计算C语言中的求和、标准差、方差和标准差等，需要加上头文件：#include

#include
#include "math.h" 
 double sum = 0;//求和
 double array[4] = {1.2,2.1,3.1,4.1};
 int length = 0;//数组长度
 double average = 0;//求平均数
 double var = 0; //求方差
 double standard = 0; //求标准差 
 int main()
 {
  for (int i = 0; i <= 3;i++)
  {
   sum += array[i];//求和
  }
 
  length = sizeof(array) / sizeof(array[0]);//求数组长度
  average = sum / length;//求平均值
 
  for (int j = 0; j <= 3;j++)
  {
   var += pow(array[j]-average,2)/length;//求方差
  } 
  standard = pow(var,0.5);//求标准差
 
  printf("sum = %g\n",sum);//这边打印也可以使用printf("%f\n",sum);这种打印默认出来的数是小数点后6位
  printf("length of array[4] = %d\n",length);
  printf("average = %g\n",average);
  printf("var = %g\n",var);
  printf("standard = %g\n",standard);
  while (1);
  return 0;
 }

补充知识：C语言：计算并输出给定10个数的方差

//计算并输出给定10个数的方差。

#include
#include
double fun(double x[10])
{
 double p = 0.0,f=0.0,g=0.0;
 for (int i = 0; i < 10; i++)
 {
  p += x[i];
 }
 p = p / 10;
 printf("%f\n", p);
 for (int j = 0; j < 10; j++)
 {
  g = x[j] - p;
  f += g*g;
  printf("%f\n", f);
 }
 f = f / 10;
 f = sqrt(f);
 return f;
}
void main()
{
 FILE *wf;
 double s,x[10]={95.0,89.0,76.0,65.0,88.0,72.0,85.0,81.0,90.0,56.0};
 int i;
 printf("\nThe original data is:\n");
 for(i=0;i<10;i++)
  printf("%6.1f ",x[i]);
 printf("\n\n ");
 s=fun(x);
 printf("s=%f\n\n ",s);
/******************************/
 wf=fopen("out.dat","w");
 fprintf(wf,"%f",s);
 fclose(wf);
/*****************************/
}