I have noticed that some people on the Rennlist are computer programmers or have programming experience. I am taking a computer science course and I am stuck on writing a program (in C) that involves printing all the prime numbers less than a number that is entered. I know this is probably a real basic program, but I have been working on it for a few hours and I can't get it right. I have the program that indicates if a number is prime, but that has not been much help yet. All I need is a step in the right direction. Any help will be appreciated. Thank you.

 

I'm taking Java at the moment, which is similar to C. What does your logic look like? Ire you having trouble with syntax or the logic part? I might be able to help.

 

It is not allowing me to post what I have. It is referring to my code as HTML tag.

 

Post your code or send it out in an E-mail. It's been awhile, but I think I might be able to remember enough to help. Not sure if I'll have the time before your deadline, but perhaps someone will.

 

Here is an example on calculating prime numbers.

<a href="http://www.syix.com/wpsjr1/primes.html" target="_blank">http://www.syix.com/wpsjr1/primes.html</a>

You just calculate all the numbers up to the value entered, have fun.

 

cut and paste the code into the message.

 

Sorry, we do not permit this HTML tag:Parenthesis in HTML tag
This is the message I am getting when trying to post my code. Let me work with it a little more. Thanks for everyone's response.

 

You need to make a loop that tests every number between 0 and the number that was entered. When the number is prime, print it out. You can do this with either a for() loop or a while() loop.

 

</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">code:</font><hr /><pre style="font-size:x-small; font-family: monospace;">#include &lt;stdio.h&gt;

void main()
{
int i,j,num;
printf(&quot;Enter the limit number: &quot;
scanf(&quot;%d&quot;, &amp;num);

for(i=3;i&lt;=num;i++)/*iterates through numbers from 3 to the number entered*/
{
for(j=i-1;j&gt;1;j--)/*iterates backwards from i-1 checking to see if i is divisible by every other number &quot;before&quot; it.*/
{
if(!(i%j)) /*if i%j==0, the number is divisible by another(not prime)*/
break;/*breaks out of the innermost for loop*/
}
if(j==1) /*if j==1 the for above finished without finding i%j == 0 therefore the number in i is prime*/
printf(&quot;%d &quot;,i);
}
printf(&quot;\n&quot;
}</pre><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva"><img border="0" alt="[thumbsup]" title="" src="graemlins/bigok.gif" />
edit: I edited the comments a bit but couldn't get italics to work <img border="0" title="" alt="[Frown]" src="frown.gif" />

 

YOU ARE MY IDOL!!! I'll be sure to put your name on the assignment. I don't like stealing credit from people.

 

Finally, after about a dozen edits I think I'm happy with the look of the comments. <img border="0" alt="[ouch]" title="" src="graemlins/c.gif" />

p.s. I won't consider it stolen if you learn from it.

 

Ok... that algorithim is quick and ditry, and it will take ages to determine larger primes. I therefore suggest the following method using a dynamic algorithim

I didn't compile/test/etc, so for all I know there are bugs, typos, etc.; but the math is good

</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">code:</font><hr /><pre style="font-size:x-small; font-family: monospace;">
/* This is the number that n cannot exceed, perhaps something
like 32767 or 65535 would be better */
#define MAX_PRIMES 10000

/* Global Variables */
/* Array to hold prime numbers */
long primes[MAX_PRIMES];

/* Current position in prime array index */
int iIndex;

/* Loop variable */
int i;

void Generate(long n)
{
/* The first prime is 1, put it in the array
This is needed as a starting point for the
algorithim */
iIndex = 0;
primes[iIndex] = 1;

for (i = 2; i &lt; n; i++)
{
if (IsPrime(i) == TRUE)
{
iIndex++;
primes[iIndex] = i;
}
}
}

bool IsPrime(long n)
{
long i;
long lRoot;

lRoot = sqrt(n);
for (i = 1; i &lt;= iIndex; i++)
{
if (primes[i] &lt;= lRoot)
{
if ((n % primes(iIndex)) == 0)
return FALSE;
} else {
return TRUE;
}
}
return TRUE;
}</pre><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">But if you're in a class where making a simple prime number generator is your assignment, then using an algorithim like above will probably get you suspected of cheating <img border="0" title="" alt="[Wink]" src="wink.gif" />

 

For the future, arstechnica.com has an excellent forum, one if which is dedicated to coding. The folks around there are often very smrt.

 

Once again, thank you everyone for your great help.