While循环不适用于c编程

Question

我正在尝试让程序允许用户输入他们想要将总成本更改为的货币。当用户确实输入了错误的输入，然后被提示再次输入正确的货币时，当他们输入正确的货币时，while循环不会中断。它不断要求用户再次输入正确的货币。

Please enter the number of gallons of gasoline: 7

7.0 gallons of gasoline produces..
7.0 gallons of gasoline requires.. 
7.0 gallons of gasoline costs..

Choose a currency you want to see your total cost in (Euro, Pound, or Yen): eurooo

You need choose one of these currencies (Euro, Pound, or Yen). Please enter one: Euro

You need choose one of these currencies (Euro, Pound, or Yen). Please enter one: Euro

You need choose one of these currencies (Euro, Pound, or Yen). Please enter one: 

代码：

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <math.h>
#include <string.h>
#include <stdbool.h>

float gas_gallons;
float cost_today_gallons;
int main()
{
    char input;
    printf("\nPlease enter the number of gallons of gasoline: ");
    scanf(" %c", &input);
    getchar();

    while (!isdigit(input))
    {
        printf("\nYou need to enter a digit. Please enter the number of gallons of gasoline: ");
        scanf("%c", &input);
        }

    if (isdigit(input))
    {
        gas_gallons = input - '0';

        float carbon_dioxide_pounds = gas_gallons * 19.64;
        printf("\n%.2f gallons of gasoline produces approximately %f pounds of carbon dioxide.", gas_gallons, carbon_dioxide_pounds );

        float barrels_crude_oil = gas_gallons/19.0;
        printf("\n%.2f gallons of gasoline requires %f barrels of crude oil.", gas_gallons, barrels_crude_oil);

        cost_today_gallons = gas_gallons*2.738;
        printf("\n%.2f gallons of gasoline costs a total average of %f US dollars today.", gas_gallons, cost_today_gallons);
        }

    char currency[100];
    printf("\nChoose a currency you want to see your total cost in (Euro, Pound, or Yen): ");
    scanf("%s", &currency);
    getchar();

    char *str1 = "Yen";
    char *str2 = "Euro";
    char *str3 = "Pound";

    while ((strcmp(currency, str1) != 0) || (strcmp(currency, str2) != 0) || (strcmp(currency, str3) != 0))
    {
        printf("\nYou need choose one of these currencies (Euro, Pound or Yen). Please enter one: ");
        scanf("%s", &currency);
    }

    if ((strcmp(currency, str1) == 0))
    {
        float yen_total_cost = cost_today_gallons*123.07;
        printf("\n%.2f gallons of gasoline costs a total average of %f Japenese Yens today.", gas_gallons, yen_total_cost);
    }
    if (strcmp(currency, str2) == 0)
    {
        float euro_total_cost = cost_today_gallons*0.92;
        printf("\n%.2f gallons of gasoline costs a total average of %f Euros today.", gas_gallons, euro_total_cost);
    }
    if (strcmp(currency, str3) == 0)
    {
        float pound_total_cost = cost_today_gallons*0.65;
        printf("\n%.2f gallons of gasoline costs a total average of %f British Pounds today.", gas_gallons, pound_total_cost);
    }


    return 0;
}

Answer 1

在这一行 while ((strcmp(currency, str1) != 0) || (strcmp(currency, str2) != 0) || (strcmp(currency, str3) != 0))

你告诉你的代码循环直到 currency 等于 str1 和 str2 和 str3 这永远不会发生。

您需要将其更改为 while ((strcmp(currency, str1) != 0) && (strcmp(currency, str2) != 0) && (strcmp(currency, str3) != 0))

Answer 2

scanf 还在末尾添加了\n。您需要将其砍掉，或将其添加到您要比较的字符串中。或者，如果您不关心有人输入“European money”而不是“Euro”，您可以使用 strncmp() 而不是 strcmp()。

请注意，如果有人输入超过 98 个字符，您的代码很容易出现缓冲区溢出（添加 \n 和 \0 后，您将位于字节 101）。为确保这不会导致问题，您应该使用 'scanf("%99s")' 代替，这将切断第 99 个字符之后的字符串（或者，使用不同的输入函数）。

Answer 3

当您应该使用 AND 时，您在 while 语句中使用了 OR 运算符。无论选择哪种有效货币都将失败 2 个条件，因此您会陷入循环。

Answer 4

有几个问题。

这个条件是错误的

while ((strcmp(currency, str1) != 0) || (strcmp(currency, str2) != 0) || strcmp(currency, str3) != 0))

替换为：

while ((strcmp(currency, str1) != 0) && (strcmp(currency, str2) != 0) && strcmp(currency, str3) != 0))

作为练习，我让你找出原因。

您对scanf 的使用过于复杂，您可能想要这样：

float gas_gallons;
float cost_today_gallons;
int main()
{
    printf("\nPlease enter the number of gallons of gasoline: ");
    scanf("%f", &gas_gallons);

    float carbon_dioxide_pounds = gas_gallons * 19.64;
    printf("\n%.2f gallons of gasoline produces approximately %f pounds of carbon dioxide.", gas_gallons, carbon_dioxide_pounds );

    float barrels_crude_oil = gas_gallons/19.0;
    printf("\n%.2f gallons of gasoline requires %f barrels of crude oil.", gas_gallons, barrels_crude_oil);

    cost_today_gallons = gas_gallons*2.738;
    printf("\n%.2f gallons of gasoline costs a total average of %f US dollars today.", gas_gallons, cost_today_gallons);


    char currency[100];
    printf("\nChoose a currency you want to see your total cost in (Euro, Pound, or Yen): ");
    scanf("%s", currency);

    char *str1 = "Yen";
    char *str2 = "Euro";
    char *str3 = "Pound";

    while ((strcmp(currency, str1) != 0) && (strcmp(currency, str2) != 0) && (strcmp(currency, str3) != 0))
    {
        printf("\nYou need choose one of these currencies (Euro, Pound or Yen). Please enter one: ");
        scanf("%s", &currency);
    }

    if ((strcmp(currency, str1) == 0))
    {
        float yen_total_cost = cost_today_gallons*123.07;
        printf("\n%.2f gallons of gasoline costs a total average of %f Japenese Yens today.", gas_gallons, yen_total_cost);
    }
    if (strcmp(currency, str2) == 0)
    {
        float euro_total_cost = cost_today_gallons*0.92;
        printf("\n%.2f gallons of gasoline costs a total average of %f Euros today.", gas_gallons, euro_total_cost);
    }
    if (strcmp(currency, str3) == 0)
    {
        float pound_total_cost = cost_today_gallons*0.65;
        printf("\n%.2f gallons of gasoline costs a total average of %f British Pounds today.", gas_gallons, pound_total_cost);
    }


    return 0;
}

奖励：变量名 str1 到 str3 的选择很差。为什么不选择名称为strEuro、strYen、strPound？