Beginners Task #2

[tomcant]

Beginners Task #2: Prime Factors

I hope most of you know what a prime factor is, but for those of you who dont: A prime factor is a factor of a number that is also a prime number. ie. 7 is a prime factor of 56 and 13 is a prime factor of 169.

Example:


So it can be said that 2, 2, 3, and 3 are the prime factors of 36. ie. When you do 2 x 2 x 3 x 3, you get 36.

Input and Output

The user input for this program should look like this:

Enter number: 36

The output of the program should look something like:

Prime factors: 2 x 2 x 3 x 3

Don't really know about a deadline.... we'll discuss that later

Happy codin'...

[RecursiveS]

Stickied for a while, as last time.

[Guest]

Okay I am in. It will be a few days before I can post anything, busy weekend.

[schloob]

Code: Select all

#include <iostream>
#include <vector>
#include <cmath>

using namespace std;

int prime(int);

int vectorisprime(vector<int> o){

   if (o.size()==0)
      return 0;
   
   for (int i=0;i<o.size();i++){

      if (prime(o[i]))
         return 0;

   }

   return 1;

}

int prime(int n){ // (!prime(x))=prime

   int i;

   for (i=2;i<=sqrt(n);i++){

      if (n%i==0)
         return i;

   }

   return 0;

}

void prime_factors(int n, vector<int> &o){

   o.clear();

   o.push_back(n);

   int i,c;

   while (!vectorisprime(o)){

      for (i=0;i<o.size();i++){

         if ((c=prime(o[i]))){

            o.push_back(c);

            o.push_back(o[i] / (double)c);

            o.erase(o.begin()+i);

            break;
                        
         }

      }

   }

}

int main(){

   vector<int> o;

   int n;

   cin>>n;

   if (!cin) return 1;

   prime_factors(n,o);

   cout<<"factors: "<<endl;

   for (int i=0;i<o.size();i++)
      cout<<o[i]<<" x "<<flush;

   cout<<"\b\b\b   "<<endl;
   
   return 0;

}

*Edit*

latest update

Code: Select all

void factor(int n){
for (int i=2;i<n/2.0;i++)
if (!(n%i)){cout<<i<<" x "<<flush;factor(n/i);return;}
cout<<n<<flush;
}

[tomcant]

Nice one, I hadn't thought of using vectors

[Guest]

schloob I can not get yours to compile it gives me errors with the sqrt.

Code: Select all

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

[C++]

Don't people comment their code anymore

[schloob]

dev cpp version:

Code: Select all

#include <iostream>
#include <vector>
#include <cmath>

using namespace std;

int prime(int);

int vectorisprime(vector<int> o){

   if (o.size()==0)
      return 0;
   
   for (int i=0;i<o.size();i++){

      if (prime(o[i]))
         return 0;

   }

   return 1;

}

int prime(int n){ // (!prime(x))=prime

   int i;

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

      if (n%i==0)
         return i;

   }

   return 0;

}

void prime_factors(int n, vector<int> &o){

   o.clear();

   o.push_back(n);

   int i,c;

   while (!vectorisprime(o)){

      for (i=0;i<o.size();i++){

         if ((c=prime(o[i]))){

            o.push_back(c);

            o.push_back((int)((double)o[i] / (double)c));

            o.erase(o.begin()+i);

            break;
                       
         }

      }

   }

}

int main(){

   vector<int> o;

   int n;

   cin>>n;

   if (!cin) return 1;

   prime_factors(n,o);

   cout<<"factors: "<<endl;

   for (int i=0;i<o.size();i++)
      cout<<o[i]<<" x "<<flush;

   cout<<"\b\b\b   "<<endl;
   
   system("pause");
   
   return 0;

}

[schloob]

Don't people comment their code anymore

commenting is for suckers :]

[Guest]

dev cpp version:

Very nice. This version worked fine for me thank you schloob.

[C++]

Don't people comment their code anymore

commenting is for suckers :]

Not when 25% of your project's grade depends on it

[schloob]

project? what project...

[C++]

Class projects....

Commenting also helps you understand your programs later on. Write a 1000 line program, don't comment it at all, go back to it 3 months later and you won't be able to tell what your program's doing, it'll take a couple of hours to figure things out. But if you comment everything, you can easily read the comments and in minutes you'll know what the code is doing. The same applies if another person is reading your code.

You should comment every variable, stating what it is for
Comment every loop stating the conditions and what it does
Comment every function stating what it takes as parameters, what it does, and what it returns, if any.
Put a comment on top of your program stating the name of the program, your name, last modification date, starting date, purpose of the program, and anything else you feel is important.

[Guest]

This is my version of the task. The program does have some limitations: if any of the primes are over 541 it will not work.

Code: Select all

#include <iostream>

using namespace std;

long primenext(long* prime, long next);

int main()
{
   for(;;)
   {
      long prime = 2;
      int answer;
      cout << endl;
      cout << "Enter a number for Prime Factors : " << endl;
      cin >> answer;
      long factor = 0;
      long next = 1;
      if (answer > 0)
      {
         do
         {
            if (answer % prime == 0)
            {
               factor = answer / prime;
               if (answer == prime)//the end if they are ==
                  answer = 0;
               else
                  answer = factor;
               cout << prime << ((0 == answer) ? "." : "x");
            }
            else                     
            {
               if (next == 100)
               {
                  cout << endl << "One of the Prime Factors is over 541" << endl;
                  break;
               }                  //next++ will decide the array number
               primenext(&prime , next++);//get next prime
            }
         }while(answer > 0);
      }
      else
      {
         cout << "That was out of range." << endl;
         return 0;
      }
   }
}

long primenext(long* prime, long next) //prime number generator
{
   long totalprimes[100] = {2,3};//keep prime to check for primes
   long count = 2;
   long test = 3;
   bool isprime = true;

   if(*prime == 2)
      *prime =3;
   else
   {
      do
      {
         test += 2;
         int i =0;
         do
         {
            isprime = test % totalprimes[i] > 0;//find next prime
         }while(++i < count && isprime);
         if (isprime)
            totalprimes[count++] = test;//store in the array
      }while(count < next && count < 100);

      *prime = test;//take new prime
      return *prime;
   }
}

[schloob]

Class projects....

Commenting also helps you understand your programs later on. Write a 1000 line program, don't comment it at all, go back to it 3 months later and you won't be able to tell what your program's doing, it'll take a couple of hours to figure things out. But if you comment everything, you can easily read the comments and in minutes you'll know what the code is doing. The same applies if another person is reading your code.

You should comment every variable, stating what it is for
Comment every loop stating the conditions and what it does
Comment every function stating what it takes as parameters, what it does, and what it returns, if any.
Put a comment on top of your program stating the name of the program, your name, last modification date, starting date, purpose of the program, and anything else you feel is important.

lol i have like a 2000-3000 line program i havent worked on for ~5-6 months, i go back to it and its still very easy to understand :-]

although my word count program is hard as hell to understand (for me, slow mind :-p)

if you want to see it, this is it

http://poop.mine.nu/wordcount.txt

anyways, comments just arent my style :-\

*edit*

also, i dont take a programming class so i just dont need to comment :-\