[C++ ] find prime number

[jasonfghx (工人)]

功能

用到的觀念

CODE
/*DESCRIPTION:find the prime number*/
#include"iostream"//標頭檔
#include"iomanip"
#include"cmath"
using namespace std;
using std::setw;
using std::fixed;
using std::setprecision;

int main()
{
        unsigned n;//n save the number of the end,
        int i,j,m=0;//declare the number for the loop
        cout<<"find the number form 1 to n, enter the n"<<endl;//顯示內容
        cin>>n;//輸入存到n
        system("cls");//clean the screen
        cout<<"the prime number are:"<<"between 1 to "<<n<<endl;//顯示內容
        if(n==2)//check whether n is equal to 2
        {
                cout<<"2";//顯示內容
        }
        if(n==3)//check whether n is equal to 3
        {
                cout<<"2"<<endl;//顯示內容
                cout<<"3"<<endl;//顯示內容
        }
        if(n>3)
        {
                cout<<"2 "<<endl;//顯示內容
                cout<<"3 "<<endl;//顯示內容
                for(i = 2; i<= n;i++)//從2檢查到n判斷是否為質數
                {

                        for(j = 2; j <=sqrt(double(i)); j++)//extract a root for i. this makes the
calculation quicker
                        {
                                if(i%j==0)//if i is divided with no remainder
                                {
                                        m=1;//save 1 to m
                                        break;//jump from this loop
                                }
                                else//i is divided with remainder
                                        m=2;
                        }
                        if(m==2)//if m=2 it proves that i is a prime number
                        {
                                cout<<i<<endl;///show the prime numbers
                        }
                }
        }

        system("pause");
        return 0;
}


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