How to test if a user inputted string is valid in do/while loop?

0
votes

So essentially, I am asking the user for a departure time, as well as if it is in the AM or PM. In the code provided, I only test for AM/am (caps or lowercase), but in my actual program I will be testing for both AM/am and PM/pm. Now, when I set my do/while loop up like this:

do {
    cout << "Please Enter a Valid Period: ";
    cin >> departure_amOrPM;
} while (departure_amOrPM != "AM");

it works just fine, and it lets the program continue when "AM" is entered in all caps. But when I add another test to my loop like this:

do {
    cout << "Please Enter a Valid Period: ";
    cin >> departure_amOrPM;
} while (departure_amOrPM != "AM" || departure_amOrPM != "am");

it does not work. I tried entering both "AM" and "am", yet it did not let me continue. I don't know how to overcome this obstacle, so any help or tips would be much appreciated. Thanks!

3
c++loopsvalidationdo-while
Covert the entire string entered to uppercase. Then the first example will work. And what if aM is entered? Is that valid? - PaulMcKenzie
Your while loop says: keep looping as long as the string isn't AM or the string isn't am. Only one of those condition needs to be true, that's what || means. Therefore, for the loop to fail both conditions must fail, and the string must be AM, and the string must also be am, at the same time. Until such time as quantum computing becomes reality, this is just not possible, sorry... - Sam Varshavchik

3 Answers

2
votes

while means to continue looping while the condition is true. You'd like to loop as long as it's neither "AM" _nor "am".

You can write it as:

while (departure_amOrPM != "AM" && departure_amOrPM != "am");
1
votes

Think about this condition

} while (departure_amOrPM != "AM" || departure_amOrPM != "am");

This condition is true of every string. Every string is either not equal to "AM" or not equal to "am" (most strings are not equal to both). That's why you couldn't proceed.

What you meant to write is this

} while (departure_amOrPM != "AM" && departure_amOrPM != "am");

It's very common to get && and || confused, especially when also dealing with negation.

0
votes

Instead of using the logical OR operator in the condition or the do-while loop

do {
    cout << "Please Enter a Valid Period: ";
    cin >> departure_amOrPM;
} while (departure_amOrPM != "AM" || departure_amOrPM != "am");

Use the logical AND operator like

do {
    cout << "Please Enter a Valid Period: ";
    cin >> departure_amOrPM;
} while (departure_amOrPM != "AM" && departure_amOrPM != "am");

To test yourself at first write a condition to which the valid input satisfies. For your code it will look like

(departure_amOrPM == "AM" || departure_amOrPM == "am")

Now make its negation like

!(departure_amOrPM == "AM" || departure_amOrPM == "am")

or

not(departure_amOrPM == "AM" || departure_amOrPM == "am")

As for example

do {
    cout << "Please Enter a Valid Period: ";
    cin >> departure_amOrPM;
} while ( not (departure_amOrPM == "AM" || departure_amOrPM == "am") );

Now you can rewrite it using the mathematical logic like

do {
    cout << "Please Enter a Valid Period: ";
    cin >> departure_amOrPM;
} while (departure_amOrPM != "AM" && departure_amOrPM != "am");