If we try to implement your idea literally we would need one more parameter to the function: the list of existing primes.<br>I'll call this function primesWRT, since it finds all primes, with respect to the second list provided.<br>
<br><span style="font-family: courier new,monospace;">primesWRT [] _ = [] -- base case</span><br style="font-family: courier new,monospace;"><span style="font-family: courier new,monospace;"><span style="font-family: courier new,monospace;">primesWRT (x:xs) existing</span><br>
</span><span style="font-family: courier new,monospace;"> -- if all x is not divided by all of the existing primes,<br> -- x itself is a prime (wrt 'existing')<br> -- add it to the primes list, and the existing primes list for the next function call<br style="font-family: courier new,monospace;">
</span><span style="font-family: courier new,monospace;"> | all (\ y -> mod x y /= 0 ) existing = x : primesWRT xs (x:existing)</span><span style="font-family: courier new,monospace;"><br> -- if x is not prime, check the rest.<br>
| otherwise = primesWRT xs existing</span><br style="font-family: courier new,monospace;"><br style="font-family: courier new,monospace;"><span style="font-family: courier new,monospace;">primes xs = primesWRT xs []</span><br style="font-family: courier new,monospace;">
<br><br>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.<br><br style="font-family: courier new,monospace;"><span style="font-family: courier new,monospace;">primes [2..20] = [2,3,5,7,11,13,17,19]</span><br style="font-family: courier new,monospace;">
<span style="font-family: courier new,monospace;">primes [2..30] = [2,3,5,7,11,13,17,19,23,29]</span><br style="font-family: courier new,monospace;"><span style="font-family: courier new,monospace;">primes [10..30] = [10,11,12,13,14,15,16,17,18,19,21,23,25,27,29] -- not bad actually, works as advertised.</span><br>
<br>Best,<br><br><div class="gmail_quote">On 12 March 2010 21:14, legajid <span dir="ltr"><<a href="mailto:legajid@free.fr">legajid@free.fr</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
Hi,<br>
i'm trying to write a function to find all primes from 2 to N.<br>
<br>
<br>
My idea is :<br>
take the first number (2)<br>
try to find whether it's a multiple of one of all existing primes ([] at first)<br>
add 2 to the list of primes<br>
<br>
take the following number (3)<br>
find if multiple of existing primes ([2])<br>
add 3 to the list of primes<br>
<br>
take the following number (4)<br>
find if multiple of existing primes ([2, 3])<br>
do not add 4 to the list of primes<br>
<br>
...<br>
<br>
take the following number (8)<br>
find if multiple of existing primes ([2, 3, 5, 7])<br>
do not add 8 to the list of primes<br>
<br>
take the following number (9)<br>
find if multiple of existing primes ([2, 3, 5, 7])<br>
do not add 9 to the list of primes (multiple of 3)<br>
<br>
and so on...<br>
<br>
So, i would like a function like :<br>
<br>
f (x : xs) = g x : f xs<br>
<br>
g would return x if x is prime, [] otherwise<br>
<br>
But g would use the preceding value of f (list of primes before the calculation for x) that is a result of g itself.<br>
f needs g that needs f : what's wrong with my mind ?<br>
Perhaps i am still under control of imperative programming ?<br>
<br>
Thanks for your help,<br>
<br>
Didier.<br>
<br>
<br>
<br>
<br>
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</blockquote></div><br><br clear="all"><br>-- <br>Ozgur Akgun<br>