Problem in Generating 1st 100 Primes

[baltimoredude1]

Hi

I was writing a simple code to generate the first 100 prime numbers.
Everything looks fine to me except the output of the program. What's
wrong with it? I am attaching the program as well as the output. Would
appreciate if someone could mail me at ba************@ gmail.com

Thanks
A M Rahman

//------------------------------------------------------------------------------------------------------------------------------------------

#include<iostre am.h>
#include<proces s.h>

int prime(int number);
int main(void)
{

int counter = 0;

for (int n = 2; n < 1000; n++)
{
if ( prime(n) == 1 )
{
cout << n << " ";
counter ++;
if (counter 99)
{
exit(0);
}

} // close if
} // close for

return 0;
} // close main

int prime(int number)
{

for (int i = 2; i <= number -1 ; i++)
{
if (number % i != 0)
{
return 1;
}

else
{
return 0;
}
} // close for
}

-----------------------------------------------------------------------------------------------------------------------------------//

output

2 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49
51 53 55
57 59 61 63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99 101
103 105
107 109 111 113 115 117 119 121 123 125 127 129 131 133 135 137 139 141
143 145
147 149 151 153 155 157 159 161 163 165 167 169 171 173 175 177 179 181
183 185
187 189 191 193 195 197 199

[BigBrian]

baltimoredude1 wrote:

int prime(int number)
{

for (int i = 2; i <= number -1 ; i++)
{
if (number % i != 0)
{
return 1;
}

else
{
return 0;
}
} // close for
}

This will return true when 'number' mod some number is not 0. 15 mod 2
is not zero ( so your function returns true ), but 15 is not prime.
Basically this prime function isn't implemented right.

-Brian

[Victor Bazarov]

baltimoredude1 wrote:

I was writing a simple code to generate the first 100 prime numbers.

Are you sure about "100"? You seem to only print out first 99, no?

Everything looks fine to me except the output of the program.

LOL... So, everything is OK, only it doesn't work, eh? That means
not everything is OK.

What's
wrong with it?

A couple of things that I immediately see. Comments below...

I am attaching the program as well as the output. Would
appreciate if someone could mail me at ba************@ gmail.com

Thanks
A M Rahman

//------------------------------------------------------------------------------------------------------------------------------------------

#include<iostre am.h>

There is no such standard header. There hasn't been for as long as we had
the Standard. Why are you still writing non-standard code? Should be:

#include <iostream>

#include<proces s.h>

This is a non-standard header. Are you using anything from it, actually?

>
int prime(int number);
int main(void)

"void" between parentheses doesn't make much sense, unless you're writing
a C program. You're not. So, it doesn't. And although it's allowed, you
should take a habit of not using it.

{

int counter = 0;

for (int n = 2; n < 1000; n++)
{
if ( prime(n) == 1 )
{
cout << n << " ";

This should be

std::cout << n << " ";

counter ++;
if (counter 99)
{
exit(0);

This should probably be simply

break;

}

} // close if
} // close for

return 0;
} // close main

int prime(int number)
{

for (int i = 2; i <= number -1 ; i++)
{
if (number % i != 0)
{
return 1;

If there is a remainder from division, shouldn't you be going on trying
to find another divisor?

}

else
{
return 0;
}
} // close for

And once you checked _all_ numbers, what do you return?

}

-----------------------------------------------------------------------------------------------------------------------------------//

output

2 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49
51 53 55
57 59 61 63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99 101
103 105
107 109 111 113 115 117 119 121 123 125 127 129 131 133 135 137 139
141 143 145
147 149 151 153 155 157 159 161 163 165 167 169 171 173 175 177 179
181 183 185
187 189 191 193 195 197 199

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[Kai-Uwe Bux]

baltimoredude1 wrote:

Hi

I was writing a simple code to generate the first 100 prime numbers.
Everything looks fine to me except the output of the program. What's
wrong with it? I am attaching the program as well as the output. Would
appreciate if someone could mail me at ba************@ gmail.com

Thanks
A M Rahman

//------------------------------------------------------------------------------------------------------------------------------------------

#include<iostre am.h>

#include <iostream>

#include<proces s.h>

Not a standard header. Not used anyway.

int prime(int number);

May I recommend

bool is_prime( unsigned number );

>

int main(void)
{

int counter = 0;

for (int n = 2; n < 1000; n++)
{
if ( prime(n) == 1 )
{
cout << n << " ";
counter ++;
if (counter 99)
{
exit(0);
}

} // close if
} // close for

return 0;
} // close main

int prime(int number)
{

for (int i = 2; i <= number -1 ; i++)
{
if (number % i != 0)
{
return 1;
}

else
{
return 0;
}
} // close for
}

Note:

a) This function never returns a value for number == 2. Thus, you have
undefined behavior in main().

b) For number 2, the prime() function is semantically broken. The for loop
never enters the second iteration, i.e., i==2 is the only case that ever
occurs: if number is not a multiple of 2, prime returns 1, if number is a
multiple of 2, prime returns 0. This matches the output.

-----------------------------------------------------------------------------------------------------------------------------------//

output

2 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49
51 53 55
57 59 61 63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99 101
103 105
107 109 111 113 115 117 119 121 123 125 127 129 131 133 135 137 139 141
143 145
147 149 151 153 155 157 159 161 163 165 167 169 171 173 175 177 179 181
183 185
187 189 191 193 195 197 199

[Mark P]

baltimoredude1 wrote:

Hi

I was writing a simple code to generate the first 100 prime numbers.
Everything looks fine to me except the output of the program. What's
wrong with it? I am attaching the program as well as the output. Would
appreciate if someone could mail me at ba************@ gmail.com

It's pretty obvious what's wrong with the output-- you've printed out
every odd number greater than 2.

