JavaArray Index out of Bounds exception w/ for loop

Question

I am getting an array index out of bounds exception while iterating over an array through the for-loop. Can someone tell me why this is happening it I have set the boolean in the for-loop to be i

public static boolean verify(int [] seq ){
  for (int i=0; i<seq.length; i++){

  //If the number is even, the next number
  //must the half the previous number
  if (seq[i] %2==0){
    if (seq[i+1] != (seq[i]/2)){
      return false;
    }
  }

  //If the number is positive, the next number
  //must be 3 times + 1 the previous number
  else if (seq[i] %2!=0){
    if (seq[i+1] != ((seq[i])*3+1)){
      return false;
    }
  }

}

}

Why don't you print out the value of i at every step? Or better yet, use a debugger. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Feb 3, 2014 at 17:56
can't call seq[i+1] when i=seq.length. Change loop condition to i<seq.length-1 — turbo
– turbo, Commented Feb 3, 2014 at 17:57
"I have set the boolean in the for-loop to be i" you have WHAT? — Ingo
– Ingo, Commented Feb 3, 2014 at 18:00
This is a good opportunity to learn how to debug. If you don't have an effective debugger, use println statements. — Hot Licks
– Hot Licks, Commented Feb 3, 2014 at 18:08

rgettman · Accepted Answer · 2014-02-03 17:58:09Z

1

The problem is when you access index i+1. If i is the last possible value (seq.length - 1), then i+1 is one beyond the end of the array, resulting in an ArrayIndexOutOfBoundsException.

Stop your for loop one iteration earlier by modifying your condition to be:

i < seq.length - 1

answered Feb 3, 2014 at 17:58

rgettman

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Comments

Kick · Accepted Answer · 2014-02-03 17:58:19Z

1

You will face exception for the maximum value of i bcoz you are increasing the value by 1 to find the index value.

    if (seq[i] %2==0){
            if (seq[i+1] != (seq[i]/2)){
---------------------^
              return false;
            }
          }

answered Feb 3, 2014 at 17:58

Kick

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Comments

Hugo Sousa · Accepted Answer · 2014-02-03 17:58:21Z

1

You're trying to access position i+1 or the Array. Since your for loop goes until the last element, you'll try to access 1 position after the last element, what causes the Out Of Bounds exception.

answered Feb 3, 2014 at 17:58

Hugo Sousa

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Mad Physicist · Accepted Answer · 2014-02-03 17:58:25Z

1

You are iterating over all elements in the array, but checking element seq[i + 1] for i == seq.lenth - 1 will always cause the exception. The last number is fully constrained by your conditions, so no need to check it. Make your loop run as follows: for (int i=0; i <seq.length - 1; i++)

answered Feb 3, 2014 at 17:58

Mad Physicist

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Ingo · Accepted Answer · 2014-02-03 17:59:19Z

1

This:

    if (seq[i+1] != (seq[i]/2)) {

cannot access an element beyond the end of the array, when i is seq.length - 1.

Another line like that is down in the else branch.

answered Feb 3, 2014 at 17:59

Ingo

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Comments

JDGuide · Accepted Answer · 2014-02-03 18:06:36Z

1

Its quite obvious , when you are passing an array (i.e. array contains 10 elements) and operating inside loop that correct.But when you are accessing seq[i+1] , there might be the you are accessing the index which is not available in the array.

When the i value reaches at 10 and you are trying to access i+1 , but this index is not in array (as we know array size is 10)

So , its caused this exception. Hope it will help you.

answered Feb 3, 2014 at 18:06

JDGuide

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