#include<stdio.h>
int sushu(int a);
main()
{    int x;
while (scanf("%d",&x)!='\n')
if(sushu(x))
     printf("%d是一个素数\n",x);
else
     printf("%d不是一个素数\n",x);
}
int sushu(int a)
{int i;
for(i=2;i<=a/2;i++)
 if(a%i==0)
     return 0;
            return 1;
 }

[此贴子已经被作者于2017-3-8 00:26编辑过]

while (scanf("%d",&x)!='\n')这句有问题
你的原意可能是输入回车就退出，但这里x接收到的是十进制的ascii码，建议你改成while (scanf("%c",&x)!='q')，输入Q就跳出。
另外，帖子最好加点分，不然没人

这个n好像和d%没半毛钱关系吧

仔细阅读一下scanf函数，一定出在返回值上

scanf()的返回值是读入的数量，返回0代表没有读入，EOF代表文件尾或者读取错误
msdn上的说法：
Return Value

Both scanf and wscanf return the number of fields successfully converted and assigned; the return value does not include fields that were read but not assigned. A return value of 0 indicates that no fields were assigned. The return value is EOF for an error or if the end-of-file character or the end-of-string character is encountered in the first attempt to read a character.

楼主理解下下面的句子
Both scanf and wscanf return the number of fields successfully converted and assigned