poj2909 | | poj2262

#include <stdio.h>#include<stdlib.h>#include<math.h>intIspri (intAintb) {    inti;  for(i=2; I <= sqrt (a); i++)    {        if(a%i = =0)            return 0; }     for(i=2; I <= sqrt (b); i++)    {        if(b%i = =0)            return 0; }    return 1;}intMain () {intn,flag,i,cout;  while(SCANF ("%d", &n)! = EOF &&N) {cout=0;  for(i=2; i< n/2+1; i++) {flag=ispri (i,n-i); if(Flag = =1) cout++; } printf ("%d\n", cout); }    return 0;}

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We use the Eratosthenes sieve method to play the table

Algorithm Description: Less than 2 is not a prime, with an array of a[10000], the element subscript indicates the number, the value of the element is the prime, C + + practice is, bool a[10000], will a[0] and A[1] assigned 0, starting from 2, 2*2,2*3 ...

2*6. These subscript corresponding element values are changed to 0, in this environment again looking for the next prime, once again, 3*2,3*3,3*4 ... product overall less than 10000

    inta[10000]; a[0]=0; a[1]=0;  for(i=2;i<10000; i++) A[i]=1;  for(i=2;i<10000; i++)    {        if(A[i] = =1)        {             for(j=2;j<10000; j + +) {A[i*J] = =0; }        }    } for(intj=2; j<=n;++j) {          if(isprimes[j]==true)               for(intm=2; j*m<=n;++m) Isprimes[j*m] =false;

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See:

http://blog.csdn.net/kingwolfofsky/article/details/7199758

poj2909 | | poj2262