Outputting Prime Numbers

[flyaway888]

Hey All,
I need a program to input a number, test if it is a prime number or not and then output it if it is prime or output the next prime number if it is not.
So far I have to input section, the function and the prime number test so it works if it is prime but I can't figure out how to output the next prime number.
Any help would be much appreciated.

[ilikepython]

Hello,
for the next prime number can't you just loop the prime number testing function from the non-prime until it finds a prime number and then return that. If that doesn't work, how does your function work then?

[flyaway888]

I wasn't sure how to loop a function. Do I just put it in a for loop like any other value?

[seninfothil]

hi friend
i red you question .
let a be the number is not a prime ...
then we go for a looping statement form a to a+100...
i am sure that there must be a prime in between that range....

[palani12kumar]

hi friend
i red you question .
let a be the number is not a prime ...
then we go for a looping statement form a to a+100...
i am sure that there must be a prime in between that range....

I think you need not to go on checking upto next 100 numbers to find the next prime of the number which is already been found not to be a prime.
its enough to try something like this:

 read the value of a;
do{
 if a is not a prime
 i)increment a by 1(ie.)a++ );
 ii)check whether a++ is prime or not
    a.)if a++ is prime, print the result and get out of the loop
    b.)else continue the process
}while(a is not a prime)
 

[JosAH]

hi friend
i red you question .
let a be the number is not a prime ...
then we go for a looping statement form a to a+100...
i am sure that there must be a prime in between that range....

*ahem* The following two numbers are both prime numbers and are more than
100 apart (they are the smallest two primes having this property)

370261 370373

kind regards,

Jos

[flyaway888]

Thanks for all your help,
I have tried doing the do...while loop and the output comes up with nothing and I can't understand why.
I use an if..else statement to check if it is prime at first and output it if it is. This all works. Then from the else part I did this loop...

else
       do
       n++;
       while (n != prime);
 
       cout << n << endl;
 

Any ideas why this wouldn't work?

[Ganon11]

You have forgotten the brackets around your loop. You need

do {
   n++;
} while (n != prime);

Also, shouldn't you be using your prime() function? This looks like you are comparing n with another number.

[flyaway888]

Oh I did have brackets at one point but I keep changing it! I change to function() to prime

bool prime;
   prime = isprime (n);
   if (prime)
   cout << n << endl;
   do
   { 
        n++;
   } while (!prime);  
   cout << n << endl;   

So its actually like this
But with brackets it still doesn't work and I don't understand why.

[Ganon11]

You should probably use a while loop rather than a do...while loop, and you should have the loop under an else clause instead of after your if statement.

[flyaway888]

So you mean something like this...

bool prime;
   prime = isprime (n);
   if (prime)
   cout << n << endl;
   else
   while (!prime)
   {
         n++;
   }  
   cout << n << endl;   

Because this does the same thing, outputs nothing.

[JosAH]

So you mean something like this...

bool prime;
   prime = isprime (n);
   if (prime)
   cout << n << endl;
   else
   while (!prime)
   {
         n++;
   }  
   cout << n << endl;   

Because this does the same thing, outputs nothing.

If variable 'prime' was false to start with it stays false in your code because
you never attempt to change it anywhere in your loop.

I'd simply do this:

for (; !isPrime(n); n++)
   ;
cout << n << endl;

kind regards,

Jos