Why am I getting an error when checking if a string can be an double

Question

Hey guys im pretty new to coding but one of my projects is to check to see if a string can be parse into a double. It keeps printing an error when trying running the program.

Here is the code:

        public static void main(String[] args) {
            SimpleReader in = new SimpleReader1L();
            SimpleWriter out = new SimpleWriter1L();

          // Constant entered in by user as a string
            out.println("Welcome to constant approximator");
            out.println("Please enter in a constant to be estimated");
            String realConstant = in.nextLine();

        //Double variable created in order to reassign later
            double test = 0;

       //FormatChecker class and canParseDouble verifies if the string is truly a double. boolean method.
            FormatChecker.canParseDouble(realConstant);
       //Test reassign and converts
            test = Double.parseDouble(realConstant);
            out.print(test);
            in.close();
            out.close();
        }

    }

Here is the error:

Exception in thread "main" java.lang.NumberFormatException: For input string: "pi"
    at sun.misc.FloatingDecimal.readJavaFormatString(Unknown Source)
    at sun.misc.FloatingDecimal.parseDouble(Unknown Source)
    at java.lang.Double.parseDouble(Unknown Source)
    at ABCDGuesser1Test.main(ABCDGuesser1Test.java:36)

Perhaps you shouldn't ignore the value returned by `FormatChecker.canParseDouble(realConstant)`. — Eran, May 27 '18 at 13:28
Shouldn't the input string only consist of numbers/digits `0-9` and a `.`? — piepi, May 27 '18 at 13:28
The thing is called a "Welcome to constant approximator" - it seems the main problem is that it is asking for a constant name (like "pi") and then trying to parse that as a double - it should instead try to approximate PI in some way. — Erwin Bolwidt, May 27 '18 at 14:21

Nikolas Charalambidis · Answer 1 · 2018-05-27T13:36:31.957

0

It happens because you type pi which is not recognized as π (pi) constant. What have you typed was a String and these characters are not convertible to a number.

If you want to enter any number including the special constant like pi, you have to check first if the input is a Number or String. In case it's String, you can try to match it with a defined constant like π or e and use their defined value in Java such as Math.PI.

edited May 27 '18 at 13:36

answered May 27 '18 at 13:30

Nikolas Charalambidis

40,893
16
117
183

I think the real problem is not checking the return value from `canParseDouble`. – M. le Rutte May 27 '18 at 13:39
M. le Rutte: How does it solve the issue when `pi` is entered as an input? – Nikolas Charalambidis May 27 '18 at 13:43
@M.leRutte without knowing the contents of the `canParseDouble` method, that's just a guess. It may allow "pi|e|sqrt2" as well. The name "constant approximator" actually suggests this. – Erwin Bolwidt May 27 '18 at 14:00

score 0 · Answer 2 · edited May 27 '18 at 13:43

0

You should use the result of canParseDouble() not just call it. Something like this, I think:

if (FormatChecker.canParseDouble(realConstant)) {
   test = Double.parseDouble(realConstant);
   out.println(test);
}

edited May 27 '18 at 13:43

M. le Rutte

3,525
3
18
31

answered May 27 '18 at 13:42

Mike B

2,756
2
16
28

score 0 · Answer 3 · answered May 27 '18 at 13:49

As you say:

//FormatChecker class and canParseDouble verifies if the string is truly a double. boolean method.
        FormatChecker.canParseDouble(realConstant);

You know that this line calls a boolean method and will then return either true or false. However, you do not do any use of this returned value. If so, what's the point of even calling the function, right?

You are trying to check if the string realConstant is a double, the method checks it but you simply ignore it, here. I believe you have an error because whether it is truly a double or not, the rest of the code will run. In the case where the string is not actually a double, an error will appear since the rest of the code cannot compile.

You should then use an if statement such as:

if (FormatChecker.canParseDouble(realConstant)) {
    test = Double.parseDouble(realConstant);
    out.println(test);
}

Also, I do not think you should expect an input of "pi" to return a double!

Why am I getting an error when checking if a string can be an double

3 Answers3