Hello Pratima,

Il 11 dicembre 2023 alle 19:24 Folsk Pratima ha scritto:

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Please explain why this fails for negative numbers. By fails I mean it starts eating RAM infinitely until either me or OOM kills it. If you replace `m1` definition with the one commented out, the code will fail for positive integers as well, which is very frustrating.

I have replaced m1 definition with the commented one, and it works on my machine.

f at x270:/tmp/prova$ ./prime -4 -4 is not primal because divisible by 2

How are you invoking the program?

A few additional notes (run `hlint` for more)

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main = getArgs >>= \argv ->

I don’t mind `>>=` but with `do` notation and `traceM/traceShowM` it is easier to debug your programs.

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--m1 = -- map -- (\x -> -- if x < 0 -- then -x -- else x) -- m0

More compact: `m1 = map abs m0`

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msg (x, (y0, y1)) = x ++ " is " ++ f y0 ++ " because " ++ y1

Ancillary functions like this one can go in the `where` section. Ciao —F

I have been just going to remedy and send the actual number I was using for testing, sorry for this. Assuming the binary is called a.out, ./a.out 4394853075948683624652562564254523466839834983 I have not immediately guessed that it overflows, so after playing a minute with it I figured another number -- 1506491439391566589 -- that breaks even while being positive. And it does not matter if `m1 = m0` or `m1 = map abs m0`. I have come up with another definition, based on the `primals` definition on haskell.org main page that I have not noticed until now. isPrime6 :: Int -> (Bool, String) isPrime6 n = test (f [2 ..]) where f (x:xs) = x : f [y | y <- xs, (y `rem` x) /= 0] test (x:xs) | x > n = (False, "it is not") | x == n = (True, "it is") | otherwise = test xs Testing it with 1506491439391566589 makes it look like the code is going to run forever, but at least it does not take all of your RAM. I dare suspect the problem is somewhere around having a separate variable for the `l` list, which does not allow haskell to forget used members. I am not sure about anything however. Also, the fact that the code runs forever nonetheless is very sad, because C equivalent taken from wiki[1] calculates the whole things almost immediately. The C code is ultra simplified: #include <stdio.h> #include <stdint.h> int IsPrime(int64_t n) { if (n == 2 || n == 3) return 1; if (n <= 1 || n % 2 == 0 || n % 3 == 0) return 0; for (int i = 5; i * i <= n; i += 6) { if (n % i == 0 || n % (i + 2) == 0) return 0; } return 1; } int main () { printf ("1506491439391566589: %d\n", IsPrime (1506491439391566589)); } I did not even do anything special to compile it, just `gcc`. With haskell I supplied the `-O2` flag to `ghc`. 1. https://en.wikipedia.org/wiki/Primality_test

Il 11 dicembre 2023 alle 20:24 Folsk Pratima ha scritto:

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Also, the fact that the code runs forever nonetheless is very sad, because C equivalent taken from wiki[1] calculates the whole things almost immediately. The C code is ultra simplified:

#include <stdio.h> #include <stdint.h> int IsPrime(int64_t n) { if (n == 2 || n == 3) return 1;

if (n <= 1 || n % 2 == 0 || n % 3 == 0) return 0;

for (int i = 5; i * i <= n; i += 6) { if (n % i == 0 || n % (i + 2) == 0) return 0; }

return 1; }

int main () { printf ("1506491439391566589: %d\n", IsPrime (1506491439391566589)); }

Let’s implement the algorithm in Haskell, shall we?

import System.Environment

isPrimeWiki :: Integer -> Bool isPrimeWiki 2 = True isPrimeWiki 3 = True isPrimeWiki n | n <= 1 || rem n 2 == 0 || rem n 3 == 0 = False isPrimeWiki n = let f i | i^2 > n = True | rem n i == 0 || rem n (i+2) == 0 = False | otherwise = True in f 5

main :: IO () main = do [n] <- getArgs print $ isPrimeWiki (read n)

then

f at x270:/tmp/prova$ time ./prime 1506491439391566589 True

real 0m0.014s user 0m0.001s sys 0m0.005s Ahem,

#include <stdio.h> #include <stdlib.h> #include <stdint.h> int IsPrime(int64_t n, char **msg) { if (n == 2 || n == 3) return 1; if (n <= 1 || n % 2 == 0 || n % 3 == 0) return 0; for (int i = 5; i * i <= n; i += 6) { if (n % i == 0) { sprintf (*msg, "divisible by %d", i); return 0; } if (n % (i + 2) == 0) { sprintf (*msg, "divisible by %d", i + 2); return 0; } } return 1; } int main () { char *msg[1]; msg[0] = malloc (sizeof (char) * 128); sprintf (msg[0], "success"); int res; int64_t num; char numstr[] = "1506491439391566589"; num = 1506491439391566589; res = IsPrime (num, msg); printf ("%s: %d: %s\n", numstr, res, msg[0]); } tells me the number is divisible by 13 and is *not* primal. The qalculate tells me pretty much the same!