[plt-scheme] HTDP: 9.5.5

From: Veer (diggerrrrr at gmail.com)
Date: Sun Sep 14 03:35:37 EDT 2008

I think this is a problem of understanding recursion than anything else.
It took me while to understand recursion.

When i was stuck , Matthias wrote this (i am substituting
'arrangements' with 'convert') :


When you go from templates to definitions, read out the PURPOSE
statement for the recursive template expression:

;;(convert a-list) produce the number that can be formed from all
;; digits in  a-list.

;; (convert (rest a-list)) therefore produces the number that can be formed
;; from all the digits in the rest of a-list
(What does this mean for a concrete example? Use the examples!!)

And then ask yourself how a primitive or a helper function can
use this and the rest of the template (line) to get the real

For example:
;;(convert a-list)
(convert (cons 1 (cons 2 (cons 3 empty))) produces what

;;(convert (rest a-list))
(convert (cons 2 (cons 3 empty))) produces what

So does this help?

On 9/14/08, Grant Rettke <grettke at acm.org> wrote:
>> Now discover Jens's hint by following the design recipe.
> Where did I go wrong? (Here is not where I expect you to be psychic,
> so let me elaborate)
> I followed the recipe by defining the recursive data (I re-used list
> of numbers), created a bunch of tests, walked through the
> implementation of the body (cond clauses, selector expressions,
> natural recursion), implemented the answer for the base case (zero),
> and then was left with:
> [else
>  ... (first a-list-of-numbers) ...
>  ... (convert (rest a-list-of-numbers) ...)]
> I think I was too preoccupied with shoe-horning in my approach than
> looking at the data. What I *didn't* do was to take something one more
> complex than the base case, like this, which is a test I had written
> before I implemented the function:
> (check-expect (convert (cons 1 empty)) 1)
> That at least would have gotten me to the first part of the equation
> in the else clause, 1 plus ...
> Then I would have looked at:
> (check-expect (convert (cons 0 (cons 1 empty))) 10)
> and maybe though zero plus something times the 2nd part, and finally plus
> zero.
> Is that the thought process, step by step satisfy the test by revising
> the calculation? You start with the simplest thing from the base
> condition?
> My approach thus far has been to write *all* of the tests up front, I
> was staring at this while I tried to implement the body:
> (check-expect (convert empty) 0)
> (check-expect (convert (cons 1 empty)) 1)
> (check-expect (convert (cons 0 (cons 1 empty))) 10)
> (check-expect (convert (cons 1 (cons 1 empty))) 11)
> (check-expect (convert (cons 0 (cons 0 (cons 1 empty)))) 100)
> (check-expect (convert (cons 1 (cons 1 (cons 1 empty)))) 111)
> This was a point where I totally didn't understand it and I felt like
> the recipe wasn't taking me there, which I was I posted a long-ish
> email.
> Please point me in the right direction to get back on track with what
> I missed from the recipe.
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