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C# C# Collections Arrays Jagged Arrays

Posted by Allison Cuyjet
Allison Cuyjet
20,689 Points

C# Collections Challenge Task (Jagged Arrays)

I seem to be stuck on this challenge. When I preview my code, I don't get any errors, but when I submit my work, I get a Bummer! message telling me that an "object reference not set to an instance of an object." So I know I've got an unhandled NullReferenceException, but I'm not clear on how to resolve it.

Math.cs 
namespace Treehouse.CodeChallenges
{
    public static class MathHelpers
    {           
        public static int[][] BuildMultiplicationTable(int maxFactor)
        {
            int[][] multiplicationTable = new int[maxFactor][];

            for (int row = 0; row <= maxFactor; row++)
            {
                for (int column = 0; column <= maxFactor; column++)
                {
                    multiplicationTable[row][column] = row * column;
                }
            }
            return multiplicationTable;
        }
    }
}

3 Answers

Lukas Dahlberg
Lukas Dahlberg
53,736 Points

Ah, right.

So, I missed a couple things above. Let's try again.

First, you didn't initialize the column rows above. And when you do the calculations, you'll find that maxFactor is one short of what you'll require because of the 0s.

So the solution looks like this: 

namespace Treehouse.CodeChallenges
{
    public static class MathHelpers
    {           
        public static int[][] BuildMultiplicationTable(int maxFactor)
        {
            maxFactor = maxFactor + 1;
            int[][] multiplicationTable = new int[maxFactor][];

            for (int row = 0; row < multiplicationTable.Length; row++)
            {
                multiplicationTable[row] = new int[maxFactor];
                for (int column = 0; column < multiplicationTable[row].Length; column++)
                {
                    multiplicationTable[row][column] = row * column;
                }
            }
            return multiplicationTable;
        }
    }
}

Personally, I find it easiest to do the solutions in Visual Studio 2015 and port them over.

Allison Cuyjet
Allison Cuyjet
20,689 Points
Allison Cuyjet
Allison Cuyjet
20,689 Points

Thank you! That makes sense.

Lukas Dahlberg
Lukas Dahlberg
53,736 Points
Lukas Dahlberg
Lukas Dahlberg
53,736 Points

You don't want to use maxFactor to set your for loop limit.

So instead of: 

for (int row = 0; row <= maxFactor; row++)
for (int column = 0; column <= maxFactor; column++)

use: 

for (int row = 0; row < multiplicationTable.Length; row++)
for (int column = 0; column < multiplicationTable[row].Length; column++)

(in their respective spots, of course.)