[racket] Rookie Question on Functional Languages

From: Luke Jordan (luke.jordan at gmail.com)
Date: Thu Dec 9 14:48:38 EST 2010

I will look at for and in-range and spend some more time exploring the
built-ins.  But I want to talk about the do-times solution, because that was
the first thing I tried, but I couldn't make it work.  In chapter 11
expression arguments are applied to something that builds up a result (as a
list or an arithmetic operation).  I don't understand how that applies to
what I'm trying to do here since I'm not building anything, just calling it
over and over and ignoring the result.  Following what I learned from
chapter 11 the code would look something like this, which doesn't make

(define (do-times n expr)
    [(< n 1) evaluate-expr-last-time]
    [else ?? evaluate-expr (do-times (sub1 n) expr)]))

On Thu, Dec 9, 2010 at 13:08, Ryan Culpepper <ryanc at ccs.neu.edu> wrote:

> On 12/09/2010 11:51 AM, Luke Jordan wrote:
>>  Here's a rookie question that stems from HtDP 29.3.2.
>> The idea is to test an expression a number of times while timing it and
>> compare to another version of the same function.  The expression finds a
>> route in a vector (graph) from node 0 to node 4.  The way I would do
>> this in C/Python is with a while loop, while the counter is not zero
>> execute the expression (ignore return value), and wrap it in the timer.
>>  In Racket I came up with:
>> (time (for-each
>>        (lambda (dest) (find-route 0 dest Graph))
>>        (make-list 1000 4)))
>> It seems unnecessarily tricky, but I couldn't think of a better way.  Am
>> I missing a piece?  Does it only seem tricky because Racket is my first
>> functional language.
> To the student of functional programming:
> You're right, it is unnecessary. You're using a list because you know
> someone has already provided "loop" functions for lists. But you just want
> to repeat an action N times---that's a natural number. Refresh yourself on
> Section 11 (Natural Numbers) and design this function:
> ;; do-times : Nat (-> Void) -> Void
> ;; Applies the given thunk N times.
> To the aspiring Racketeer:
> Racket has loop support for natural numbers, just not as a single function
> like 'for-each'. Look at 'for' and 'in-range'.
> Ryan
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