Number lines and modulus C++

Question

I am trying to make the opposite of case 3. So the idea is that you can say "x amount of days from sat will be _" . This works well in my case 3 so I figured the same would work if you wanted to solve "x days before sat will be _ ". Now my case 4 only works till a input of 6 then you enter negative numbers which I did not thick about when making this... And my program is based off a array char dayChar[7][4] {"Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"}; so has you can see case 4 does not really work. Any suggestions on how to what I could do that would work? Thanks a ton in advance.

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

A simple solution is to do: dayNum = ((temp % 7) + 7) % 7;

Since temp % 7 is guaranteed to be greater than -7, adding 7 is guaranteed to give you a positive number, equivalent to the original (modulo 7). The resulting number, though, may be greater than 7, which is why an additional modulo operation is necessary.