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Is this a joke question? Generate a four digit number and add an extra number to the end, being either 5 or 0. Then don't check it because it will be correct!!!
e.g. 4678; add 5 on the end to give 46785.
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Exactly my take on it 
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Random random = new Random();
return (random.Next(17999) + 2000) * 5;
this gives a number divisable by 5 between 10000 and 99995
Paulo Gomes
Over and Out 
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Generate a five digit random number, convert to string, replace the right most digit with 5 (or 0), parse the string back to an int.
No need to check as anything ending in 5 or 0 is divible by 5.
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This would work as an alternative but the math approach is better (more performant) unless you want the final value to be a string. And, even then the math approach would be faster.
(nnnn*10)=nnnn0
It's the same as appending a "0" to a four-digit number but it's much faster.
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I'm sorry but a lot of the answers made me laugh...
By definition any number that has been multiplied by 5 is divisible by 5. So the solution is:
x=random(low_limit, high_limit)*5; no need to check this!
The five digit condition is a little bit more complicated, because it is not clearly defined. is 00005 a five digit number by this definition? Low limit will then be 0, high limit 19999 (the highest 5 digit number fullfilling the condition is 99995).
To get a number that is divisable by 5, the ending needs to be 0 or 5. If you need checking, the modulo division is correct.
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// To generate as random numbers as possible, this variable must only
// be initialized once and then reused as much as possible.
Random rand = new Random();
/// <summary>
/// Generates a random 5 digit number that is always divisible by 5.
/// </summary>
/// <returns>Returns a random integer 5 digits long that is evenly divisible by 5.</returns>
private int GetDivBy5()
{
// rnd.Next(minValue, maxValue) returns a random between minValue
// and maxValue, inclusive. Use min/max values of 1000 and 9999 to
// yield a random 4 digit number between 1000 and 9999 then multiply
// by 10 to get a 5 digit number. This number will always be
// divisible by 5 but will always end in 0.
int divBy5 = rand.Next(1000, 9999) * 10;
// To get more random and double the possible result set, randomly
// add on 5. This will yield the total possible set of integers
// between 10000 and 99995 that are divisible by 5.
if (rand.NextDouble() < 0.5)
return divBy5;
else
return divBy5 + 5;
}
Stephen, you mention you are a coding noobie. So, a short word about random number generators - they're not really very random.
All they do is take a seed (beginning) value and run it through a mathmatical algorithm that generates a new number. If you use the same seed value every time, you will get the same "random" number every time. Most random number generators can either self-seed - generally using the current clock value - or you can pass them a seed value when you initialize them. This is why you only want to initialize your random number generator once and reuse it over and over.
Once you get past that, the rest is pretty easy. Just use the math several others have already provided. See the comments in the code...
<edit>
Um, just looked at the thread in the other forum where you defined your requirements a little better. It seems you would like to include 0-9999 in your result set too. You can do this easily enough by changing the min value passed to rand.Next(minValue, maxValue) to 0 instead of 1000. This last bit, is a bit tricky in that in programming terms "00005" is actually not a number but is really a string. To get the leading zeros, you need to convert your final value to a string and use a formatting mask to append the leading zeros. This also assumes "00000" is valid. If it's not, test for it with an "if" and call GetDivBy5 again.
So, to get a string with leading zeros use the following;
private void button1_Click(object sender, EventArgs e)
{
this.textBox1.Text = this.GetDivBy5().ToString("00000");
}
</edit>
modified on Tuesday, August 23, 2011 11:03 AM
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Thank you very much.
Sorry I missed this post, and to be honest I am a little p***** off with the negative comments I am receiving, that is why I stopped looking at the responses as it upset me a little that people on here can be so damn rude and insulting, oh, and judgmental, for no apparent reason?
Anyway, thank you very much.
