#include<stdio.h>
int main()
{
    int counter = 0;/*输入次数*/
    float liter, km, m, average;/*油量，里程，每公升油量的里程，所有油箱情况平均值*/
    float total = 0;

    printf("enter the liters used, -1 to end::");
    scanf_s("%f", &liter);/*第一次输入油量*/
    printf("enter the killometers driven:");
    scanf_s("%f", &km);/*第一次输入里程*/

    while (liter != -1){
        m = km / liter;/*计算每公升跑了多少公里*/
        counter += 1;/*输入次数计数*/
        total += m;
        printf("the km / liter is %f\n\n", m);
        printf("enter the liters used, -1 to end:");
        scanf_s("%f", &liter);
        printf("enter the killometers driven:");
        scanf_s("%f", &km);
    }
    average = total / counter;
    printf("The overall average km / liter is:%f\n\n", average);/*输出总的平均每公升油量跑了多少公里*/

    return 0;
}

书上的练习题，书上给出了结果，显示出来的结果是这样的：

Enter the gallons used (-1 to end): 12.8
Enter the miles driven: 287
The miles / gallon for this tank was 22.421875
Enter the gallons used (-1 to end): 10.3
Enter the miles driven: 200
The miles / gallon for this tank was 19.417475
Enter the gallons used (-1 to end): 5
Enter the miles driven: 120
The miles / gallon for this tank was 24.000000
Enter the gallons used (-1 to end): -1
The overall average miles/gallon was 21.601423

但是我写的那个while结构里边printf("enter the liters used, -1 to end:");输入-1之后还继续运行到了printf("enter the killometers driven:");这一步，我想是因为这个-1还没运行到条件liter != -1那里，所以会继续运行后边的。但是，我不知道怎么才能解决它，达到书上的那种效果，输入完-1之后就不会再出现多余的东西？

scanf_s("%f", &liter);
这里加上if(liter==-1)break;
printf("enter the killometers driven:");

哦哦，还没学到break，刚去查了一下。原来是这样啊。谢谢你。