cout prime numbers

Victor89 (62)
I want to write a program that will print out prime numbers in order.
My program right now all it does is checking if the number is prime or not, and prints "1" for prime, "0" if it's not prime, and prints out instead of prime numbers, only 0's and 1's.

I have included <math.h> library, so why do I get the error for sqrt(n)?

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#include <iostream>
#include <conio.h>
#include <math.h>
using namespace std;

int PrimeNumber(int);

int main()
{
	int n;
	cout<<"n="; cin>>n;
	for(int i=0; i<n; i++)
	cout<<PrimeNumber(i)<<" ";

	_getch();
}

int PrimeNumber(int n)
{
	for(int i=2;i<sqrt(n); i++)  // error C2668: 'sqrt' : 
                                      //  ambiguous call to overloaded function
		if(n%i==0)
		{
			return false;   //If it isn't a prime number
		}
		
		
			return true; //If it is a prime number
		

}
cronopio (95)
as you are using Visual Studio IDE?
cronopio (95)
the sqrt function receives as a parameter data type double, and you're passing an int...?
you need to make a cast.
 
 
sqrt((double)n)
Victor89 (62)
Thanks that worked for sqrt, but still I can't figure out a way to print the prime numbers.
cronopio (95)
I recommend you read this article, and when you find the solution, the insert in the post to see what works for you:

http://www.thegeekstuff.com/2014/03/cpp-prime-number-checker/

@cronopio
Josue Molina (281)
You don't need to import a library for such a simple problem. If you can do some math, you'll see that i < √n is the same as i² < n. Therefore, your prime number function can look like this:

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int PrimeNumber(int n)
{
    for(int i = 2; i * i < n; i++)
        if(n % i == 0)
            return false;    
    return true;
}

Also, check only odd numbers; every even number except for two is not prime.
Victor89 (62)
I read that article and I implemented it, but it doesn't work for all numbers, I would still stick to my code.
Jacobhaha (153)
This will cout prime number from 2 to 100. i hope this helps

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#include <iostream>
using namespace std;

int main ()
{
   int i, j;

   for(i=2; i<100; i++) {
      for(j=2; j <= (i/j); j++) {
        if(!(i%j)) break; // if factor found, not prime
      }
      if(j > (i/j)) cout << i << " is prime\n";
   }
   return 0;
}
Last edited on
giblit (3750)
I would suggest a sieve of Eratosthenes. http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes as it is trivial to implement and is fairily fast for what you are doing. Though if you want a lot like > 2 billion primes you should use a more efficient sieve.


Something like:

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#include <vector>
#include <iostream>
#include <iomanip>

int main()
{
    std::size_t const largest = 10000;
    std::vector<bool> sieve(largest + 1, true);
    std::size_t const sqrt = 100;
    for(std::size_t i = 3; i < sqrt; i += 2)
    {
        if(sieve.at(i))
        {
            for(std::size_t j = i * i; j <= largest; j += i)
            {
                sieve.at(j) = false;
            }
        }
    }
    
    std::cout << std::setw(5) << 2;
    
    std::size_t count = 1;
    for(std::size_t i = 3; i <= largest; i += 2)
    {
        if(sieve.at(i))
        {
            std::cout << std::setw(5) << i;
            if(++count % 20 == 0)
            {
                std::cout << std::endl;
            }
        }
    }
    
