How to resolve Exception in thread error (ArrayIndexOutOfBoundsException) in Java?

Question

This code is to input n integer values and calculate the no. of even and odd values among those inputted values.

This java code shows ArrayIndexOutOfBoundsException when used do..while loop, however when used for loop it worked fine. Haven't changed any thing just rearranged the syntax so as to convert for loop into do while loop.

FOR LOOP:

import java.util.*;

public class EvenOddCount
{
    public static void main(String args[]) throws Exception
{
    System.out.print("Enter the no. of inputs to be taken : ");
    int evenCount=0, oddCount=0;
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int a[] = new int[n];
    System.out.println("Enter the inputs : ");
    for(int i=0;i<n;i++)
        a[i] = sc.nextInt();
    for(int i=0;i<a.length;i++)
    {
        if(a[i]%2==0)
            evenCount++;
        else 
            oddCount++;
    }
    System.out.println("\nThe number of even numbers in input numbers are : "+evenCount);
    System.out.println("The number of odd numbers in input numbers are : "+oddCount);
}
}

The above code works fine and gives suitable output.

DO...WHILE LOOP:

import java.util.*;

public class EvenOddCount
{
public static void main(String args[]) throws Exception
{
    System.out.print("Enter the no. of inputs to be taken : ");
    int evenCount=0, oddCount=0, i=0;
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int a[] = new int[n];
    System.out.println("Enter the inputs : ");
    do
    {
        a[i] = sc.nextInt();
        i++;
    } while (i<n);
    do
    {
        if(a[i]%2==0)
            evenCount++;
        else 
            oddCount++;
        i++;
    } while (i<a.length);
    System.out.println("\nThe number of even numbers in input numbers are : "+evenCount);
    System.out.println("The number of odd numbers in input numbers are : "+oddCount);
}
}

The above code has a runtime exception i.e.

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 5
at EvenOddCount.main(EvenOddCount.java:20)

That's because do-while loops always execute at least once due to the condition being evaluated after. In your second do while loop, `int i` is still the same value as it was on the last iteration of the first do-while, and thus out range when doing `a[i]%2` — Andrew Li, Sep 23 '18 at 17:54

score 5 · Accepted Answer · answered Sep 23 '18 at 17:54

In your original code, you had two separate i variables:

for(int i=0;i<n;i++)
...

for(int i=0;i<a.length;i++)

In your do/while version, you've got one i variable. Once you've gone through the first loop, the value of i will be n - but you start on the second loop without resetting it to 0, so on the very first iteration it will be out of range.

You can fix that by adding:

i = 0;

just before the second do/while loop, but be aware that you'll still have a problem (even in the first loop) if n is 0, because you're not checking the condition until the end of the iteration. If you use:

while (i < n)

and

while (i < a.length)

instead, that will check the condition before the first iteration, so it will execute 0 times when n is 0. (You still need to reset i to 0 before the second loop though.)

Thanks A Lot Sir! Actually I have to specifically code using do..while loop hence I can't use while hence for the problem about input n=0 I'll have to use if-else. — meisner97, Sep 23 '18 at 18:09

score 0 · Answer 2 · answered Sep 23 '18 at 18:04

0

U need to reset the value of i :

do
{
    a[i] = sc.nextInt();
    i++;
} while (i<n);
i =0;
do
{
    if(a[i]%2==0)
        evenCount++;
    else 
        oddCount++;
    i++;
} while (i<a.length);

answered Sep 23 '18 at 18:04

Abdalnassir Ghzawi

99
9

Doesn't the loop exits as soon as i becomes equal to n. – meisner97 Sep 23 '18 at 18:15
@meisner97 yes but i is declared as a class variable, which means the value of i is updated and " remembered ", that is why u need to reset. unlike local variables; which get destroyed as soon as the method finish executing – Abdalnassir Ghzawi Sep 23 '18 at 18:42
@abdalnaseerghzawi okay I understood. Thanks – meisner97 Sep 23 '18 at 19:07
@AbdalnassirGhzawi The i variable in the code you've shown is still a local variable. – Jon Skeet Sep 23 '18 at 21:12
@JonSkeet yes my bad, forget it is in main method – Abdalnassir Ghzawi Sep 23 '18 at 21:16

How to resolve Exception in thread error (ArrayIndexOutOfBoundsException) in Java?

2 Answers2