如何在Java中从命令行平均数

Question

This is a school assignment. I truly want to understand what I am doing. We are instructed to make a program in Java that runs from the command line. After a user enters numbers the program should then start, send a welcome message, and then tell the user what the average of his numbers is.

I believe the point of this is to illustrate how args is used in the program and to help familiarize us with Java.

This is the explanation I have currently. I can run this from the command line, I know how to add a welcome message as well... But I do not understand the code well enough to be able to add in the ability to average the numbers when a user adds the arguments.

public class ArgumentExample {

    public static void main(String[] args) {

        if( args.length == 1 || args.length > 1 ){
            System.out.println( args[0]);
        }

        if( args.length > 1 ) {
            for( int i = 1; i < args.length; i++ ){
                System.out.println(args[i]);
            }
        }
    }
}

Answer 1

public class ArgumentExample {

        public static void main(String[] args) {
            int i = 0; // declare a counter here so you can use it in the catch
            double tot = 0.0d; // declare the accumulator variable
            try {
                for (i = 0; i < args.length; i++) { // iterate over arguments...if only one is present returns just that one
                    tot += Double.parseDouble(args[i]); // sum
                }
            } catch (Exception ex) {
                System.out.println("Argument " + i + " is not a number");// print which argument is not a number if any
            }
            System.out.println("Sum is: " + tot/(args.lenght == 0 ? 1 : args.lenght); // final print statement
        }
    }

Answer 2

Right now you are simply printing out the arguments that are put into the args array for you. It's not a big step from there to transforming the String arguments into numbers (say, doubles), adding those to a running sum in the loop, and dividing by the number of arguments after the loop:

    double sum = 0;
    if (args.length >= 1) {
        for (int i = 0; i < args.length; i++) {
            sum += Double.parseDouble(args[i]);
        }
        System.out.println("Average = " + sum/args.length);
    }

Double is the wrapper class for the primitive type double. It has the convenient method parseDouble to which you can pass a String and it will try to convert it to a double type. We should use doubles here instead of ints because of the higher precision of doubles - eg if you divide 5/2 you get 2, not 2.5, unless you make sure the 5 and 2 are treated as doubles.

Note that this has no error checking so it will fail by throwing an exception if any of the arguments is not parseable to a Double.

Answer 3

args is a reference to the String array that holds all of the arguments passed into the program when it is started.

Arrays use a 0 base system for referencing the object in that position, so args[0] is the first argument passed to the program.

The for( int i = 1; i < args.length; i++ ) loop is going through the array until it reaches the end ( i = length - 1 ) after printing each argument

Answer 4

Students tend to put too much stuff into main.

I'd recommend that you put the real work into sensible methods that you might use someday and pass data in and out, like this:

public class StatisticsUtils {
    public double average(double [] values) {
        double average = 0.0;
        if ((values != null) && (values.length > 0)) {
            for (double value : values) {
                average += value;
            }
            average /= values.length; 
        }
        return average;
    }
}

Answer 5

To get the average of the numbers you have to build the sum of them and then divide through the number of arguments. For example

public class ArgumentExample 
{
  public static void main(String[] args) {

  int sum = 0;

  // step through all arguments
  for(String arg : args)
  {
    try
    {
      // try to convert the argument to an integer
      int number = Integer.parseInt(arg);
      // sum it
      sum += integer;
    }
    catch(NumberFormatException e)
    {
      // the currently processed argument couldn't be converted
      // to a number
      System.out.println(arg + " is not a number");
    }
  }

  // average = sum / valuecount
  double average = sum / args.length;

  System.out.println("average = " + average);
}

java -jar ArgumentExample 1 2 3 4 5 would result into 3. Hope that helps