Need help wrapping my head around lazy substituion errors

[Spencer Weight]

This is some code I'm toying with...

(define (extend-primes n)
  (begin
    (define p (last primes))
    (for ([i (in-range p n)])
      (cond
        [(andmap (lambda (x)
                   (not (zero? (modulo i x))))
                 primes)
         (append primes (list i))]))))


I'm trying to get (last primes) to evaluate and store the result in p so that I can iterate up to n

(last) is a function I defined to get the last element in a list

When I try to do something like the code above, because of the lazy language substitution I get this error
in-range: contract violation
  expected: real?
  given: #<promise:p>

How can I get (last primes) to evaluate so that I can use the result in the next part of the procedure?

[Spencer Weight]

I'm calling the code with (extend-primes 13)
and primes is defined as (list 2)

[Andrew Kent]

You need a strictness point that *forces* the promise to give you what you want.

http://docs.racket-lang.org/lazy/

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-Andrew

[Jay McCarthy]

This code is in the totally wrong direction. 

When I get back to my desk I'll give you more low level details. 

Sent from my iPhone

[Dan Burton]

It's the wrong direction because it is an imperative approach to the problem: build a list, fetch the last prime, append to that list, repeat.

Don't think of it that way. Laziness works best with a declarative approach.

-- Dan Burton

[Jay McCarthy]

Yes... meaning try to define what the primes ARE /not/ "how to get them".

Jay

--
Jay McCarthy <j...@cs.byu.edu>
Assistant Professor / Brigham Young University
http://faculty.cs.byu.edu/~jay

"The glory of God is Intelligence" - D&C 93

[Spencer Weight]

Okay, so I know what primes are now, but I'm struggling to figure out how to program prime? and prime in a declarative manner.

I've been looking at the examples for nats and fibs and I'm still a little shaky on how to accomplish what I need.

I guess the thing I am hung up on is to test if a number is prime, then I need to see if it is divisible by any the primes before it.  I'm having trouble trying to understand how I would figure out all the primes before the prime I'm testing so that I can get an accurate answer.

For example if I called (prime? 13), then the procedure should try dividing 13 by 11, 7, 5, 3, and 2 and if they don't divide evenly, then the number is prime.  How do I calculate for 11, 7, 5, 3, and 2

If I have this all wrong, please let me know.

[Andrew Kent]

A number is prime if it is not divisible by primes less than it - this is because of the definition of prime and the fundamental theorem of arithmetic.

A stronger requirement which implies the above is a number is prime if it is not divisible by *any* number less than it.

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-Andrew

[Jay McCarthy]

Maybe a function like take-while would be useful to pull the prefix of a list before a certain number. :)

Sent from my iPhone

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[Willard Hagen]

---------- Forwarded message ----------
From: Willard Hagen <willar...@gmail.com>
Date: Wed, Dec 11, 2013 at 1:32 PM
Subject: Re: Need help wrapping my head around lazy substituion errors
To: Jay McCarthy <jay.mc...@gmail.com>

I used a for-each in my take-while, wouldn't this take away from the non-imperative programming model?

--
-Willard

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-Willard

[Jay McCarthy]

Yes, that's a sign your take-while is wrong