关于error C2065: 'U' : undeclared identifier是怎么回事啊？在程序头定义过的

#include <math.h>
main()
{ clrscr();
float a[]={3.6124,3.6127,3.6122,3.612};
float x=0.0002;
float sum1=0.0000,sum2=0.0000;
float average,S-x,t,S;
float U;
int i,j,k;
for(i=0;i<=100;i++)
{ sum1=sum1+a[i];
printf(" %d %f\n",i+1,a[i]);
}

average=sum1/i;
printf("\n平均值（average）=%f\n",average);
for(j=0;j<i;j++)
{ sum2=sum2+(a[j]-average)*(a[j]-average);
}
S-x=sqrt(sum2/i/(i-1));
printf("平均值的标准误差(S-x)=%f",S-x);
switch(i)
{ case 2: t=1.84;break;
case 3: t=1.32;break;
case 4: t=1.20;break;
case 5: t=1.14;break;
case 6: t=1.11;break;
case 7: t=1.09;break;
case 8: t=1.08;break;
case

error C2065: 'CString' : undeclared identifier error C2065: 'cvNormalize' : undeclared identifier error C2065的问题 高手来啊,多少会都不在乎error C2065: 'DirectInputCreateEx' : undeclared identifier 关于SynthCore Error 关于runtime error 关于VC的Error 关于Runtime Error 的问题 关于stop: c000021a Hard error 紧急！电脑问题关于error!