ASP.NET/C# Prime Numbers

[Harry2000]

Am try to tell if a number is prime or not. I only can get this to work with even number. It say that 15,21,25 35 ect. it said thatthey are prime number but there not. Prime number are only divisible by it self and one. Here is the code its in C#. This is for a math apllication that am work on. int iPrime = Int32.Parse(TextBox1.Text) for (int icount = 2; icount <= (iPrime / 2); icount++) { if((iPrime % icount) == 0) { Label1.Text = "False"; return; } Label1.Text = "True"; return; } Harrison Brock

[Ian Darling]

Your return for true is in the wrong place. try:for (int icount = 2; icount <= (iPrime / 2); icount++) { if((iPrime % icount) == 0) { Label1.Text = "False"; return; } } // end of for loop Label1.Text = "True"; return;
-- Ian Darling "The moral of the story is that with a contrived example, you can prove anything." - Joel Spolsky

[Asad Hussain]

Good code. A few mathematics pointers though. You dont really need to check up to number / 2. If the numbers up til Ceiling(sqrt(iPrime/2)) dont return a mod result of 0 then the higher numbers wont either. e.g lets say u r testing 30 for prime (I know its not). Using the num/2 method the candidates are all numbers 1 through 15. Using the sqrt method the numbers are 1 through 6. Lets pick a number > 6 say 10 and the mod returns 0. But you have already tested 10 when you divided by 3 (3 * 10 = 30) Secondly, an even number cannot be prime so you can short circuit your code. Apologies for the not so major corrections but efficiency and performance are key to any successful coding endeavor.int test = Ceiling(sqrt(iPrime)); if ((iPrime % 2) == 0) { Label1.Text = "False"; return; } else for (int icount = 2; icount <= test; icount++) { if((iPrime % icount) == 0) { Label1.Text = "False"; return; } } Label1.Text = "True"; return;

[Harry2000]

Thank It work good now. Harrison Brock