Out of range index in if condition

Question

I have a sentence, and I want to check for duplication letters in order to add a 'x' as a separator between them, however I debugged and kept getting an exception in here:

for (int i = 0; i < res.Length; i++)
{
    t += res[i];

    if (res[i] == res[i + 1]) //Index out of range exception here
    {
        t += 'x';
    }
}

what goes wrong in here?

when `i == res.Length - 1` you have `res[i + 1]` being `res[res.Length]` which throws exception. — Dmitry Bychenko, Feb 27 '19 at 13:19

Dmitry Bychenko · Answer 1 · 2019-02-27T13:38:32.007

The cause of misbehavior is in the if:

  if (res[i] == res[i + 1])

when i == res.Length - 1 (for loop last iteration) you have

  if (res[res.Length - 1] == res[res.Length])

and res[res.Length] throws OutOfRangeException since valid range is [0..res.Length - 1] (please, note - 1).

Your code corrected:

    for (int i = 0; i < res.Length; i++)
    {
        Nplaintext += res[i];
        // we don't want to check (and insert 'x') for the last symbol
        if (i < res.Length - 1 && res[i] == res[i + 1]) 
        {
            Nplaintext += 'x';
        }
    }

Often we work with string with a help of regular expressions (let .Net loop over the string for you):

  using System.Text.RegularExpressions;

  ...

  string source = "ABBBACCADBCAADA";

  // Any symbol followed by itself should be replaced with the symbol and 'x'
  string result = Regex.Replace(source, @"(.)(?=\1)", "$1x");

  Console.Write(result);

Outcome:

  ABxBxBACxCADBCAxADA

Thank you. Finally, someone with a brain. Too many answers suggesting to change the for to `Length - 1`, completely unaware that nothing will happen with the last element of the array. — ThePerplexedOne, Feb 27 '19 at 13:38

Manoj Choudhari · Accepted Answer · 2019-02-27T13:53:47.060

-1

i+1 us causing this.

in the last iteration, i+1 refers to a location which is not inside that array.

Better change the condition in for loop as below:

for (int i = 0; i < res.Length - 1; i++)
{
    t += res[i];

    if (res[i] == res[i + 1]) //Index out of range exception here
    {
        t += 'x';
    }
}

t += res[res.Length -1 ];

Hope this helps.

edited Feb 27 '19 at 13:53

answered Feb 27 '19 at 13:17

Manoj Choudhari

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worked out, thanks – AspiringLilly Feb 27 '19 at 13:33
Not the correct answer, but good luck anyway. – ThePerplexedOne Feb 27 '19 at 13:37
1

It seems that the *last character* will not be *added* . – Dmitry Bychenko Feb 27 '19 at 13:37
@ThePerplexedOne - i agree that last character would not be added, i will update the full code here, but the question was mainly about the out of index exception, I believe that this answers the question. – Manoj Choudhari Feb 27 '19 at 13:52
It doesn't, though, does it? Because you've given birth to another problem. If you change the for, `t += res[i];` will never occur for the last element. – ThePerplexedOne Feb 27 '19 at 13:57

score -1 · Answer 3 · answered Feb 27 '19 at 13:21

Sure you get this exception. In the case i=res.Length -1 (what is exactly the last position) you ask for res[Length] with the i +1 but because of starting with 0 the element you are asking for does not exist. try something like

if(i+i < res.Length)

before you ask for that element or even better start counting with i=1 and use

if (res[i] == res[i - 1]) 
        {
            Nplaintext += 'q';
        }

Out of range index in if condition

3 Answers3