[RESOLVED] Prime number code

Quote:

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Originally Posted by incidentals

Kaliman makes and observation that i had missed

he is correctly states that the test does not need to go right upto the value being tested

but instead only half way

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Nice catch Khali... :thumb:

Me too missed that one. :)

@shardooni: Glad to hear that you got the solution. It would be a good thing if you have marked this thread as RESOLVED. You could use the "Thread Tools" found at the top of this page and choose "Mark as RESOLVED" or you could edit the very first post and add "[RESOLVED]" in the beginning of the title. The reason is that, when others view your thread, they could easily understand that you have got it solved.

:wave:

here's my solution:

vb Code:

Dim uBound As Integer = 999
Dim primes() As Integer = Enumerable.Range(2, uBound - 1).Where(Function(i1) i1 = 2 OrElse Not Enumerable.Range(2, i1 - 3).Any(Function(i2) i1 Mod i2 = 0)).ToArray
ListBox1.Items.AddRange(Array.ConvertAll(primes, Function(x) x.ToString))

best thing to do then is

set out you requirements

and make a little plan of action and then we can help you build it from the
start

outline plan and fill in bit by bit

here to talk

here's an improved Prime number finder:

http://www.vbforums.com/showthread.p...45#post4108845

Quote:

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Originally Posted by incidentals

best thing to do then is

set out you requirements

and make a little plan of action and then we can help you build it from the
start

outline plan and fill in bit by bit

here to talk

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soundss great. should I post the new part for this assignment in another thread or can I just talk to you guys through this one? The next part of the assignment involves embedding another loop of primes into the loop the first "prime number" loop.

okay so the flow chart for the next part is called " simple prime sifter". i have to add this part INSIDE the loop of the first prime number loop you guys helped me on. I am going to attach a picture of the loop to this message. I am going to start working on it meanwhile, but hopefully you guys can help me out with how to start off. flow chart

http://tinypic.com/r/23ux2mh/5

heres the pic. sorry i dont know why those others pic i posted arent working
flow chart

i have a feeling that flow chart is the code we just did yesterday !! is it?

Quote:

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Originally Posted by incidentals

Kaliman makes and observation that i had missed

he is correctly states that the test does not need to go right upto the value being tested

but instead only half way

so function isprime can work much faster ( potentially twice as fast ) if we change the limits of the test loop

and the divisible by 2 doenot need to be tested as the candidates are all odd

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I realize I'm late to the party, but in case this thread ever gets read by anyone else, I wanted to point out a couple items.

First, the method described for primality testing in this thread is a variant of what is called the Sieve of Eratosthenes. This technique is fine for determining if small numbers are prime, but is horrible for large numbers (by which I mean the kinds of prime numbers used in modern encryption algorithms). For those kinds of numbers you should use other methods.

That said, when using the Sieve, you only need to count up to the square root of the number being tested, not half the number, which can make a significant difference. Thus, line 25 in akhileshbc's last example of code should be:

Code:

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For j As Integer = 3 To Math.Floor(Math.Sqrt(i))

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Thanks for sharing it with us Lenggries :wave:

Quote:

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Originally Posted by incidentals

glad she has got it

didn't we do well

the problem with my code is its for vb6 not net

so
listbox.additem x
is now
listbox.items.add x

oh well

btw kaliman the mod operator is the modulus (the remainder due to whole number division) not the module

I have this need to teach... sorry

now if she would just remember to rate out posts

here to talk

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Thank you, its kind of embarassing, but keep in mind English is not my native language, in Spanish is "Modulo", which translates both to "Modulus" and "Module".