程序代码：

// 从文本文件"data.txt"中取出数据，并计算平均值，方差

#include<stdio.h>
#include <math.h>
#define hh printf("\n=======================================\n");
void main()
{
    FILE *fp; 
    float x,avr,fc,a[520],sum = 0.0;
    int i,k = 0;
    fp = fopen("D:\\data.txt","r");
    if(fp == NULL)
    {
        printf("文件打开失败!\n");
    }
    else
    { 
        printf("读取数据：\n");
        fscanf(fp,"%f",&x);

        while(!feof(fp))
        {
            a[k++] = x;
            printf("%.0f ",x);
            sum += x;
            fscanf(fp,"%f",&x);
        }

/*      fscanf(fp,"%f",&x);
        a[k++] = x;
        printf("%.0f ",x);
        sum += x;            */

        avr = sum / k;             // k 为读到的数据个数
        sum = 0; 
        for(i = 0;i < k;i++)
        {
            sum += (a[i] - avr) * (a[i] - avr);
        }
        fc = sum / (k - 1);
        hh
        printf("  读取个数：%d\n",k);
        printf("  平 均 值：%.2f\n",avr);
        printf("  方    差：%.2f\n",fc);

    }
    hh
    fclose(fp);
}

程序代码：

#include <stdio.h>

int main( void )
{
    FILE* fp = fopen("D:\\data.txt","r");
    if(fp == NULL)
    {
        printf("文件打开失败!\n");
        return 1;
    }

    puts( "读取数据：" );
    size_t k = 0;
    float sum=0.0f, sum2=0.0f;
    for( float x; fscanf(fp,"%f",&x)==1; ++k )
    {
        sum2 += x*x;
        sum += x;
        printf( "%.0f ", x );
    }
    fclose( fp );

    puts( "\n=======================================" );
    float avr = sum / k;
    float fc = (sum2 - sum*sum/k)/(k-1);
    printf("  读取个数：%d\n",k);
    printf("  平 均 值：%.2f\n",avr);
    printf("  方    差：%.2f\n",fc);
}