Need help correcting my code for calculating sexy prime pairs

**BinaryPenguin14** · November 22nd, 2011, 11:43 PM

So I have been given a problem in class stating that I had to calculate the sexy prime pairs. I have to use the sieve of eratosthenes alogorithm and a array to calculate the prime numbers and then use that same array to display the sexy prime pairs in between boundaries that are specified by the user. Much help will be appreciated thanks.

This is the sample run.

Please enter a lower boundary and an upper boundary and I will print all of the
sexy prime pairs between those boundaries.
Please enter the lower boundary (between 1 and 50000): 0
Please enter the lower boundary (between 1 and 50000): 50001
Please enter the lower boundary (between 1 and 50000): 150
Please enter the upper boundary (between 1 and 50000): 0
Please enter the upper boundary (between 1 and 50000): 50001
Please enter the upper boundary (between 1 and 50000): 100
Your upper boundary cannot be smaller than your lower boundary
Please enter the lower boundary (between 1 and 50000): 8997
Please enter the upper boundary (between 1 and 50000): 9200
Here are all of the sexy prime pairs in the range 8997 to 9200, one pair per line:
9001 and 9007
9007 and 9013
9043 and 9049
9103 and 9109
9127 and 9133
9151 and 9157
9181 and 9187
There were 7 sexy prime pairs displayed between 8997 and 9200.

**Junky** · November 23rd, 2011, 12:04 AM

Any chance you could explain what your problem is and ask a specific question?

**Mr.777** · November 23rd, 2011, 12:09 AM

for (int e = 2; e <= 223; e++)
        {
            if (primes[e])
            {
                for (int f = e; f <= 223; f++)
                {
                    int g = (e * f);
                    primes[g] = true;
                }
 
            }
 
 
        }

And

                    int g = (e * f);
                    primes[g] = true;

What do you expecting it to do?

int g = e*f;

Let's say e=5 and f = 10, so?

I have looked on your code. A real immature approach. May be with some fixes it will lead you to the solution but in the end, not efficient enough.

**BinaryPenguin14** · November 23rd, 2011, 01:30 AM

My major problem is calculating the prime numbers using the sieve of eratosthenes algorithm. How do I Loop through the factors and mark them like and unmark them if they are prime?

**BinaryPenguin14** · November 23rd, 2011, 01:37 AM

My problems are the prime number algorithm and my sexy prime pairs im a little unsure as to how to calculate them and I understand my attempts to do so have not been very good.

**Mr.777** · November 23rd, 2011, 04:05 AM

I will recommend you to take a pencil and a paper and draw, how will you do it manually.
Write all the steps and then code them.

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