>
Thanks
A M Rahman

//------------------------------------------------------------------------------------------------------------------------------------------

#include<iostre am.h>
#include<proces s.h>

int prime(int number);
int main(void)
{

int counter = 0;

for (int n = 2; n < 1000; n++)
{
if ( prime(n) == 1 )
{
cout << n << " ";
counter ++;
if (counter 99)
{
exit(0);
}

} // close if
} // close for

return 0;
} // close main

Time for lesson 1 of debugging. Step through the code! Pick a value
where you're getting the wrong result and see what happens. We know 9
isn't a prime so put 9 into your function and watch what it does. When
it says 9 *is* a prime figure out what must be wrong with your logic.

int prime(int number)
{

for (int i = 2; i <= number -1 ; i++)
{
if (number % i != 0)
{
return 1;
}

else
{
return 0;
}
} // close for
}

Here's a hint: For a single call to prime() what's the most number of
times that the for loop can run? Remember that the function (and hence
the loop) will end as soon as a return is encountered.

>
-----------------------------------------------------------------------------------------------------------------------------------//

output

2 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49
51 53 55
57 59 61 63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99 101
103 105
107 109 111 113 115 117 119 121 123 125 127 129 131 133 135 137 139 141
143 145
147 149 151 153 155 157 159 161 163 165 167 169 171 173 175 177 179 181
183 185
187 189 191 193 195 197 199

[Panks]

Hi

Your prime function should be like below : -

int prime(int number)
{

for (int i = 2; i <= number -1 ; i++)
{
if (number % i == 0)
{
return 0;
}

} // close for

return 1;

}

I hope this willl solve your problem

Pankaj

[baltimoredude1]

Victor Bazarov wrote:

baltimoredude1 wrote:

I was writing a simple code to generate the first 100 prime numbers.

Are you sure about "100"? You seem to only print out first 99, no?

HMM... OUTPUT IS POSTED.. PLEASE DO THE COUNTING...

>

Everything looks fine to me except the output of the program.

LOL... So, everything is OK, only it doesn't work, eh? That means
not everything is OK.

LOOKS OK TO ME AND I KNOW IT'S NOT OK....

>

What's
wrong with it?

A couple of things that I immediately see. Comments below...

I am attaching the program as well as the output. Would
appreciate if someone could mail me at ba************@ gmail.com

Thanks
A M Rahman

//------------------------------------------------------------------------------------------------------------------------------------------

#include<iostre am.h>

There is no such standard header. There hasn't been for as long as we had
the Standard. Why are you still writing non-standard code? Should be:

#include <iostream>

#include<proces s.h>

This is a non-standard header. Are you using anything from it, actually?

MAYBE YOU SHOULD DO SOME RESEARCH ON THIS ISSUE.

>

int prime(int number);
int main(void)

"void" between parentheses doesn't make much sense, unless you're writing
a C program. You're not. So, it doesn't. And although it's allowed, you
should take a habit of not using it.

AGAIN DO SOME RESEARCH ON THIS ISSUE.

>

{

int counter = 0;

for (int n = 2; n < 1000; n++)
{
if ( prime(n) == 1 )
{
cout << n << " ";

This should be

std::cout << n << " ";

counter ++;
if (counter 99)
{
exit(0);

This should probably be simply

break;

}

} // close if
} // close for

return 0;
} // close main

int prime(int number)
{

for (int i = 2; i <= number -1 ; i++)
{
if (number % i != 0)
{
return 1;

If there is a remainder from division, shouldn't you be going on trying
to find another divisor?

ANYWAY I FIGURED OUT THE PROBLEM AND IT'S WORKING FINE.

>

}

else
{
return 0;
}
} // close for

And once you checked _all_ numbers, what do you return?

}

-----------------------------------------------------------------------------------------------------------------------------------//

output

2 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49
51 53 55
57 59 61 63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99 101
103 105
107 109 111 113 115 117 119 121 123 125 127 129 131 133 135 137 139
141 143 145
147 149 151 153 155 157 159 161 163 165 167 169 171 173 175 177 179
181 183 185
187 189 191 193 195 197 199

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[baltimoredude1]

I figured it out and it's working fine

Panks wrote:

Hi

Your prime function should be like below : -

int prime(int number)
{

for (int i = 2; i <= number -1 ; i++)
{
if (number % i == 0)
{
return 0;
}

} // close for

return 1;

}

I hope this willl solve your problem

Pankaj

[Victor Bazarov]

baltimoredude1 wrote:

[..]
HMM... OUTPUT IS POSTED.. PLEASE DO THE COUNTING...
[..]
LOOKS OK TO ME AND I KNOW IT'S NOT OK....
[..]
MAYBE YOU SHOULD DO SOME RESEARCH ON THIS ISSUE.
[..]
AGAIN DO SOME RESEARCH ON THIS ISSUE.
[..]
ANYWAY I FIGURED OUT THE PROBLEM AND IT'S WORKING FINE.

For a netiquette violator, you're being too polite. You need
to call me an ass or something. To complete the picture...

[galeon]

I guess it's more efficient!
............... ....
bool is_Prime( unsigned candidate )
{
if ( candidate == 2 )
return ( true );
int lim = sqrt( ( double )candidate );
for ( int index = 3; index <= lim; index += 2 )
if ( candidate % index == 0 )
return ( false );
return ( ( candidate % 2 ) );
}
............... ...