Re: [Haskell-beginners] Primes calculation

primes [10..30] = [10,11,12,13,14,15,16,17,18,19,21,23,25,27,29] -- not bad actually, works as advertised.

10 is not prime. On Fri, Mar 12, 2010 at 7:49 PM, Ozgur Akgun wrote:

If we try to implement your idea literally we would need one more parameter to the function: the list of existing primes. I'll call this function primesWRT, since it finds all primes, with respect to the second list provided.

primesWRT [] _ = [] -- base case primesWRT (x:xs) existing -- if all x is not divided by all of the existing primes, -- x itself is a prime (wrt 'existing') -- add it to the primes list, and the existing primes list for the next function call | all (\ y -> mod x y /= 0 ) existing = x : primesWRT xs (x:existing) -- if x is not prime, check the rest. | otherwise = primesWRT xs existing

primes xs = primesWRT xs []

Please keep in mind that this function assumes its parameter to be in form [2..n]. But this is an assumption coming from your description.

primes [2..20] = [2,3,5,7,11,13,17,19] primes [2..30] = [2,3,5,7,11,13,17,19,23,29] primes [10..30] = [10,11,12,13,14,15,16,17,18,19,21,23,25,27,29] -- not bad actually, works as advertised.

Best,

On 12 March 2010 21:14, legajid wrote:

...

Hi, i'm trying to write a function to find all primes from 2 to N.

My idea is : take the first number (2) try to find whether it's a multiple of one of all existing primes ([] at first) add 2 to the list of primes

take the following number (3) find if multiple of existing primes ([2]) add 3 to the list of primes

take the following number (4) find if multiple of existing primes ([2, 3]) do not add 4 to the list of primes

...

take the following number (8) find if multiple of existing primes ([2, 3, 5, 7]) do not add 8 to the list of primes

take the following number (9) find if multiple of existing primes ([2, 3, 5, 7]) do not add 9 to the list of primes (multiple of 3)

and so on...

So, i would like a function like :

f (x : xs) = g x : f xs

g would return x if x is prime, [] otherwise

But g would use the preceding value of f (list of primes before the calculation for x) that is a result of g itself. f needs g that needs f : what's wrong with my mind ? Perhaps i am still under control of imperative programming ?

Thanks for your help,

Didier.

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-- Ozgur Akgun

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