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Thread: square root and prime numbers
 09192010, 03:59 AM #1Member
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square root and prime numbers
Hello
It is the first time that I post on this site, so I do not know if it really
good category. I try to write a small java program that determines if a number is
prime or not, it looks like this:
if (nombreEntré % 2 != 0)
System.out.println(nombreEntré +" est un nombre premier"); // prime
else if(nombreEntré % 2 == 0 && nombreEntré != 2)
System.out.println(nombreEntré +" n'est pas un nombre premier"); // not prime
but if I write 51 or 25 (which are not prime number) it said me prime because it is odd. My question is what way should we change the program to detect odd numbers compounds as nonprime?
And I try to express the square root of a number, but Math.sqrt () instead of loop with if ... else or while statement. is there a way?
Sorry for my bad english
Thank you in advanceLast edited by roud9; 09192010 at 04:02 AM.
 09192010, 04:50 AM #2
For the square rooting, and you want to use a loop, you could check every number up to the number to see if it's a square root. This isn't a very good method though; Math.sqrt() is much better.
Java Code:Example: for (i = 0; i < num; i++) { if (i*i == num) return i; // If some number times itself is our number, return that. } // Note that this method will only find square roots of squares, such as 49, 64, etc.
Java Code:Example (informal psuedocode): for (i from 0 to num) { if (num % i == 0) { // % checks the remainder of a division; if it's 0, then it's evenly divisible // num is evenly divisible by i, so this number cannot be prime } }
Hope that helps a bit.
 09192010, 09:17 AM #3
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Nope, for a check for primality for a number n, you need to run through every number up to sqrt(n). A bit of trickery in the forloop code itself can avoid the Math.sqrt(n) call completely:
Java Code:for (int i= 2; i*i <= n; i++) ...
kind regards,
Jos
 09192010, 12:31 PM #4Senior Member
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 09192010, 01:38 PM #5
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Yup, it complicates the preamble of a prime testing method a bit but it speeds up the loop:
Java Code:boolean isPrime(int n) { if (n < 2) return false; // one isn't prime by definition if (n == 2  n == 3) return true; // filter out the simple cases if (n%2 == 0  n%3 == 0) return false; // those can't be prime int inc= 2; for (int i= 5; i*i <= n; i+= inc, inc= 6inc) { // test if n can be divided by i; if so n is not prime } return true; }
kind regards,
JosLast edited by JosAH; 09192010 at 03:00 PM.
 09212010, 12:03 AM #6Member
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Thanks for your reply I find what I want and how to do it. But I want a while loop instead of for loop
Java Code:for (int k = 0; k < 20; k++) racineCarrée = (racineCarrée + (nombreEntré / racineCarrée)) / 2; if (racineCarrée * racineCarrée != nombreEntré) { System.out.println("La racine entière approximative est " + racineCarrée); racineApprox ++; compteurRacines ++; } else { System.out.println("La racine entière exacte est " + racineCarrée); racineExacte ++; compteurRacines++; }
 09212010, 12:37 AM #7
A for loop is a hybrid construct of a while loop:
Java Code:a; while (b) { d; c; }
Java Code:for (a; b; c) { d; }
 09212010, 12:59 AM #8Member
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Thanks :)
when I write 15, the square root give me 4 instead of 3
Java Code:racineCarrée = nombreEntré / 2; int k = 0; while (k > 20) { k ++; } racineCarrée = (racineCarrée + (nombreEntré / racineCarrée)) / 2; if (racineCarrée * racineCarrée != nombreEntré) { System.out.println("La racine entière approximative est " + racineCarrée); racineApprox ++; compteurRacines ++; } else { System.out.println("La racine entière exacte est " + racineCarrée); racineExacte ++; compteurRacines++; }
 09212010, 01:22 AM #9Senior Member
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you need this:
Java Code:racineCarrée = (racineCarrée + (nombreEntré / racineCarrée)) / 2;
 09212010, 01:28 AM #10Member
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The same thing give me 7 now :confused:
 09212010, 01:30 AM #11Senior Member
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oh, your while loop whould be:
Java Code:k=0; while(k<20) { racineCarrée = (racineCarrée + (nombreEntré / racineCarrée)) / 2; k++; }
 09212010, 01:33 AM #12Member
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Omg thanks a lot I put racineCarrée = (racineCarrée + (nombreEntré / racineCarrée)) / 2; after k ++;
 09212010, 09:27 AM #13
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 09212010, 04:50 PM #14Member
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because I don't need the decimal part
 09212010, 04:59 PM #15
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 09212010, 05:01 PM #16Member
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Okay thanks I will check this
 09222010, 04:20 PM #17
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