如何检查数组中没有字母？

Question

This is a simple calculator program. I just need something to check my array and prevent any letters being in it before my program continues "adding" the two arguments entered. The input is taken from the command line eg java adder 1 2

public class Adder {
    public static void main(String[] args) {
        //Array to hold the two inputted numbers
        float[] num = new float[2];
        //Sum of the array [2] will be stored in answer
        float answer = 0;

        /*
            some how need to check the type of agruments entered...
        */

        //If more than two agruments are entered, the error message will be shown
        if (args.length > 2 || args.length < 2){
            System.out.println("ERROR: enter only two numbers not more not less");
        }

        else{
        //Loop to add all of the values in the array num 
            for (int i = 0; i < args.length; i++){
                num[i] = Float.parseFloat(args[i]);
                //adding the values in the array and storing in answer
                answer += Float.parseFloat(args[i]);
            }

            System.out.println(num[0]+" + "+num[1]+" = "+answer);
        }
    }
}

Answer 1

While you can't "prevent" the user from inputting letters, you can write your code so that you can handle the letters. Here are a couple ways to do this:

1) Parse for the letters, and if you find any, throw them out.

2) Parse for the letters, and if you find any, return an error message and ask the user to try again

3) Parse for the numbers , and catch the NFE (NumberFormatException) thrown, then return an error message and ask the user to try again

try {
    // your parsing code here
} catch (NumberFormatException e) {
    // error message and ask for new input
}

On a side note, I probably would rewrite that program so that it runs in a while loop, using a Scanner object to take input. That way, you don't have to run the program using java from command line everytime you want to add something, you can just run the program once, and accept input until the user wants to quit. It would look something like this:

public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);

    while (true) {
        // ask for input
        System.out.println("insert 2 numbers separated by a space or quit to quit:")
        //scanner object to take input, reads the next line
        String tempString = scan.nextLine();
        // break out of the loop if the user enters "quit"
        if (tempString.equals("quit") {
            break;
        }
        String[] tempArray = tempString.split(" ");
        // add the values in tempArray to your array and do your calculations, etc. 
        // Use the Try/catch block in 3) that i posted when you use parseFloat()
        // if you catch the exception, just continue and reloop up to the top, asking for new input.

    }
}

Answer 2

You can use regex to check for patterns.

String data1 = "d12";
String data2 = "12";
String regex = "\\d+";
System.out.println(data.matches(regex)); //result is false
System.out.println(data.matches(regex)); //result is true

Answer 3

I would probably just try parsing the values and then handle the exception.

public class Adder {
    public static void main(String[] args) {
        //Array to hold the two inputted numbers
        float[] num = new float[2];
        //Sum of the array [2] will be stored in answer
        float answer = 0;

        /*
            some how need to check the type of agruments entered...
        */

        //If more than two agruments are entered, the error message will be shown
        if (args.length > 2 || args.length < 2){
            System.out.println("ERROR: enter only two numbers not more not less");
        }

        else{
            try {
                //Loop to add all of the values in the array num 
                for (int i = 0; i < args.length; i++){
                    num[i] = Float.parseFloat(args[i]);
                    //adding the values in the array and storing in answer
                    answer += Float.parseFloat(args[i]);
                }

                System.out.println(num[0]+" + "+num[1]+" = "+answer);
            } catch (NumberFormatException ex) {
                System.out.println("ERROR: enter only numeric values");
            }
        }
    }
}

Answer 4

I suggest that you use a Regular Expression

// One or more digits
Pattern p = Pattern.compile("\d+");
if(!p.matcher(input).matches())
   throw new IllegalArgumentException();

For more about regular expression see: http://docs.oracle.com/javase/8/docs/api/java/util/regex/Pattern.html

Answer 5

No need to loop:

public static void main(String[] args) {
    // length must be 2
    if (args.length != 2) {
        System.out.println("we need 2 numbers");
        // regex to match if input is a digit
    } else if (args[0].matches("\\d") && args[1].matches("\\d")) {
        int result = Integer.valueOf(args[0]) + Integer.valueOf(args[1]);
        System.out.println("Result is: " + result);
        // the rest is simply not a digit
    } else {
        System.out.println("You must type a digit");
    }
}