For loop with if inside not working

Question

I am having with my studies, I have problem where I am supposed to get an IP address from an user and then iterate it from right most number and if that number will be equal or more than 256 then I should iterate the number -1 place before this and this one set to 0.

I tried to solve it by simply making primitive code at first which would do it one time and only by user input and after that I would add more complexity like original more than one iteration, error checks and put code into propper .java files and classes. I understand that this would be better with ArrayList but I intended to add ArrayList instead of simple Array later.

Could anyone please tell me why the loop with condition put outofarraybound exception when I am not trying to iterate "i"?

    for (i = 3; i >= 0; i--) {
        pomoc = zasobnikIPadresa[i];

        if (pomoc > 255) {              
            zasobnikIPadresa[i] = 0;
            zasobnikIPadresa[i-1] = pomoc + 1;
        }           
    }

So far I was able to analyze that I dont have proper knowledge of Arrays and I think that solution to my issue would help me to finish my problem and to better understand them.

here is full code so far:

    package com.ipadresa.classes;

    import java.util.Scanner;

    public class Hlavni {

    public static void main(String[] args) {

        int i = 0;

        int[] zasobnikIPadresa = new int[4];        
        Scanner ctecka = new Scanner(System.in);

        for (i = 0; i < zasobnikIPadresa.length; i++) {
            zasobnikIPadresa[i] = ctecka.nextInt();
        }

        System.out.print("Original IP adress: ");
        for (i = 0; i < zasobnikIPadresa.length; i++) {

            if (i < zasobnikIPadresa.length - 1) {
                System.out.print(zasobnikIPadresa[i] + ".");
            } else {
                System.out.print(zasobnikIPadresa[i]);
            }           
        }   System.out.println();


        int pomoc = 0;  

        for (i = 3; i >= 0; i--) {
            pomoc = zasobnikIPadresa[i];

            if (pomoc > 255) {              
                zasobnikIPadresa[i] = 0;
                zasobnikIPadresa[i-1] = pomoc + 1;
            }           
        }


        System.out.print("Final IP adress: ");
        for (i = 0; i < zasobnikIPadresa.length; i++) {

            if (i < zasobnikIPadresa.length - 1) {
                System.out.print(zasobnikIPadresa[i] + ".");
            } else {
                System.out.print(zasobnikIPadresa[i]);
            }           
        }       

        ctecka.close();
    }
}

Hovercraft Full Of Eels · Accepted Answer · 2015-09-20 21:40:16Z

6

Since by this for loop condition, for (i = 3; i >= 0; i--) {, the variable i is allowed to == 0, then let's see what the array index is here when i is 0:

zasobnikIPadresa[i-1] = pomoc + 1;

it's -1. Ouch.

answered Sep 20, 2015 at 21:40

Hovercraft Full Of Eels

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Comments

Vishwas · Accepted Answer · 2015-09-20 21:41:04Z

2

What if the condition

pomoc > 255

is true when

i==0.

Then you'll be accessing zasobnikIPadresa[-1] i.e. out of bound.

answered Sep 20, 2015 at 21:41

Vishwas

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