CODEWITHSUNDEEP
javado-while
caleb wheeler
caleb wheeler

Reputation: 57

java do while loop keeps repeating and I can't figure out why

So I've attached a debugger, and tried different inputs and I can't seem to figure out why this won't get past the loop. When ran I enter "l" or "L", then entry gets set to that, then input is set to the capitalized version and then it repeats.

public static char displayMenu(){
    char input;
    sc.nextLine();//clear junk
    do {
        System.out.println();
        System.out.println("\t\t Enter L to (L)oad ");
        String entry = sc.nextLine();
        input = entry.toUpperCase().charAt(0);
    } while (input != 'L' || input!='M' || input != 'P' || input != 'Q');

Upvotes: 2

Views: 2355

Answers (6)

candied_orange
candied_orange

Reputation: 7334

De Morgan's laws tell us that

   (input != 'L' || input!='M' || input != 'P' || input != 'Q')

is the same as

! (input == 'L' && input=='M' && input == 'P' && input == 'Q')

which must always be true because

    (input == 'L' && input=='M' && input == 'P' && input == 'Q')

must always be false because

     input can only be equal to one thing at a time.

Use De Morgan's laws to avoid extra NOT's when you can. Computers don't care but humans don't process NOT's very well.

Upvotes: 0

Sindhoo Oad
Sindhoo Oad

Reputation: 1194

your have used logical or condition it needs just one true statement to run, even though you enter 'L' , at this point your one statement is false but other statements became true that why it keeps on repeating.

Upvotes: 1

Elliott Frisch
Elliott Frisch

Reputation: 201447

Your boolean || is incorrect. If a value is L it is then not M, P or Q so your loop will continue to iterate. I think you wanted something like,

while (input != 'L' && input != 'M' && input != 'P' && input != 'Q');

or

while (!(input == 'L' || input == 'M' || input == 'P' || input == 'Q'));

consider when input is L, clearly L is not M and so your initial while condition would continue to iterate.

Upvotes: 1

chinna_82
chinna_82

Reputation: 6403

public static char displayMenu(){
    char input;
    sc.nextLine();//clear junk
    do {
        System.out.println();
        System.out.println("\t\t Enter L to (L)oad ");
        String entry = sc.nextLine();
        input = entry.toUpperCase().charAt(0);
    } while ((input != 'L') && (input!='M') && (input != 'P') && (input != 'Q'));

try this

Upvotes: 0

Mergades
Mergades

Reputation: 11

Like 

public static void main(String[] args) {
    char input;
    Scanner sc = new Scanner(System.in);
    input = sc.nextLine().charAt(0);//clear junk
    do {
        System.out.println();
        System.out.println("\t\t Enter L to (L)oad ");
        String entry = sc.nextLine();
        input = entry.toUpperCase().charAt(0);
    } while (input != 'L' && input!='M' && input != 'P' && input != 'Q');
}

Upvotes: 0

mattm
mattm

Reputation: 5949

input will only have one value. That value cannot be both L and M. You need to change the termination condition.

Upvotes: 2

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