How can I better check whether two char variables are in some set of values?

Question

Recently, our professor has requested that we use two char variables (day) to receive the input from the user.

The code below works fine as a check to ensure that either Mo, Tu, We, Th, Fr, Sa, Su are the only two characters which are entered together as a pair. If anything else is received as input, it'll loop and ask the user for valid input.

The input should be case-insensitive, meaning that, for example, "mO" and "tu" are acceptable. It seems like there is a lot of repetition that is happening. Is there a way to clean this up?

cout << "Please enter the day of the week did you made the long distance call (Mo Tu We Th Fr Sa Su): ";
cin >> dayOne >> dayTwo;

while ((dayOne != 'M' && dayOne != 'm' || dayTwo != 'O' && dayTwo != 'o') &&
       (dayOne != 'T' && dayOne != 't' || dayTwo != 'U' && dayTwo != 'u') &&
       (dayOne != 'W' && dayOne != 'w' || dayTwo != 'e' && dayTwo != 'E') &&
       (dayOne != 'T' && dayOne != 't' || dayOne != 'H' && dayTwo != 'h') &&
       (dayOne != 'F' && dayOne != 'f' || dayTwo != 'R' && dayTwo != 'r') &&
       (dayOne != 'S' && dayOne != 's' || dayTwo != 'A' && dayTwo != 'a') &&
       (dayOne != 'S' && dayOne != 's' || dayTwo != 'U' && dayTwo != 'u'))
{
    cin.clear();
    cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
    cout << endl << "You have entered an invalid day. Please re-enter a day in the correct format (Mo Tu We Th Fr Sa Su): ";
    cin >> dayOne >> dayTwo;
}

Answer 1

You could write a fold-expression that compares 2 characters to a string:

template<typename ...Days>
bool any_of(char a, char b, Days ...days)
{
    return (... || (a == days[0] && b == days[1]));
}

and then use it like this:

while (! any_of(std::tolower(dayOne), std::tolower(dayTwo), "mo", "tu", "we", "th", "fr", "sa", "su"))
    // keep asking for input

Here's a demo.

This should satisfy the requirement of using 2 char inputs.

Answer 2

You typically use tolower or toupper to convert your char variable to the correct case first. I like using tolower - it looks marginally better.

dayOne = tolower(dayOne);
dayTwo = tolower(dayTwo);

while (
    (dayOne != 'm' || dayTwo != 'o') &&
    (dayOne != 't' || dayTwo != 'u') &&
    (dayOne != 'w' || dayTwo != 'e') &&
    (dayOne != 't' || dayTwo != 'h') &&
    (dayOne != 'f' || dayTwo != 'r') &&
    (dayOne != 's' || dayTwo != 'a') &&
    (dayOne != 's' || dayTwo != 'u'))
{
    ...
}

You can further change it by using memcmp to compare both characters at once, but I am not sure it would simplify the code.