    return 0;
}
 
    2    3    5    7   11   13   17   19   23   29   31   37   41   43   47   53   59   61   67   71
   73   79   83   89   97  101  103  107  109  113  127  131  137  139  149  151  157  163  167  173
  179  181  191  193  197  199  211  223  227  229  233  239  241  251  257  263  269  271  277  281
  283  293  307  311  313  317  331  337  347  349  353  359  367  373  379  383  389  397  401  409
  419  421  431  433  439  443  449  457  461  463  467  479  487  491  499  503  509  521  523  541
  547  557  563  569  571  577  587  593  599  601  607  613  617  619  631  641  643  647  653  659
  661  673  677  683  691  701  709  719  727  733  739  743  751  757  761  769  773  787  797  809
  811  821  823  827  829  839  853  857  859  863  877  881  883  887  907  911  919  929  937  941
  947  953  967  971  977  983  991  997 1009 1013 1019 1021 1031 1033 1039 1049 1051 1061 1063 1069
 1087 1091 1093 1097 1103 1109 1117 1123 1129 1151 1153 1163 1171 1181 1187 1193 1201 1213 1217 1223
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 1523 1531 1543 1549 1553 1559 1567 1571 1579 1583 1597 1601 1607 1609 1613 1619 1621 1627 1637 1657
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 2293 2297 2309 2311 2333 2339 2341 2347 2351 2357 2371 2377 2381 2383 2389 2393 2399 2411 2417 2423
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 2621 2633 2647 2657 2659 2663 2671 2677 2683 2687 2689 2693 2699 2707 2711 2713 2719 2729 2731 2741
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 3259 3271 3299 3301 3307 3313 3319 3323 3329 3331 3343 3347 3359 3361 3371 3373 3389 3391 3407 3413
 3433 3449 3457 3461 3463 3467 3469 3491 3499 3511 3517 3527 3529 3533 3539 3541 3547 3557 3559 3571
 3581 3583 3593 3607 3613 3617 3623 3631 3637 3643 3659 3671 3673 3677 3691 3697 3701 3709 3719 3727
 3733 3739 3761 3767 3769 3779 3793 3797 3803 3821 3823 3833 3847 3851 3853 3863 3877 3881 3889 3907
 3911 3917 3919 3923 3929 3931 3943 3947 3967 3989 4001 4003 4007 4013 4019 4021 4027 4049 4051 4057
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 4241 4243 4253 4259 4261 4271 4273 4283 4289 4297 4327 4337 4339 4349 4357 4363 4373 4391 4397 4409
 4421 4423 4441 4447 4451 4457 4463 4481 4483 4493 4507 4513 4517 4519 4523 4547 4549 4561 4567 4583
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 5449 5471 5477 5479 5483 5501 5503 5507 5519 5521 5527 5531 5557 5563 5569 5573 5581 5591 5623 5639
 5641 5647 5651 5653 5657 5659 5669 5683 5689 5693 5701 5711 5717 5737 5741 5743 5749 5779 5783 5791
 5801 5807 5813 5821 5827 5839 5843 5849 5851 5857 5861 5867 5869 5879 5881 5897 5903 5923 5927 5939
 5953 5981 5987 6007 6011 6029 6037 6043 6047 6053 6067 6073 6079 6089 6091 6101 6113 6121 6131 6133
 6143 6151 6163 6173 6197 6199 6203 6211 6217 6221 6229 6247 6257 6263 6269 6271 6277 6287 6299 6301
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 6481 6491 6521 6529 6547 6551 6553 6563 6569 6571 6577 6581 6599 6607 6619 6637 6653 6659 6661 6673
 6679 6689 6691 6701 6703 6709 6719 6733 6737 6761 6763 6779 6781 6791 6793 6803 6823 6827 6829 6833
 6841 6857 6863 6869 6871 6883 6899 6907 6911 6917 6947 6949 6959 6961 6967 6971 6977 6983 6991 6997
 7001 7013 7019 7027 7039 7043 7057 7069 7079 7103 7109 7121 7127 7129 7151 7159 7177 7187 7193 7207
 7211 7213 7219 7229 7237 7243 7247 7253 7283 7297 7307 7309 7321 7331 7333 7349 7351 7369 7393 7411
 7417 7433 7451 7457 7459 7477 7481 7487 7489 7499 7507 7517 7523 7529 7537 7541 7547 7549 7559 7561
 7573 7577 7583 7589 7591 7603 7607 7621 7639 7643 7649 7669 7673 7681 7687 7691 7699 7703 7717 7723
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 7927 7933 7937 7949 7951 7963 7993 8009 8011 8017 8039 8053 8059 8069 8081 8087 8089 8093 8101 8111
 8117 8123 8147 8161 8167 8171 8179 8191 8209 8219 8221 8231 8233 8237 8243 8263 8269 8273 8287 8291
 8293 8297 8311 8317 8329 8353 8363 8369 8377 8387 8389 8419 8423 8429 8431 8443 8447 8461 8467 8501
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 8681 8689 8693 8699 8707 8713 8719 8731 8737 8741 8747 8753 8761 8779 8783 8803 8807 8819 8821 8831
 8837 8839 8849 8861 8863 8867 8887 8893 8923 8929 8933 8941 8951 8963 8969 8971 8999 9001 9007 9011
 9013 9029 9041 9043 9049 9059 9067 9091 9103 9109 9127 9133 9137 9151 9157 9161 9173 9181 9187 9199
 9203 9209 9221 9227 9239 9241 9257 9277 9281 9283 9293 9311 9319 9323 9337 9341 9343 9349 9371 9377
 9391 9397 9403 9413 9419 9421 9431 9433 9437 9439 9461 9463 9467 9473 9479 9491 9497 9511 9521 9533
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 9739 9743 9749 9767 9769 9781 9787 9791 9803 9811 9817 9829 9833 9839 9851 9857 9859 9871 9883 9887


[edit]added formatting to the output[/edit]
Last edited on
Victor89 (62)
Thank you all for your answers, I implemented Jacobhaha solution.
But still I have a small problem making a function out of it.

This is what I have so far:

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#include <iostream>
#include <conio.h>
using namespace std;


int PrintPrime(int);

int main()
{
	
	int n;
	cout<<"n="; cin>>n;
	for(int i=2; i<=n; i++)
	cout<<PrintPrime(i)<<" ";  // calling the function should print out the prime numbers
	_getch();
}


int PrintPrime(int n)
{
	int i, j;
	
	for(i=2; i<=n; i++)
		for(j=2; j<(i/j); j++)
		{
			if((i%j) == 0) break;
		}
		if(j>n)
			return i;


}
Victor89 (62)
After working a little more on this exercise I finally came out with a solution.

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int PrimeNumber(int);

int main()
{
	int n;
	cout<<"n="; cin>>n;
	for(int i=2; i<n; i++)
	if (PrimeNumber(i)==true)
		cout<<i<<" ";

	_getch();
}

int PrimeNumber(int n)
{
	
		for( int i=2; i<n; i++) //for(int i=2;i<sqrt(double(n)); i++) , strange that it   
                                     // prints out numbers that are not prime
		if(n%i==0)
		{
			return false;   //If it isn't a prime number
		}
		
		
			return true; //If it is a prime number
		

}


But still I don't understand why it doesn't show only primes when I use the divisors until sqrt(n), I also commented the code there.
wildblue (1505)
If you call the Primenumber function on 4...

for(int i=2;i<sqrt(double(n)); i++)

The square root of 4 is 2 - i starts at 2 and 2 is not less than 2, so the if statement will not be run and the Primenumber function will return true.

There's really no need to check even numbers greater than 2, since those aren't going to be prime.
Victor89 (62)
for(int i=2;i<=sqrt(double(n)); i++)

It works perfectly for this code, I put "<" instead of "<=".
dhayden (5798)
Your last example there will compute sqrt(n) each time through the loop. It's better to do it once: Also, if you do a special purpose check for even numbers then you can skip them in the loop. To me, this is a nice trade off between a simple algorithm and fast runtime:

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if (n%2 == 0) return false;  // it's even
int sqrtn = sqrt(double(n));
for (int i=3; i<sqrtn; i += 2)

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