用 double 换就行了
#include <stdio.h>
int main()
{
double v1,v2;
           printf("*****************************************\n");
           printf("*                                       *\n");
         printf("*   please input ______ km              *\n");
         printf("*   is equal to  ______ m               *\n");
          printf("*                                       *\n");
         printf("*                                       *\n");
          printf("*****************************************\n");
        scanf("%lf",&v1); v2 = 1000*v1;
//         clrscr();
           printf("*****************************************\n");
           printf("*        1 Length   conversion          *\n");
         printf("*        2 Area     conversion          *\n");
         printf("*        3 Pressure conversion          *\n");
        printf("*          %lf km = %lf m               *\n",  v1,v2  );
         printf("*****************************************\n");
}

正确代码:
#include <stdio.h>
int main()
{  float v1,v2;
           printf("*****************************************\n");
           printf("*                                       *\n");
         printf("*   please input ______ km              *\n");
         printf("*   is equal to  ______ m               *\n");
          printf("*                                       *\n");
         printf("*                                       *\n");
          printf("*****************************************\n");
        scanf("%f",&v1);
        v2 = 1000*v1;
         
           printf("*****************************************\n");
           printf("*        1 Length   conversion          *\n");
         printf("*        2 Area     conversion          *\n");
         printf("*        3 Pressure conversion          *\n");
        printf("*          %f km = %f m               *\n",  v1,v2  );
         printf("*****************************************\n");
}

以后有问题加我的编程群 108668542

以后有问题加我的编程群 108668542

没用啊。

10L正解！！

你有没有试过？ 不行呢-NAN

你的v2不能是int型的，改成无符号型的简单的方法就是吧下面的输出换成无符号unsigned就行了
    #include <stdio.h>
int main()
{ int v1;int v2;
           printf("*****************************************\n");
           printf("*                                       *\n");
         printf("*   please input ______ km              *\n");
         printf("*   is equal to  ______ m               *\n");
          printf("*                                       *\n");
         printf("*                                       *\n");
          printf("*****************************************\n");
        scanf("%d",&v1); v2 = 1000*v1;
         clrscr();
           printf("*****************************************\n");
           printf("*        1 Length   conversion          *\n");
         printf("*        2 Area     conversion          *\n");
         printf("*        3 Pressure conversion          *\n");
        printf("*          %d km = %um               *\n",  v1,v2  );
         printf("*****************************************\n");
         getch();

}

收到的鲜花

• loveisaac 于 2010-04-25 21:08 送鲜花  3朵   附言：我很赞同

很不错解决了。50时候的问题。但是继续增加值 达到66时候又出现错误了。。。66KM不能转换·
还有哪有误吗？

换个数据类型