#include<conio.h>
#include<math.h>
#include<stdio.h>
double countValue(int n)
{
    int i;
    double s=0;
    for(i=1;i<n;i++)
        if(i%3==0&&i%7==0)
            s+=i;
        return sqrt(s);
}
main()
{
    printf("自然数之和的平方根=%f\n",countValue(1000));
    progReadWrite();
}

progReadWrite()
{
   FILE*fp,*wf;
   int i,n;
   float s;
   fp=fopen("in.dat","r");
   if(fp=NULL)
   {
       printf("数据文件不存在");
       return;
   }
   wf=fopen("out.dat","w");
   for(i=0;i<10;i++)
   {
      fscanf(fp,"%d\n",&n);
      s=countValue(n);
      fprintf(wf,"%f\n",s);
   }
   fclose(fp);
   fclose(wf);
}

--------------------Configuration: 试验一 - Win32 Debug--------------------
Compiling...
试验一.cpp
F:\考试作业\试验一\试验一.cpp(16) : error C2065: 'progReadWrite' : undeclared identifier
F:\考试作业\试验一\试验一.cpp(17) : warning C4508: 'main' : function should return a value; 'void' return type assumed
F:\考试作业\试验一\试验一.cpp(20) : error C2373: 'progReadWrite' : redefinition; different type modifiers
F:\考试作业\试验一\试验一.cpp(28) : warning C4508: 'progReadWrite' : function should return a value; 'void' return type assumed
F:\考试作业\试验一\试验一.cpp(34) : warning C4244: '=' : conversion from 'double' to 'float', possible loss of data
执行 cl.exe 时出错.

试验一.obj - 1 error(s), 0 warning(s)

主调函数在被调函数之前，应该对被调函数进行说明。否则需要调一下函数的位置顺序。如下：（  另：if(fp=NULL)--->if(fp==NULL))
#include<conio.h>
#include<math.h>
#include<stdio.h>
double countValue(int n)
{
    int i;
    double s=0;
    for(i=1;i<n;i++)
        if(i%3==0&&i%7==0)
            s+=i;
        return sqrt(s);
}
void progReadWrite()
{
   FILE*fp,*wf;
   int i,n;
   float s;
   fp=fopen("in.dat","r");
   if(fp==NULL)
   {
       printf("数据文件不存在");
       return;
   }
   wf=fopen("out.dat","w");
   for(i=0;i<10;i++)
   {
      fscanf(fp,"%d\n",&n);
      s=countValue(n);
      fprintf(wf,"%f\n",s);
   }
   fclose(fp);
   fclose(wf);
}

main()
{
    printf("自然数之和的平方根=%f\n",countValue(1000));
    progReadWrite();
}