Kind Regards,
Stephen
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Everyone's a Legend in Their Own Lunchtime. You've learned a lesson - whether you wished to or not - about being precise in framing your question, so I hope that at least that, and the sensible answers you did get, helped.
As to the others who felt it was easier to ridicule than to stop for a second and realise there was more to it, I apologise.
cheers,
Chris Maunder
The Code Project | Co-founder
Microsoft C++ MVP
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Chris Maunder wrote: Everyone's a Legend in Their Own Lunchtime.
Agreed!
Chris Maunder wrote: You've learned a lesson - whether you wished to or not
Actually I learned two. Yes; one was indeed to think allot more before I pose my question!
Chris Maunder wrote: As to the others who felt it was easier to ridicule than to stop for a second and realise there was more to it, I apologise.
You have nothing to apologise for, and it was really just the one statement that I was not a scientist that caused me to react.
Never mind, I have moved on
Kind Regards,
Stephen
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You're welcome, Stephen.
Please keep in mind that mad geniuses as many programmers may be, few people tend to lump us programmers into the "high social skills" cliche. It's been a while since I worked with scientists but I seem to recall from my university days they can be somewhat socially lacking too. I remember this one physicist... Ahem...  Uh, anyway, its probably best to just take the "water off a ducks back" approach - no one really means to insult - especially if you want any of us social deviants to actually help you. 
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Michael A. Cochran wrote: It's been a while since I worked with scientists but I seem to recall from my university days they can be somewhat socially lacking too.
Could not agree more! At the end of the day we are all human.
Michael A. Cochran wrote: it’s probably best to just take the "water off a ducks back" approach
I normally do, however, being told by someone that I had never met or spoke to that I was not a scientist was what got to me. It was not just an insult and ignorant statement, but it was not even to do with programming and in my own opinion was a personal attack that was not called for.
Anyway, I have taken what I need from this post, got my code to work and moved on 
Kind Regards,
Stephen
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x = generate random number between 10000 and 99995
remainder to be added = 5 - x MOD 5
Therefore, the resultant random number should be x + 5 - x MOD 5
Here's a sample code: http://ideone.com/LxgrC[^]
a.k.a. Vite Phoenix and Vite Zeus...
Proud member and co-founder of OlympianZ
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Why didn't I think of that?  Too much business programming, I guess. We don't use modulus in business programming much - at least I haven't had the need.
Correct, of course. Much more elegant.
If Stephen is still listening, in C# it becomes;
Random rand = new Random();
private int GetDivBy5()
{
int divBy5 = rand.Next(0, 99999);
return divBy5 - (divBy5 % 5);
}
private void button1_Click(object sender, EventArgs e)
{
this.textBox1.Text = this.GetDivBy5().ToString("00000");
}
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nice, the shortest and the best so far
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You are getting all sorts of advice. Multiplying by 10 can result in overflow. The easiest is: num -= num%5;
IE subtract the remainder if you divided by five from the result.
Another way: num = (num/5) * 5;
That will NEVER cause an overflow in most languages. If you are using vb.net, besides the syntax replace "/" with "\".
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I guess a simple expression like
int random= (rand()%18000)*5 + 10000
should do the trick where rand() is a function that returns any positive random number for C# it can be replaced with
int random= (new Random().Next()%18000)*5 + 10000
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The solution I found most elegant had already been posted by David1984 but is deleted. So I'm going to post it now:
You want a number X that can be divided by 5.
Any number Y can be multiplied by 5. The result X is divisible by definition (and by 5).
You want X to have 5 digits.
5-digit-numbers range from 10000 to 99999. Divide both by 5 and throw away the result's decimal places to get the limits for Y (2000 to 19999).
Supposed you already have an instance of Random obeying the tips on its use, you get X by using this code (X changed to a meaningful name)
int randomDivisibleByFive = random.Next(2000, 19999) * 5;
Ciao,
luker
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Hey guys, I'm just looking for someone to point me in the right direction here, not necessarily provide a solution..
