C primer plus (第六版）第十二章 编程练习第3题

题目：

3．重新设计编程练习2，要求只使⽤⾃动变量。该程序提供的⽤⼾界⾯不变，即提⽰⽤⼾输⼊模式等。但是，函数调⽤要作相应变化。

思路1：

• 因为在编程练习2中用的是全局变量在函数set_mode(); get_info(); show_info();之间传递mode值，fuel_consume值与distance值，相对简单，如果限定只使用自动变量，需要注意的是get_info()函数，因为正常函数只能返回一个值，但是get_info()之前的功能是需要读入fuel_consume与distance这2个值并在show_info()函数中计算并显示，这种情况下需要将计算步骤迁移至get_info()函数，并让get_info()返回计算好的一个值，show_info()函数只负责显示最终的结果即可；
• 演示程序：

//需要存为pe12-3a.h
int set_mode(int mode);
double get_info(int mode);
void show_info(int mode, double result);

//pe12-3a.c
#include <stdio.h>
// set_mode()函数目的是直接将mode值设为0或1（>0的都设为1）
int set_mode(int mode)
{
    if (mode == 0)
        return 0;
    else
        return 1;
}
// get_info()函数的目的是根据mode的值，输入里程和油耗并计算，返回计算值
double get_info(int mode)
{
    float distance;  
    float fuel_consume;
    int mode_code;
    double mode_0_result;
    double mode_1_result;
    mode_code = mode;

    if (mode_code == 0)
    {
        puts("Enter distance traveled in kilometers:");
        while (scanf("%d", &distance) == 1 && distance > 0)
        {
            puts("Enter fuel consumed in liters:");
            break;
        }
        while (scanf("%d", &fuel_consume) == 1 && fuel_consume > 0)
        {   
            mode_0_result = 100.0*fuel_consume/distance;
            break;
        }
        return mode_0_result;    //因为函数没法返回两个值，所以只能提前计算好结果返回
    }

    if (mode_code == 1)
    {
        puts("Enter distance traveled in miles:");
        while (scanf("%d", &distance) == 1 && distance > 0)
        {
            puts("Enter fuel consumed in gallons:");
            break;
        }
        while (scanf("%d", &fuel_consume) == 1 && fuel_consume > 0)
        {
            mode_1_result = distance/fuel_consume;
            break;
        }
        return mode_1_result;   //因为函数没法返回两个值，所以只能提前计算好结果返回
    }    
}
//show_info()函数根据mode值显示计算结果
void show_info(int mode, double result)
{
    int mode_code = mode;
    double mode_result = result;

    if (mode_code == 0)
    {
        printf("Fuel consumption is %.2f liters per 100 km.\n",mode_result);
    }
    if (mode_code == 1)
    {
        printf("Fuel consumption is %.2f miles per gallon.\n",mode_result);
    }    
}

//需要存为pe12-3b.c
#include <stdio.h>
#include "pe12-3a.h"
int main(void)
{
    int mode;
    printf("Enter 0 for metric mode, 1 for US mode: ");
    scanf("%d", &mode);
    while (mode >= 0)
    {
        int mode_return = set_mode(mode);
        double result_return = get_info(mode_return);
        show_info(mode_return, result_return);
        printf("Enter 0 for metric mode, 1 for US mode");
        printf(" (-1 to quit): ");
        scanf("%d", &mode);
    }
    printf("Done.\n");
    return 0;
}

思路2：

• 可以参考原书答案，通过指针的形式传递mode值，fuel_consume值与distance值；
• 参考代码如下：

//存为pe12-3a.h
#define METRIC 0
#define US 1
#define USE_RECENT 2

void check_mode(int *pm);               //point of mode 缩写
void get_info(int mode, double *pd, double *pf);        //point of distance 与point of fuel缩写
void show_info(int mode, double distance, double fuel);

//存为pe12-3a.c
//compile with pe12-3b.c
#include <stdio.h>
#include "pe12-3a.h"

void check_mode(int *pm)
{
    if (*pm != METRIC && *pm != US)     //处理非0与非1的mode数值
    {
        printf("Invalid mode specified. Mode %d\n", *pm);
        printf("Previous mode will be used.\n");
        *pm = USE_RECENT;
    }
}

void get_info(int mode, double *pd, double *pt)     //通过指针形式存入距离与油耗参数
{
    if (mode == METRIC)
        printf("Enter distance traveled in kilometers: ");
    else
        printf("Enter distance traveled in miles: ");
    scanf("%lf",pd);
    if (mode == METRIC)
        printf("Enter fuel consumed in liters: ");
    else
        printf("Enter fuel consumed in gallons: ");
    scanf("%lf",pf);
}

void show_info(int mode, double distance, double fuel)  
{
    printf("Fuel consumption is ");
    if (mode == METRIC)
        printf("%.2f liters per 100 km.\n", 100 * fuel / distance);
    else
        printf(".1f miles per gallon.\n", distance / fuel);
}

//pe12-3b.c
//complie with pe12-3a.c
#include <stdio.h>
#include "pe12-3a.h"
int main()
{
    int mode;
    int prev_mode = METRIC;
    double distance, fuel;

    printf("Enter 0 for metric mode, 1 for US mode: ");
    scanf("%d", &mode);
    while (mode >= 0)
    {
        check_mode(&mode);
        if (mode == USE_RECENT)
            mode = prev_mode;
        prev_mode = mode;       //根据第一次输入结果重置prev_mode默认mode值
        get_info(mode, &distance, &fuel);
        show_info(mode, distance, fuel);
        printf("Enter 0 for metric mode, 1 for US mode");
        printf(" (-1 to quit): ");
        scanf("%d", &mode);
    }
    printf("Done.\n");

    return 0;
}