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13.7: Validating Input

  • Page ID
    18821
  • \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

    Here is a method that uses printLogarithm from the previous section:

    public static void scanDouble(Scanner in) {
        System.out.print("Enter a number: ");
        double x = in.nextDouble();
        printLogarithm(x);
    }
    

    This example uses nextDouble, so the Scanner (provided by the main method) tries to read a double. If the user enters a floating-point number, the Scanner converts it to a double. But if the user types anything else, the Scanner throws an InputMismatchException.

    We can prevent this error by checking the input before parsing it:

    public static void scanDouble(Scanner in) {
        System.out.print("Enter a number: ");
        if (!in.hasNextDouble()) {
            String word = in.next();
            System.err.println(word + " is not a number");
            return;
        }
        double x = in.nextDouble();
        printLogarithm(x);
    }
    

    The Scanner class provides hasNextDouble, which checks whether the next token in the input stream can be interpreted as a double. If so, we can call nextDouble with no chance of throwing an exception. If not, we display an error message and return.


    This page titled 13.7: Validating Input is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by Allen B. Downey (Green Tea Press) .

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