#include<stdio.h>
void main()
{
    int i,m;
    scanf("%d",&m);
    for(i=2;i<=m-1;i++)
        if(m%i==0) break;
            printf("%d is not a prime number\n",m);
        else
            printf("%d is a prime number\n",m);
}

你的代码应该是
#include<stdio.h>
void main()
{
    int i,m;
    scanf("%d",&m);
    for(i=2;i<=m-1;i++)
        if(m%i==0) {
            printf("%d is not a prime number\n",m);return；
        }
     printf("%d is a prime number\n",m);
}

[ 本帖最后由 ileelsai 于 2011-10-22 21:00 编辑 ]

不行啊   如果输入7的话   输出的是5行7 is a prime number啊

[ 本帖最后由 A13433758072 于 2011-10-22 21:11 编辑 ]

看题目 你的意思是这样吧 ：
我加个flag标识符
#include<stdio.h>
void main()
{
    int i,m,flag;
   
    scanf("%d",&m);
   
    for(i=2;i<=m-1;i++)

       if(m%i==0)
        {
           flag=1;
           break;
        }
        
         if(flag==1)   printf("%d is not a prime number\n",m);
          else  printf("%d is a prime number\n",m);
}

#include<stdio.h>
 void main()
 {
     int i,m;
     scanf("%d",&m);
     for(i=2;i<=m-1;i++)
     {if(m%i==0)
     {
     printf("%d is not a prime number\n",m);
     break;
     }
         else
             printf("%d is a prime number\n",m);}
 }

问题很简单，你那个else没有和任何一个if配对，被这条语句（ printf("%d is not a prime number\n",m);
）隔开了与if的联系，编译器不认识，
#include<stdio.h>
void main()
{
    int i,m;
    scanf("%d",&m);
    for(i=2;i<=m-1;i++)
        if(m%i==0)
        {
            break;
            printf("%d is not a prime number\n",m);
         }
        else
            printf("%d is a prime number\n",m);
}