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Building Array....Compile error.

Posted on 2004-09-22
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Last Modified: 2010-03-31
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Code begin
--------------------------------------------------------------------------------------------------
/*
instance variables
-----------------------------------------
private double[][] M                  array representing the Matrix

private final int MAX_SIZE             constant representing the maximum size of a Matrix;
                        it should be set to 100
-----------------------------------------

The Code:
public methods
-----------------------------------------
Matrix(int sizeN)                  A constructor that creates a Matrix of
                        size N x N filled with zeros
                        N must be in the range [0, MAX_SIZE].
                        If N is negative, create a Matrix of size 0 x 0.
                        If N is larger than MAX_SIZE, create a Matrix
                        of size MAX_SIZE x MAX_SIZE.

Matrix(int sizeN, double[][] initValues)      A constructor that creates a Matrix of size N x N
                        and fills it with the values found in the array initValues.
                        N must be in the range [0, MAX_SIZE].  If N is negative,
                        create a Matrix of size 0 x 0.  If N is larger than MAX_SIZE,
                        create a Matrix of size MAX_SIZE x MAX_SIZE.
-----------------------------------------
*/

public class Matrix
{
      //initialize array M
      private double[][] M;

      //variable to set max size of array
      private final int MAX_SIZE = 100;

      //Constructor One
      //Create a new Matrix, initializing it to zeros
      //The size must be a value in the range [0, MAX_SIZE]
      public Matrix(int sizeN)
      {
            if (sizeN > MAX_SIZE)
            {
                  System.out.println("The maximum size of a matrix is " +
                        MAX_SIZE);
                  sizeN = MAX_SIZE;
            }
            else if (sizeN < 0)
            {
                  System.out.println("The size of a matrix cannot be negative");
                  sizeN = 0;
            }

            //build array M with sizeN x sizeN initalized to 0
            M = new double[sizeN][sizeN];
            for (int i = 0; i < sizeN; i++)
                  for (int j = 0; j < sizeN; j++)
                        M[i][j] = 0.0;
      }

      //Constructor Two
      //Create a new Matrix, initializing it with the above parameters
      //The dimensions must be in the range [1, MAX_SIZE]
      public Matrix(int sizeN, double[][] initValues)
      {
            initValues = new double[sizeN][sizeN];
            double fillArray;
            for (int i = 1; i < MAX_SIZE; i++)
                  fillArray = 0;
                  for (int j = 1; j < MAX_SIZE; j++)
                        fillArray += initValues [i][j];
      }
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code end
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Problem: (Compile error)
--------------------------------------------------------------------------------------------------
cannot resolve symbol
symbol  : variable i
location: class Matrix
fillArray += initValues [ i ] [ j ];
                                 ^

What's the problem?  Up to "//Constructor Two" the code compiles without error.
As you probably know I'm writing a "matrix" class.
Last do you see any flaws with the rest of the build either in code itself or my approach?

Thanks.
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Question by:smkJackstraw
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Accepted Solution

by:
CI-Ia0s earned 100 total points
ID: 12129653
Try putting brackets around the body of that last for loop...
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Author Comment

by:smkJackstraw
ID: 12129705
You know....syntax errors will be the death of me.  It worked!  Before I award the points, would you answer this last question:

Why didn't I get the syntax error with the previous loop:  //build array M with sizeN x sizeN initalized to 0

There nearly identical.  Which is more 'proper' syntax? (with reguards to Java AND the idea that I'm learning Java as a stepping stone to C++)

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Expert Comment

by:CI-Ia0s
ID: 12129771
Unfortunately, the Java compiler often doesn't catch errors with brackets and loops/if-statements (I've had similar issues when I've forgotten brackets, though with an if-statement). I always use brackets and I have never seen it done without them. The reasons for this are:
1. No chance of having the compiler miss it and give you some other error/get a run time error.
2. It's easier to follow. If you can see where every loop begins and ends with two clearly defined markers it makes it easier to skim the code (an important skill when you start writing long code). It's also easier to follow for others who may read your code. :)
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Expert Comment

by:objects
ID: 12129779
> Why didn't I get the syntax error with the previous loop:  //build array M with sizeN x sizeN initalized to 0

Because there was only a single statement in the loop, so brackets were optional.
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by:CI-Ia0s
ID: 12129780
P.S. I'd add brackets to the previous "//build array M with sizeN x sizeN initalized to 0" to make sure there's no confusion. Even if it runs correctly now, you may end up doing some editing to find it not working later.
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by:CI-Ia0s
ID: 12129788
>>Because there was only a single statement in the loop, so brackets were optional.

i.e. The compiler sees this:
          for (int i = 1; i < MAX_SIZE; i++){
               fillArray = 0;
          }
          for (int j = 1; j < MAX_SIZE; j++){
               fillArray += initValues [i][j];
          }
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by:CI-Ia0s
ID: 12129824
:)
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