CODEWITHSUNDEEP
c++loopswhile-loopdo-whilelogical-operators
Rishi K.
Rishi K.

Reputation: 141

Use of Logical Operator in Loop Condition

In the below given code, why the || logical doesn't work, instead the loop terminates specifically when && is used ?

int main() {
    char select {};
    do {
        cout<<"Continue the loop or else quit ? (Y/Q): ";
        cin>>select;
    } while (select != 'q' && select != 'Q'); // <--- why || (or) doesn't work here ??
    return 0;
}

Upvotes: 4

Views: 2319

Answers (3)

463035818_is_not_an_ai
463035818_is_not_an_ai

Reputation: 122486

Consider the following diagrams:

enter image description here

The full ellipse are all characters. The white dots is q and Q respectively. The black filled area depicts characters that will make the expression true. First line is select != 'q' && select != 'Q', second line is select != 'q' || select != 'Q'.

&& means both conditions must be true. The resulting black area is the overlap of the two areas on the left.

|| means either of the conditions must be true. The resulting black area is the sum of the two areas on the left.

Upvotes: 2

Ted Lyngmo
Ted Lyngmo

Reputation: 117298

This loop will go on while select is not q and it's not Q:

while (select != 'q' && select != 'Q');

This loop will go on while select is not q or it's not Q. 

while (select != 'q' || select != 'Q');

Since one of them must be true, it'll go on forever.

Examples:

  1. The user inputs q

select != 'q' evaluates to false
select != 'Q' evaluates to true
false || true evaluates to true

  1. The user inputs Q

select != 'q' evaluates to true
select != 'Q' evaluates to false
true || false evaluates to true

Upvotes: 5

Vlad from Moscow
Vlad from Moscow

Reputation: 310980

You want to terminate the loop when select is equal either to 'q' or 'Q'.

The opposite condition can be written like

do {
    cout<<"Continue the loop or else quit ? (Y/Q): ";
    cin>>select;
} while ( not ( select == 'q' || select == 'Q' ) );

If to open the parentheses then you will get

do {
    cout<<"Continue the loop or else quit ? (Y/Q): ";
    cin>>select;
} while ( not( select == 'q' ) && not ( select == 'Q' ) );

that in turn is equivalent to

do {
    cout<<"Continue the loop or else quit ? (Y/Q): ";
    cin>>select;
} while ( select != 'q' && select != 'Q' );

Upvotes: 2

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