NumberFormatException when parsing String to Int

Question

I am writing code to get the coefficients and exponents of a user entered polynomial. For an example, I am entering "-3x^4+2x^3+1x^2-1x^1+8x^0" into the console. I keep getting this error:

Exception in thread "main" 

java.lang.NumberFormatException: For input string: ""
at java.lang.NumberFormatException.forInputString(NumberFormatException.java:48)
at java.lang.Integer.parseInt(Integer.java:470)
at java.lang.Integer.parseInt(Integer.java:499)
at polynomials.Driver.main(Driver.java:31)

However, when I enter a polynomial without any negative numbers (ex: "3x^4+2x^3+1x^2+1x^1+8x^0"), there is no error. I believe the error is somehow coming from the Integer.parseInt(str2), but shouldn't it be able to parse a negative integer? This makes no sense at all to me, and I couldn't find anything anywhere else.. any help would be a great help. Thanks everyone!

public static void main(String[] args) {

    Scanner userInput = new Scanner(System.in);
    System.out.println("Enter a polynomial: ");

    String polynomial = userInput.nextLine();

    String newPoly = polynomial.replace("-", "+-").trim();

    String[] arr = newPoly.split("\\+");

    for (int i = 0; i<arr.length; i++) {
        System.out.println(arr[i]);

        String str = arr[i].trim();
        String[] arr2 = str.split("x");

        String str2 = arr2[0];

        System.out.println(str2); //prints coefficient
        System.out.println(str.substring(str.indexOf('^')+1)); //prints exponent
        System.out.println(Integer.parseInt(str2));
    }
}

You can't parse expression to Integer. That's what error means — Vimal Bera
– Vimal Bera, Commented Aug 6, 2014 at 8:38
No the exception means that he's trying to parse an empty String. — Kayaman
– Kayaman, Commented Aug 6, 2014 at 8:39
And it doesnt make sense that it works when there are no negative integers used in the polynomial — Ybarra
– Ybarra, Commented Aug 6, 2014 at 8:43

Vimal Bera · Accepted Answer · 2014-08-06 08:43:10Z

2

Here your input string is -3x^4+2x^3+1x^2-1x^1+8x^0 which will be converted into +-3x^4+2x^3+1x^2-1x^1+8x^0 after following line.

String newPoly = polynomial.replace("-", "+-").trim();

So when you are trying to split it using + as a delimiter, First value will be generated as "" input string which is causing the exception.

Just add one if condition in for loop and you will be fine

for (int i = 0; i<arr.length; i++) {
    if(arr[i].length()>0){
        System.out.println(arr[i]);

        String str = arr[i].trim();
        String[] arr2 = str.split("x");

        String str2 = arr2[0];

        System.out.println(str2); //prints coefficient
        System.out.println(str.substring(str.indexOf('^')+1)); //prints exponent
        System.out.println(Integer.parseInt(str2));
    }
}

answered Aug 6, 2014 at 8:43

Vimal Bera

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1 Comment

Ybarra Over a year ago

how do i get rid of this empty string while still maintaining this method of replacing and splitting?

Unihedron · Accepted Answer · 2014-08-06 08:47:42Z

0

Don't pre-process like that. You can use a bit of intelligent regex for the .split() argument:

{

    Scanner userInput = new Scanner(System.in);
    System.out.println("Enter a polynomial: ");

    String polynomial = userInput.nextLine();

    String[] arr = polynomial.split("\\+|(?=-)");

}

Then you would have to get rid of the empty string in the resulting String[]. Don't worry, you can modify the for loop, it's easier:

for (int i = ("".equals(arr[0]) ? 1 : 0); i < arr.length; i++)

This way the empty string in the first element of the array will be ignored, if it's there.

answered Aug 6, 2014 at 8:47

Unihedron

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2 Comments

Ybarra Over a year ago

I solved my problem in my current code by just starting the loop at 1 instead of 0. I will definitely give your code a try as well. Thank you! @Unihedron

Unihedron Over a year ago

@Ybarra You're welcome. Please note that my main concern is that polynomial.replace("-", "+-") consumes unnecessary resources performing the regex replacement, which was not beneficial as you then break down the string (and throw away the effort). As .split() essentially does regex match and consumption, it would be better code.

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