Errata

RobC · August 28th, 2006, 06:15 PM

There are a couple of problems with the example on this page. The first one isn't really a code bug, but a logic bug:

Code:

while (rdr.Read())
        {
            goalsFor += Convert.ToInt32(rdr["GoalsFor"]);
            goalsAgainst += Convert.ToInt32(rdr["GoalsAgainst"]);

            if (goalsFor > goalsAgainst)
                wins++;
            else if (goalsFor < goalsAgainst)
                losses++;
            else
                draws++;
        }

Checking for wins and losses this way doesn't make any sense, because the code is evaluating the cumulative goal totals. It should be comparing the goalsFor and goalsAgainst for each individual game:

Code:

while (rdr.Read())
        {
            int thisGameGoalsFor = Convert.ToInt32(rdr["GoalsFor"]);
            int thisGameGoalsAgainst = Convert.ToInt32(rdr["GoalsAgainst"]);

            if (thisGameGoalsFor > thisGameGoalsAgainst)
                wins++;
            else if (thisGameGoalsFor < thisGameGoalsAgainst)
                losses++;
            else
                draws++;

            goalsFor += thisGameGoalsFor;
            goalsAgainst += thisGameGoalsAgainst;
        }

The second problem is this line:

Code:

winRatio = Convert.ToInt32(wins / losses * 10);

Because the wins and losses variables are integers, this expression will always evaluate to zero if there are more losses than wins. In order for this work, you have to do a conversion to a floating point number. There's probably a more elegant way to do this, but here's what I did to get it to work:

Code:

double winPercent = wins * 1.0 / losses;
winRatio = Convert.ToInt32(winPercent * 10);

Finally, there's a problem with the downloaded code in Begin\Chapter09. If you look at the data in the Fixtures table in WroxUnited.mdf, you'll see that there are NULL values in the GoalsFor and GoalsAgainst in the tenth record. You'll have to either delete this record or enter numbers for these fields - otherwise, you'll get a runtime error.

DaveSussman · August 30th, 2006, 02:59 AM

Rob

Right on both counts - these are logic errors. The ration problem is simply explained - the whole concept of WroxUnited was that they virtually never won a game, so I never actually tested with more wins than losses. This shows an important point about application testing, in that you should always test with a variety of values, even ones you don't expect.

Dave

tbroom · September 8th, 2006, 02:08 PM

Could you explain why this part

"Finally, there's a problem with the downloaded code in Begin\Chapter09. If you look at the data in the Fixtures table in WroxUnited.mdf, you'll see that there are NULL values in the GoalsFor and GoalsAgainst in the tenth record. You'll have to either delete this record or enter numbers for these fields - otherwise, you'll get a runtime error."

is not in the Errata as something one needs to do so that you do not get this "Object cannot be cast from DBNull to other types. " when attempting to run the page?

DanM · October 25th, 2006, 04:17 PM

tbroom,
You're right of course. My best guess as to why this happened is that one of the authors added an extra row to that table by mistake. It should be deleted.

Wrox United never have the energy to play ten games in a season.

Dan

Dan Maharry
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RomeyRome · November 23rd, 2007, 08:28 PM

How do I delete this 10th record so I can get rid of the runtime error??