c++ - 数行和模数 C++

Question

我正在尝试与案例 3 相反。所以我的想法是你可以说“x amount of days from sat will be _”。这在我的案例 3 中效果很好，所以我认为如果您想解决“x days before sat will be _”，同样的方法也适用。现在我的 case 4 只工作到输入 6 然后你输入负数，我在做这个的时候并没有详细说明......我的程序基于数组 char dayChar[7 ][4] {"Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"}; 所以你可以看到case 4 并没有真正起作用。关于我该怎么做的任何建议都行得通吗？提前致谢。

case 3: // Add days
                cout << "Day selected is " << day << endl;
                cout << "How many days would you like to add?" << endl;
                cin >> addOrSub;

                temp = (dayNum + addOrSub);
                dayNum = (temp % 7);

                cout << addOrSub << " days from " << day << " will be " << dayChar[dayNum] << endl; 


                break;

            case 4: // Subtract days
                cout << "Day selected is " << day << endl;
                cout << "How many days would you like to subtract?" << endl;
                cin >> addOrSub;

                temp = (dayNum - addOrSub);
                dayNum = (temp % 7);

                cout << addOrSub << " before " << day << " was " << dayChar[dayNum] << endl;

Answer 1

一个简单的解决方案是:dayNum = ((temp % 7) + 7) % 7;

因为 temp % 7 保证大于 -7，所以加 7 保证给你一个正数，等于原来的数(模 7)。不过，生成的数字可能大于 7，这就是为什么需要额外的模运算的原因。