Writing Functions in Scheme. Writing Functions in Scheme. Checking My Answer: Empty List. Checking My Answer: Empty List

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Britton Gibbs
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1 Writing Functions in Scheme Writing Functions in Scheme Suppose we want a function ct which takes a list of symbols and returns the number of symbols in the list (ct (a b c)) 3 (ct ()) 0 (ct (x y z w t)) 5 How can we write this function? Answer #1: Have the instructor write it ;; ct : <list-of-sym> -> <num> ;; (ct ()) 0 ;; (ct (a b c)) 3 [else (ct (cdr l)))])) Checking My Answer: Empty List Checking My Answer: Empty List [else (ct (cdr l)))])) [else (ct (cdr l)))])) [else (ct (cdr l)))])) [else (ct (cdr l)))])) (ct ()) [(null? ()) 0] [else (ct (cdr ())))]) [(null? ()) 0] [else (ct (cdr ())))]) [#t 0] [else (ct (cdr ())))]) 1-4

Answer #1: Have the instructor write it ;; ct : <list-of-sym> -> <num> ;; (ct ()) 0 ;; (ct (a b c)) 3 [else (ct (cdr l)))])) Checking My Answer: Empty List

2 Checking My Answer: Empty List [else (ct (cdr l)))])) [#t 0] [else (ct (cdr ())))]) [else (ct (cdr l)))])) 0 [else (ct (cdr l)))])) (ct (a b c)) [else (ct (cdr l)))])) [(null? (a b c)) 0] [else (ct (cdr (a b c))))]) [else (ct (cdr l)))])) [else (ct (cdr l)))])) [else (ct (cdr l)))])) [else (ct (cdr l)))])) [(null? (a b c)) 0] [else (ct (cdr (a b c))))]) [else (ct (cdr (a b c))))]) [else (ct (cdr (a b c))))]) (ct (cdr (a b c)))) 5-8

(a b c)) 0] [else (ct (cdr (a b c))))]) [else (ct (cdr l)))])) [else (ct (cdr l)))])) [else (ct (cdr l)))]))

3 [else (ct (cdr l)))])) [else (ct (cdr l)))])) [else (ct (cdr l)))])) [else (ct (cdr l)))])) (ct (cdr (a b c)))) (ct (b c))) (ct (b c))) [(null? (b c)) 0] [else (ct (cdr (b c))))])) [else (ct (cdr l)))])) [else (ct (cdr l)))])) [else (ct (cdr l)))])) [else (ct (cdr l)))])) [(null? (b c)) 0] [else (ct (cdr (b c))))])) [else (ct (cdr (b c))))])) [else (ct (cdr (b c))))])) (ct (cdr (b c))))) 9-12

4 [else (ct (cdr l)))])) [else (ct (cdr l)))])) [else (ct (cdr l)))])) [else (ct (cdr l)))])) (ct (cdr (b c))))) (ct (c)))) (ct (c)))) [(null? (c)) 0] [else (ct (cdr (c))))]))) [else (ct (cdr l)))])) [else (ct (cdr l)))])) [else (ct (cdr l)))])) [else (ct (cdr l)))])) [(null? (c)) 0] [else (ct (cdr (c))))]))) [else (ct (cdr (c))))]))) [else (ct (cdr (c))))]))) (ct (cdr (c)))))) 13-16

5 [else (ct (cdr l)))])) [else (ct (cdr l)))])) [else (ct (cdr l)))])) [else (ct (cdr l)))])) (ct (cdr (c)))))) (ct ())))) (ct ())))) [(null? ()) 0] [else (ct (cdr ())))])))) [else (ct (cdr l)))])) [else (ct (cdr l)))])) [else (ct (cdr l)))])) [else (ct (cdr l)))])) [(null? ()) 0] [else (ct (cdr ())))])))) [#t 0] [else (ct (cdr ())))])))) [#t 0] [else (ct (cdr ())))])))) 0))) 17-20

6 [else (ct (cdr l)))])) [else (ct (cdr l)))])) [else (ct (cdr l)))])) [else (ct (cdr l)))])) 0))) 1)) 1)) 2) Writing Functions in Scheme: Answer #2 [else (ct (cdr l)))])) 2) [else (ct (cdr l)))])) 3 Answer #2: Use the general design recipe Locate or write a Write a contract Write examples Create a template that follows the shape of the Convert the template to the final function Run examples as tests 21-25

Answer #2: Use the general design recipe Locate or write a Write a contract Write examples Create a

7 Writing Functions in Scheme: Answer #2 Data Definitions Answer #2: Use the general design recipe Locate or write a Write a contract Write examples Create a template that follows the shape of the Convert the template to the final function What is a "list of symbols"? Sometimes the is given, somtimes you have to create it Usually include it in your code as a comment Run examples as tests works 90% of the time Contracts Examples A contract is a comment that identifies set of input values and output values Examples (usually in comments at first) help clarify the purpose of the function ;; ct: <list-of-sym> -> <num> All mentioned data sets should have a somewhere ;; (ct ()) 0 ;; (ct (a b c)) 3 Make sure that every case in the is covered at least once 26-34

Sometimes the is given, somtimes you have to create it Usually include it in your code as a comment Run examples as tests works 90% of the time Contracts Examples A contract is a comment

8 Template Template Two cases in implies cond with two cond-lines Template Template Corresponding predicate for each data case Extract parts in cases with meta-variables 35-38

9 Template Template Recursive call for self-references in A template depends only on the input data; it ignores the function s purpose (Nevertheless, generating a template, which is fairly automatic, usually provides most of the function) Template to Function Template to Function Transform template to function line-by-line Transform template to function line-by-line 39-43

fairly automatic, usually provides most of the function) Template to Function Template to

10 Template to Function Reminder: Recipe Transform template to function line-by-line [(pair? l) (ct (cdr l)) )])) Sometimes, a part of the template isn t needed Locate or write a Write a contract Write examples Create a template that follows the shape of the Convert the template to the final function Run examples as tests Reminder: Template Steps More Examples Create a cond expression with one line for each case in the Write down a predicate for each case For the answer, extract parts in cases with meta-variables For each self-reference in the, add a recursive call (more examples in class) Shape of template shape == Shape of 44-47

of the Convert the template to the final function Run examples as tests Reminder: Template Steps More Examples Create a cond expression with one line for each

11 Generalized Recipe Locate or write s Write contracts Write examples Create a template that follows the shape of the data definition, one for each Convert the templates to the final functions Run examples as tests 48-49

shape of the data definition, one for each Convert the

Party Time! Computer Science I. Signature Examples PAIR OBJ. The Signature of a Procedure. Building a Set. Testing for Membership in a Set

Party Time! Computer Science I. Signature Examples PAIR OBJ. The Signature of a Procedure. Building a Set. Testing for Membership in a Set Computer Science I Professor Tom Ellman Lecture 21 Party Time! Whom shall you invite? We will write the Party Planner program. It will select SETS of guests. By reasoning with RELATIONS. Before we party,