I have been tasked with writing a lightweight program for sending out email newsletters. This in itself has been easy enough to do.. but obviously I want to be able to send to as many recipients as fast as is possible.
At the moment my solution sends 1 email at a time (the to address is provided by a stored procedure that returns 1 at a time), but this is a little slow.
Having looked into it a little bit, I know I can send up to 5 emails concurrently using my smtp server before they start being throttled, and that I should be recycling the connections rather than creating and dropping them each time. I have been trying to get my head round the problem and seem to have a mental block as to the best way to create these new connections and recycle them.
Some pointers would be great if anyone can advise.
Thank you!
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markymark82 wrote: Having looked into it a little bit, I know I can send up to 5 emails concurrently using my smtp server before they start being throttled, and that I should be recycling the connections rather than creating and dropping them each time.
Huh?
Did you write your own email class rather than using the one already in .Net?
Presumably the answer is yes since otherwise the question makes no sense.
If you did then normally it is going to be marginally faster to keep the connection rather than creating it each time in terms of network traffic. However as you seem to have already determined the server itself can impact this.
Exactly what sort of volume do you anticipate because if it is a small volume then it shouldn't matter how fast you are going. If a large volume then I wonder why your company doesn't just adjust the smtp server to be more accommodating for this required business task.
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I'm using the System.Net.Mail class, so it's entirely likely my question doesn't make sense.
The sort of volume it could have to handle is up to around 80k messages + more as the list grows.
Each email is customised to the address with individual tracking strings on the URLs etc. hence being provided 1 at a time from a stored procedure.
Unfortunately, we are using authsmtp as our smtp server for improved deliverability, hence the limit to 5 connections at a time, and that can't be changed.
I have observed that actually recycling the connections gives a very minimal speed improvement, so I'm prepared to overlook that if need be, but I'm unsure of how to restrict the number of smtp connections open at any 1 time, waiting to send the next email until the number of open connections drops to fewer than 5 again.
I'm sure it must be fairly simple, and that I'm missing a key point somewhere, but as yet I've not found it. What has already been written for this task, initially started as a test, but it proved suitable for sending the few emails that it needed to, giving us the flexibility we needed, however, now that people have seen that it can be useful, they want to use it for higher volumes, so now I need to look at changing it and getting it right.
Thanks for the reply
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A ThreadPool should be able to do this, I think. If not, you can write a custom queue processor that holds five connections and pulls items off the queue into each one when it becomes idle. For 80,000 messages you can just push them all into the queue at the start.
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It's funny you should start with that, I was just looking at the possibility of Threading & ThreadPools, but that's not something I've ever dealt with, as I'm really a web developer. This is my first attempt at writing a winforms app and I think I've jumped in at the deep end! I guess it's going to be a steep learning curve from here on!
At least someone else has confirmed one of the ideas I've had for it
Thank you!
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markymark82 wrote: I have observed that actually recycling the connections
Far as I know there is no way to do that with SmtpClient. That class uses an internal connection pool.
markymark82 wrote:
The sort of volume it could have to handle is up to around 80k messages + more as the list grows.
Volume/seconds gives the rate.
So 100,000/1 second gives 27 hours.
To make that smaller you have to change the numbers.
Reduce the total number. Reduce the send time (processing or connections)
markymark82 wrote: they want to use it for higher volumes
Simple math - there is going to be an absolute limit to how fast you can send with 5 connections.
You can profile your code. That should insure that you are at least using the connections as fast as they are available. After that the only improvement is by using more connections (up to the point where you saturate some part of the network or reach a processing bottleneck.)
You can simulate what you can actually do by setting up your own SMTP pseudo server to test with. All it should do is access the email (it doesn't send it anywhere.)
markymark82 wrote: Unfortunately, we are using authsmtp
If they want to send more then the company is going to need to pay for a different solution. Certainly looks like that authsmtp can be negotiated with. There are other providers.
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