#include <stdafx.h>
#include "iostream"
#include <cmath>
#include <stdlib.h>
using namespace std;
void main(){
    cout<<"please input a number.\n";
    int m;
    double n;
    cin>>m;
    n=sqrt(m*1.0);
    for(int i=1;i<=n;++i)
        if(m%i==0){
            cout<<m<<" isn't a prime.\n ";
             }
        else cout<<m<<"  is a prime.\n";
        system("pause");
        }
搞不懂。。会出现这样的结果
please input a number.
4
4  isn't a prime.
 4 isn't a prime.

输入5时有不一样了

please input a number.
5
5 isn't a prime.
 5 is a prime.

这么说吧
你运行
 if(m%i==0){
            cout<<m<<" isn't a prime.\n ";
             }
        else cout<<m<<"  is a prime.\n";
有问题
因为i在变化，所以会运行根号n次
当输入4时候
i=1:输出isn't一次
i=2:输出isn't第2次
当输入5时候
i=1:输出isn't一次
i=2:因为！=0，所以输出is了
你可以吧 for(int i=1;i<=n;++i)改成 for(int i=2;i<=n;++i)

for(int i=1;i<=n;++i)
        if(m%i==0){
            cout<<m<<" isn't a prime.\n ";
             }
        else cout<<m<<"  is a prime.\n";
        system("pause");
显然的逻辑错误
C++有break吗 没学过

你那是具体问题具体分析
n变了，就错了

可以改为  if(m%i==0)break;
if(i>m)cout<<m<<"  is a prime.\n";
else
cout<<m<<" isn't a prime.\n ";   
你试试

for(int i=1;i<=n;++i)
        
        if(m%i==0)
            
    {    cout<<m<<" isn't a prime.\n ";break;
            
        }
        else     
    cout<<m<<"  is a prime.\n";
那我把后天改成这样，，为什么不会多次输出is