Why doesn't my else statement work when I put only 2 inputs?

Question

Ok so I'm working on my accounting application and heres what i got so far:

public class Accounting {
    public static void main(String[] args) {
        while(true){
            Scanner input = new Scanner(System.in);
            String userinput = input.nextLine();

            String[] parts = userinput.split(" ");
            String part1 = parts[0];
            String part2 = parts[1];
            String part3 = parts[2];

            int a = Integer.parseInt(part1);
            float r = Float.parseFloat(part2);
            int t = Integer.parseInt(part3);
            double Total = a*Math.pow(1.0+(r/100.0), t);

            String[] result = userinput.split(" ");
            if (result.length == 3) { 
                System.out.println(Total);
            } else {
                System.out.println("Usage: a r t (a is the amount, r is the rate, and t is the time)");
            }
        }
    }
}   

I made it so that if the user puts more than 3 inputs, it will give a usage prompt. However, when I put in 2 inputs, I get an error instead of the usage prompt, even though 2 is not equal to 3.

Heres the error message:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 2 at Accounting.main(Accounting.java:15)

How can I fix this?

EDIT: My issue here isn't that the part of the array is not there because there is only two inputs, but that it won't give the usage prompt.

Answer 1

The reason is because you try to access part of an array that doesn't exist:

String part3 = parts[2];

Where parts.length == 2

I assume you get an index out of bounds error?

Answer 2

When you have only 2 inputs the range of your parts array is [0..1], thus when you try to access parts[2] here:

   String part3 = parts[2];

An error will be thrown.

Answer 3

"My issue here isn't that the part of the array is not there because there is only two inputs, but that it won't give the usage prompt."

No, you're wrong. The point is that you need to check the length of the array before using it with your assumed length. This is a simple rearrangement that should work correctly (besides that presumably you're missing a few things as well like an exit condition and trying for NumberFormatException from the parsing):

Scanner input = new Scanner(System.in);

while(true){
    String userinput = input.nextLine();

    String[] parts = userinput.split(" ");

    // check first
    if (parts.length != 3){
        System.out.println(
            "Usage: a r t (a is the amount, r is the rate, and t is the time)"
        );
        continue; // skip the computation if the input is wrong
    }

    String part1 = parts[0]; // these lines WILL throw an error
    String part2 = parts[1]; // if you attempt to access elements
    String part3 = parts[2]; // of the array that do not exist

    int a = Integer.parseInt(part1);
    float r = Float.parseFloat(part2);
    int t = Integer.parseInt(part3);
    double total = a*Math.pow(1.0+(r/100.0), t);

    System.out.println(total);
}