 2 (se 4 (se 6 ()) • (2 4 6)

  3. Find all the even numbers (define (find-evens sent) (cond ((empty? sent) ;base case '() ) ((odd? (first sent)) ;rec case 1 (find-evens (bf sent)) ) (else ;rec case 2: even (se (first sent) (find-evens (bf sent))) ) ))

  4. (define (pascal C R) (cond ((= C 0) 1) ;base case ((= C R) 1) ;base case (else ;tree recurse (+ (pascal C (- R 1)) (pascal (- C 1) (- R 1)) )))

  5. > (pascal 2 5) (pascal 2 5) (pascal 2 4) (+ (pascal 2 3) (+  1 (pascal 2 2) (+ (pascal 1 2) (+ (pascal 1 1)  1 (pascal 0 1)  1 (pascal 1 3) (+ (pascal 1 1)  1 (pascal 1 2) (pascal 0 1)  1 (pascal 0 2)  1 (pascal 1 4) (pascal 1 3) (+ (+ (pascal 1 1) (pascal 1 2)  1 (pascal 0 1)  1  1 (pascal 0 2) (pascal 0 3)  1

  6. What is a procedure? (or, a function).

  7. Treating functions as things • “define” associates a name with a value • The usual form associates a name with a object that is a function (define (square x) (* x x)) (define (pi) 3.1415926535) • You can define other objects, though: (define *pi* 3.1415926535) (define *month-names* ‘(january february march april may june july august september october november december))

  8. Consider two forms of “month-name”: (define (month-name1 date) (first date)) (define month-name2 first)

  9. Why have procedures as objects? Other programming languages don’t (often)

  10. Procedures can be takenas arguments… (define (apply-to-5-and-3 func) (func 5 3)) (define (math-function? func) (or (equal? func +) (equal? func -) (equal? func *) (equal? func /)))

  11. …and procedures can be returned from procedures (define (choose-func name) (cond ((equal? name ‘plus) +) ((equal? name ‘minus) -) ((equal? name ‘divide) /) (else ‘sorry))) (define (make-add-to number) (lambda (x) (+ number x))) (define add-to-5 (make-add-to 5))

  12. Higher order function (HOFs) • A HOF is a function that takes a function as an argument. • There are three main ones that work with words and sentences: • every – do something to each element • keep – return only certain elements • accumulate – combine the elements

  13. Patterns for simple recursions • Most recursive functions that operate on a sentence fall into: • Mapping: square-all • Counting: count-vowels, count-evens • Finding: member, first-even • Filtering: keep-evens • Testing: all-even? • Combining: sum-evens

  14. (define (square-all sent) (if (empty? sent) ‘() (se (square (first sent)) (square-all (bf sent)) )) (square-all ‘(1 2 3 4 5)) (every square ‘(1 2 3 4 5))

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