程序代码：

#include<stdio.h>
#include<stdlib.h>

float average(int *arr,int i);//求平均值

int main(void)
{
    int arr[5];
    int i=0,j;
    FILE *fp;

    if ((fp = fopen("yy.txt","r")) == NULL)
    {
        fprintf(stdout,"Can't open yy.txt.\n");
        exit(1);
    }
    while(fscanf(fp,"%d",&arr[i])==1)
    {
        i++;
        if(i==5)
        {
           
            for(j=0;j<5;j++)
                printf("%d ",arr[j]);
            printf("的平均值是：%.3f\n",average(arr,i));
            i=0;
        }
    }
    if(i>0 && i!=5)
    {
        for(j=0;j<i;j++)
            printf("%d ",arr[j]);
        printf("的平均值是：%.3f\n",average(arr,i));
    }
    if(fclose(fp)!=0)
        fprintf(stderr,"Error closeing file.\n");

    return 0;
}

float average(int *arr,int i)
{
    int j,sum=0;
    for(j=0;j<i;j++)
        sum+=arr[j];
    return (float)sum/(float)i;
}