C++ program.....

C++ Im dead · 11-03-2002

This is the program....

An integer is said to be prime if it is divisible by only 1 and itself. For example 2,3,5 and 7 are prime, but 4,6,8 and 9 are not.

---A) Write a function that determines weather a number is prime.

---B) Use this function in a program that determines and prints all the prime numbers between 1 and 10,000. HOw many of these 10,000 numbers do you really have to test before being sure that you have found all the primes.

---C) Initially, you might think that n/2 is the upper limit for which you must test to see whether a number is prime, but you need only go as high as the square root of n. WHY? Rewrite a program and run it both ways. Estimate the performance improvement.

........PLEASE HELP!!!!!!!! I was able to do every other program for the exception of this one........and this is the most important of them all..............HELP!!!!!!!!!!!!!!!!!!!!!!

**Hunter2** · 11-03-2002

This really isn't that hard. You should give it a shot yourself! Here's some tips to get started though...

a) if it's 2, it's prime. If it's divisible by anything between 2 and itself, it's not prime. You can use a loop to check that.
b) Use a loop to print out all the primes from 1 to 1000.
for(int i = 1; i <= sqrt(10000); ++i)
if(isPrime(i))
cout << i;

You should at least have figured out (b). I answered, just to be nice for a change, but in the future, don't expect answers to homework if you haven't demonstrated that you've at least tried to figure it out.

The answer to part 2 of (b) is part of question (c), by the way. Just thought I'd point that out, since you didn't seem to have noticed it. And for part C, a useful point to ponder is the fact that x*y = y*x. I won't go farther than that, otherwise I'd have done your entire homework for you. Oh yeah, for the last part of (c), try using a profiler if you have one. It'll greatly improve your estimate's accuracy. Good luck anyways, and don't try to mooch off of us again

**Davros** · 11-03-2002

[groan]
Not another prime number question. Do your own homework.
[/groan]

**Nippashish** · 11-03-2002

Just for the record, any non prime number is divisible by a prime number that is < itself. Also checking to sqrt(n) rather than n/2 to figure out if it's a prime ups speed by a LOT.

**Hunter2** · 11-03-2002

Also checking to sqrt(n) rather than n/2 to figure out if it's a prime ups speed by a LOT.

He could have figured that out just by looking at question (C)

**Nippashish** · 11-03-2002

Originally posted by Hunter2
He could have figured that out just by looking at question (C)

That he could, that he could. Maybe I should finish reading before I hit reply next time.

**Hunter2** · 11-03-2002

Oh well, it might do some good to spam dumb answers at people who come with dumb questions. Keep up the good work!

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