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POSimply Scheme. Chapter 08. Higher—Order Functions
 

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    <p>Greets,</p> <p>Summary</p> <p>having trouble passing '(+) or '(-) as data to a cond (non evaluated). On their own, they return (+) or (-) which, as an argument returns the identity element (0).</p> <p>HELP!</p> <h2>Background.</h2> <p>For the non standard scheme in the code.</p> <p>In this book; sentences are flat lists and words are sybmols and strings. There are three higher order functions/procedures in simply.scm, part of the <a href="http://www.eecs.berkeley.edu/~bh/downloads/simply/" rel="nofollow">library</a> to illustrate the topic, every, keep and accumulate;</p> <ol> <li>(every function data) [do this function to every element of data]</li> <li>(keep predicate? data) [keep the elements of data that pass predicate? test]</li> <li>(accumulate function data) [collect all data into the form of function — combine with keep to remove invalid data] eg (accumulate + (keep number? data)) [remove non numbers then add the remaining numbers together, zero if no numbers found]</li> </ol> <h2>Data Flow.</h2> <p>Exercise <a href="http://www.eecs.berkeley.edu/~bh/ssch8/higher.html" rel="nofollow">8.11</a> is a gpa calculator procedure. By instruction, no lambda or recursion allowed (not yet taught if read sequentially).</p> <p>The first implementation I tried takes multiple grades in a single sentence and outputs individual sentences, each with a single grade. It then passes this output to a helper procedure.</p> <p>If the single grade output has a + or - it is separated, for example '(a+) into '(a) and '(+) and all output is then passed to a further helper procedure.</p> <p>then a cond allocates scores</p> <pre><code>a 4 b 3 c 2 d 1 e 0 + 0.33 - -0.33 </code></pre> <p>This, only worked in my head (why don't computers work like minds?) When a grade like '(a+) or '(a-) is seperated, the '(a) is processed properly but the '(+) or '(-) evaluate to the identity element (0) and fail to add to the gpa.</p> <p>Is there a way to make '(+) and '(-) passable as data instead of as an expression? Alternatively, can I convert them to some arbitrary data usable in the cond before they return (0)?</p> <p>The current version, a lengthy cond for each grade, works, but is hideous. Makes the implementation feel like imperative instead of functional programming.</p> <h2>Code.</h2> <p>returns the wrong gpa (doesn't add 0.33 or -0.33): also, input type check in (gpa-helper) failed spectacularly.</p> <pre><code>(define (gpa gradesset) (/ (accumulate + (every gpa-helper gradesset)) (count gradesset)) ) (define (gpa-helper gradewrd) (cond ((or (&lt; (count gradewrd) 1) (&gt; (count gradewrd) 2)) '(Please use valid grade input)) ((= (count gradewrd) 1) (gpa-allocator (keep valid-grade? gradewrd))) ((= (count gradewrd) 2) (every gpa-helper (keep valid-grade? gradewrd))) (else '(Please check that all grades entered are valid)) ) ) (define (gpa-allocator gradeletter+-) (cond ((equal? gradeletter+- 'a) 4) ((equal? gradeletter+- 'b) 3) ((equal? gradeletter+- 'c) 2) ((equal? gradeletter+- 'd) 1) ((equal? gradeletter+- 'e) 0) ((equal? gradeletter+- +) .33) ((equal? gradeletter+- -) (- .33)) (else 0) ) ) (define (valid-grade? gradein) (if (member? gradein '(+ - a+ a a- b+ b b- c+ c c- d+ d d- e)) #t #f) ) </code></pre> <p>redone version that returns a sentence of the individual scores. The 0 returned by '(+) and '(-) is visible here. Implements successful input type checking but introduces new problems. (accumulate + ing the result for one)</p> <pre><code>(define (gpa gradesset) (every gpa-cleaner gradesset) ) (define (gpa-cleaner gradewrd) (cond ((or (&lt; (count gradewrd) 1) (&gt; (count gradewrd) 2)) 0) (else (every gpa-accumulator gradewrd)) ) ) (define (gpa-accumulator gradewrd) (/ (accumulate + (every gpa-helper gradewrd)) (count gradewrd)) ) (define (gpa-helper gradewrd) (cond ((= (count gradewrd) 1) (gpa-allocator (keep valid-grade? gradewrd))) ((= (count gradewrd) 2) (every gpa-helper (keep valid-grade? gradewrd))) (else '(Please check that all grades entered are valid)) ) ) (define (gpa-allocator gradeletter+-) (cond ((equal? gradeletter+- 'a) 4) ((equal? gradeletter+- 'b) 3) ((equal? gradeletter+- 'c) 2) ((equal? gradeletter+- 'd) 1) ((equal? gradeletter+- 'e) 0) ((equal? gradeletter+- +) .33) ((equal? gradeletter+- -) (- .33)) (else 0) ) ) (define (valid-grade? gradein) (if (member? gradein '(+ - a b c d e)) #t #f) ) </code></pre> <p>Using SCM version 5e7 with Slib 3b3, the additional libraries supplied with Simply Scheme (link provided under background above — simply.scm, functions.scm, ttt.scm, match.scm, database.scm) and the library where I input my answers for every exercise loaded.</p>
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