For loop causes "out of bound" error

Question

I've been attempting to make a ludo game to learn a bit more about C# Unfortunately I have run into a for loop that I can't figure out, it gives me an out of bound error. What I am trying to do in this code is make the board( Which is the 15x15 array) I am assigning all the blank spots on the game board with the value 9, so that I can play with it easily later on.

Quadrant 1 does what it is intended to, but it gives me the "Out of bound" error in Quadrant 2.

// Make the 2D array (Game board)
int[,] GameBoard = new int[15, 15];

// Quadrant 1
for(int Blankspotx1 = 0; Blankspotx1 < 7; Blankspotx1++)
{
    GameBoard[Blankspotx1, 0] = 9;
    int Blankspoty1 = 0;
    for (Blankspoty1 = 0; Blankspoty1 < 7; Blankspoty1++)
    {
        GameBoard[Blankspotx1, Blankspoty1] = 9;
    }
    Blankspoty1 = 0;
}

// Quadrant 2
for (int Blankspotx2 = 10; Blankspotx2 < 16; Blankspotx2++)
{
    GameBoard[Blankspotx2, 0] = 9;

    int Blankspoty2 = 0;
    for (Blankspoty2 = 0; Blankspoty2 < 7; Blankspoty2++)
    {
        GameBoard[Blankspotx2, Blankspoty2] = 9;
    }
    Blankspoty2 = 0;
}

Come to think of it, that was one of the very first errors I made when I started with C++ decades ago — Panagiotis Kanavos, Mar 28 '18 at 14:24

Zohar Peled · Accepted Answer · 2018-03-28T14:30:26.103

6

for (int Blankspotx2 = 10; Blankspotx2 < 16; Blankspotx2++) should be
for (int Blankspotx2 = 10; Blankspotx2 < 15; Blankspotx2++)

The last index of any zero-based index collection is it's length - 1.

As a rule of thumb, it's always best to use the collection's length-1 instead of hard coding the number, to prevent such mistakes from happening.

For multi-dimensional arrays, use the GetLength(Int32) method to get the length of the specified dimension, so it's either:

for (int Blankspotx2 = 10; Blankspotx2 < GameBoard.GetLength(0); Blankspotx2++)

Or

for (int Blankspotx2 = 10; Blankspotx2 <= GameBoard.GetLength(0)-1; Blankspotx2++)

edited Mar 28 '18 at 14:30

answered Mar 28 '18 at 14:23

Zohar Peled

79,642
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@PanagiotisKanavos correct... I'm used to `<= x.Length-1`, so I've typed that automatically... – Zohar Peled Mar 28 '18 at 14:27
3

Never underestimate the viciousness and cunning of an off-by-one error – Panagiotis Kanavos Mar 28 '18 at 14:27
@PanagiotisKanavos nor the viciousness of automatic typing, for that matter :-) How many times you wanted to type `service` and ended up with `server`? I can't count my own it's so many.... – Zohar Peled Mar 28 '18 at 14:28
That too: what should int * int return? When talking about *DVD or Blueray* file offsets ? Somehow -1234567 isn't the correct answer – Panagiotis Kanavos Mar 28 '18 at 14:30

score 0 · Answer 2 · answered Mar 28 '18 at 14:25

0

Arrays are zero based counted. so you need to get the for loop up to 15 and not to 16. The fifteenth element is the one who is "out of Bounds"

 // Quadrant 2
for (int Blankspotx2 = 10; Blankspotx2 < 15; Blankspotx2++)
{
    GameBoard[Blankspotx2, 0] = 9;

    int Blankspoty2 = 0;
    for (Blankspoty2 = 0; Blankspoty2 < 7; Blankspoty2++)
    {
        GameBoard[Blankspotx2, Blankspoty2] = 9;
    }
   Blankspoty2 = 0;
 }

answered Mar 28 '18 at 14:25

Matthias Neubert

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And the correct answer is to *not* hard-code the length – Panagiotis Kanavos Mar 28 '18 at 14:31
Never hard code the length when its sitting right in front of you in a nice method of the `GameBoard` variable. – maccettura Mar 28 '18 at 14:35
@PanagiotisKanavos I see. I wasn't that strict yet. Maybe I should be in future. Thanks for your hint – Matthias Neubert Mar 28 '18 at 14:39

For loop causes "out of bound" error

2 Answers2