程序代码：

int set_mode(int mode)     
{
    
    if(mode != 0 && mode != 1)
    {
        printf("Invalid mode specified.mode \"1\" (us) used.\n");
        mode = 1;
    }
    return mode;
}

//  按照模式获取相应的距离和油耗
double * get_info(int mode)    
{
    double distance;
    double oil;
    double ch[3];
    double * ptr;

    if(mode != 1)
    {
        printf("Enter distance traveled in kilometers: ");
        while(1 != scanf("%lf",&distance))
            if('\n' != getchar())
                continue;
        printf("Enter fuel consumed in liters: ");
        while(1 != scanf("%lf",&oil))
            if('\n' != getchar())
                continue;
    }
    else
    {
        printf("Enter distance traveled in miles: ");
        while(1 != scanf("%lf",&distance))
            if('\n' != getchar())
                continue;
        printf("Enter fuel consumed in gallons: ");
        while(1 != scanf("%lf",&oil))
            if('\n' != getchar())
                continue;
    }
    ch[0] = distance;
    ch[1] = oil;
    ch[2] = mode;
    return ch;      //  此时ch[0]  ch[1]  ch[2]  还是输入的正确值
}

//  计算并显示燃料消耗值
void show_info(double ptr[])
{
    double mode;

    mode = ptr[2];
    if(mode != 1)
    {
        printf("Fuel consumption is %.2f liters per 100 km\n",(ptr[1] / ptr[0]) * 100);    //  这边ptr[0]ptr[1]ptr[2]  三个值错误了  这是为什么？
        
    }
    else
        printf("Fuel consumption is %.2f liters per gallon\n",ptr[0] / ptr[1]);
}

//=============================================
#include<stdio.h>
int main(void)
{
    int mode;

    printf("Enter 0 for metric mode, 1 for US mode: ");
    while(scanf("%d",&mode) && mode >= 0)
    {

        show_info(get_info(set_mode(mode)));
        printf("enter 0 for metric mode, 1 for US mode");
        printf("(-1 to quit): ");
    }
    printf("Done.\n");
    return 0;
}