Bug Vanquisher

16 September 2009

The Off-by-1 Game

Filed under: Bugz, S&G, Tips — Tanveer Badar @ 7:54 PM

Consider this:

for( int i = list.Count – 1 ; i >= list.Count – 50 ; –i , )
.     array [ i ] = list [ i ];

Intention was the get the last 50 terms from list. Ignoring the facts that it will throw a null reference exception if the list is null and an index-out-of-range exception if it contains no elements it also fails to copy all 50 elements. The condition is to be blamed.

Correct way is not to invent your own logic and use what the framework offers.

if( list != null && list.Count > 50 )
    list.RemoveRange( 50 , list.Count – 50 );

For added bonus, consider this paging code.

for( int i = 0 , index = 0 ; i < some_huge_number ; ++i )
{
    page.Results.Add( results [ i ] );
    if( index == 10 )
    {
        pages.Add( page );
        page = new Page( );
        index = 0;
    }
    ++index;
}

This code strives to divide results into pages each with 10 results. Can you imagine what this will do if there were only 9 results?

It will proceed to add those results to page, then, do nothing. Because of the incorrect if condition. Again, the correct way is divide-and-remainder approach as demonstrated here.

Advertisements

Leave a Comment »

No comments yet.

RSS feed for comments on this post. TrackBack URI

Leave a Reply

Please log in using one of these methods to post your comment:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Create a free website or blog at WordPress.com.

%d bloggers like this: