做..虽然循环不断重复，但扫描仪问题

Question

do {   
    System.out.println("Set the A param: ");
    if(input.hasNextDouble() == true) {
        A = input.nextDouble();
        if(A == 0) {
            System.out.println("Param A cannot be a 0!");
        }
    } else if(input.hasNextDouble() == false) {
        System.out.println("Param A must be a number!");
    }
} while(A == 0 || input.hasNextDouble() == false);

Hello, I'm really new in Java and I found an obstacle I can't resolve by myself.

Everything is okay until I enter some letter instead of number, then this do..while loop keeps repeating itself.

After some search I suppose this might be a problem with a Scanner buffer becouse I should clear it before every loop with input.nextLine() but I don't really know where in code should I put it.

Thanks for any help.

Answer 1

You only actually consume data from the scanner if input.hasNextDouble() is true .

Currently, if A == 0 and there are non-numeric data in the buffer then you'll indeed loop indefinitely.

You need to consume data from the buffer on all control paths. In particular, if there is something non-numeric in the buffer, then you need to consume and immediately discard it: input.next(); would be adequate.

Answer 2

Seems like you just want to get the value of A which should not be equal to 0 . Read comments

double A=0;
do {   
    System.out.println("Set the A param: ");
    if(input.hasNextDouble() == true) { //check for valid value 
        A = input.nextDouble();
        if(A == 0) {
            System.out.println("Param A cannot be a 0!");
        }
    } else{ // no valid value found , print msg and jump over the previous input
        input.nextLine();
        System.out.println("Param A must be a number!");
    }
} while(A == 0); // just check , if the desired value is received 
                 // previously input.hasNextDouble() had no use cuz we already              
                 // checked no double value found so will be false, don't use it 

Answer 3

First, you should not write the opposing check in an else if . Just use else . And you shouldn't check == true , since the value is already a boolean.

Now, for you infinite loop problem, when hasNextDouble() is false, it means that the user entered something wrong (ignoring the potential end-of-stream issue). In that case you need to discard that bad input, which is best done by calling nextLine() .

Java naming convention states that variables should start with lowercase letter, so A should be a .

Your code then becomes:

double a = 0;
do {   
    System.out.println("Set the A param: ");
    if (input.hasNextDouble()) {
        a = input.nextDouble();
        if (a == 0) {
            System.out.println("Param A cannot be a 0!");
        }
    } else {
        input.nextLine(); // discard bad input
        System.out.println("Param A must be a number!");
    }
} while (a == 0);

Answer 4

Thank you for both answers, after first I wrote this:

        do
        {   

            System.out.println("Podaj wartość parametru A: ");

            if(input.hasNextDouble() == true)
            {
                A = input.nextDouble();
                if(A == 0)
                {
                    System.out.println("Parametr A nie może być zerem!");
                }
            }
            else if(input.hasNextDouble() == false)
            {
                System.out.println("Parametr A musi być liczbą!");
                input.nextLine();
                A = 0;
            }

        } while(A == 0);

And it worked but thanks to the second answer now I know how to do it better. :) Thanks both of you once again.