Question

有人可以向我解释下面的语法我做错了吗？我的教科书没有任何用户定义的字符串函数的例子，我无法弄清楚我应该做什么。提前致谢！

#include <iostream>
#include <cmath>
#include <cstdlib>
#include <string>

string dayOfWeek(int day);

using namespace std;    

int main()
{
    cout << "Please enter a day of week (0 for Sunday, 1 for Monday, etc): ";
    cin >> day;
    cout << "The name of the day of the week is: " dayOfWeek(day) << endl;
}

string dayOfWeek(int day)
{
    if (day == 0)
        return "SUNDAY";
    else if (day == 1)
        return "MONDAY";
    else if (day == 2)
        return "TUESDAY";
    else if (day == 3)
        return "WEDNESDAY";
    else if (day == 4)
        return "THURSDAY";
    else if (day == 5)
        return "FRIDAY";
    else if (day == 6)
        return "SATURDAY";
}

Answer 1

#include <string>
#include <iostream>

std::string dayOfWeek(int day)
{
    if (day == 0)
        return "SUNDAY";
    else if (day == 1)
        return "MONDAY";
    else if (day == 2)
        return "TUESDAY";
    else if (day == 3)
        return "WEDNESDAY";
    else if (day == 4)
        return "THURSDAY";
    else if (day == 5)
        return "FRIDAY";
    else if (day == 6)
        return "SATURDAY";

    return "Invalid input!"; // May be you should guarantee a return value.
 }

int main() {
    std::cout << dayOfWeek(5) << std::endl;
}

应该按预期工作。

Answer 2

问题的关键在于：

string dayOfWeek(int day);

using namespace std;

using位于依赖于它的代码之后，因此编译器会查找string并找不到它。它不会去寻找std :: string，因为它尚未被告知。

您可以将using向上移动几行，但由于Why is "using namespace std" considered bad practice?的答案中涵盖的众多原因以及一些未涵盖的原因，您可能会更好不放

using namespace std;

在您的代码中。明确地加上std::的前缀，你就可以避免一连串令人讨厌的惊喜。

我还建议关注函数所在的@πάνταῥεῖ示例。向前声明然后声明会给你两个地方来改变代码，另外一个地方有一个bug。声明将要使用它的功能意味着您只需要更改一个声明。

将所有内容与其他一些小调整结合在一起：

#include <string>
#include <iostream>

std::string dayOfWeek(int day)
{
    switch (day) // for stuff like this, I like a switch. I find it looks nicer.
    {
        case 0:
            return "SUNDAY";
        case 1:
            return "MONDAY";
        case 2:
            return "TUESDAY";
        case 3:
            return "WEDNESDAY";
        case 4:
            return "THURSDAY";
        case 5:
            return "FRIDAY";
        case 6:
            return "SATURDAY";
        default:
            return "No such day"; // functions must always return something. 
                                  // in this case what will cout print if you 
                                  // don't return a string? Don't know?
                                  // Don't feel bad. Neither do I.
                                  // Welcome to Undefined Behaviour

    }
 }

int main()
{
    int day; //this was missing
    std::cout << "Please enter a day of week (0 for Sunday, 1 for Monday, etc): ";
    std::cin >> day;
    std::cout << "The name of the day of the week is: " << dayOfWeek(day) << std::endl;
}

您可以使用另一个技巧来完全摆脱if或switch

#include <string>
#include <iostream>


// define an array of days of the week to print. const means dayOfWeek cannot be changed
const std::string dayOfWeek[] = 
{
    "SUNDAY",
    "MONDAY",
    "TUESDAY",
    "WEDNESDAY",
    "THURSDAY",
    "FRIDAY",
    "SATURDAY"
 };

int main()
{
    unsigned int day; 
    //Note unsigned int. Negative numbers are now disallowed

    std::cout << "Please enter a day of week (0 for Sunday, 1 for Monday, etc): ";

    // keep looping until user provides a good number
    while (!(std::cin >> day) // user MUST input a number
           || day > 6) // number is a usable number
    {
        std::cin.clear(); // clean up bad input
        std::cout << "Please enter a day of week (0 for Sunday, 1 for Monday, etc): ";
    }
    // since we know day is a usable (0..6) number we can read the day out of the array   
    std::cout << "The name of the day of the week is: " << dayOfWeek[day] << std::endl;
}

字符串函数定义

2 个答案: