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I was solving this question, trying to generate spiral matrix but repeatedly getting error only when the length of the matrix is odd. It is working fine when the length is even

This is the code:

public class spiral {

public static void main(String[] args) {

    int n = 5;
    generateSpiralMat(n);
}

static void generateSpiralMat(int n) {

    int[][] mat = new int[n][n] ;
    int topRow =0, rightCol = n-1, bottonRow = n-1, leftCol = 0 ;
    int num = 1 ;

    while( num < n*n + 1 ) {

        //topRow -> leftCol to rightCol
        for(int i=leftCol; i<rightCol; i++) {
            mat[topRow][i] = num;
            num ++ ;
        }

        //rightCol -> topRow to bottomRow
        for(int j =topRow; j<bottonRow; j++){
            mat[j][rightCol] = num ;
            num++ ;
        }

        //bottomRow -> rightCol to leftCol
        for(int i=rightCol; i> leftCol; i--) {
            mat[bottonRow][i] = num;
            num++ ;
        }

        //leftCol -> bottomRow to topRow
        for(int j=bottonRow; j>topRow; j--) {
            mat[j][leftCol] = num ;
            num ++ ;
        }

        topRow += 1 ;
        bottonRow -= 1 ;
        leftCol += 1 ;
        rightCol -= 1 ;
    }

    printMat(mat);
}

static void printMat(int[][] mat) {

    for(int i=0; i< mat.length; i++) {
        for(int j=0; j< mat[i].length; j++) {
            System.out.print( mat[i][j] + " ");
        }
        System.out.println();
    }
}

}

Even length matrix working well but code is showing error in runtime only for odd length matrix

I was expecting that the code would generate a spiral matrix as per the given question

Debashis Baral
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    Well, in your generate function you increase `num` by 4 before checking the size. That works well with squares of even numbers, because those are divisible by 4. Squares of odd numbers aren't though. Which means that at least one value of `num` is outside of the valid range – Ronald Sep 02 '23 at 05:16
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    Hint: use a debugger to step your code... you will see that `leftCol` is greater than `rightCol` at line 59 (`for(int i=leftCol; i – user22471702 Sep 02 '23 at 06:11
  • BTW sooner than later, same will happen with top and bottom, and the code will be looping the outer loop without entering any of the inner loops until, due to an integer overflow, the limits - `leftCol` first - swaps to negative (`-2147483648`) – user22471702 Sep 02 '23 at 11:02

1 Answers1

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void spiral( int side ) {
   int matriz[][] = new int[ side ][ side ];
   int length = side * side;
   int count = 0;
   int top = side - 1;
   int floor = 0;
   int sector = 0;
   int j = 0, k = 0;
   for( int i = 0; i < length; i++ ) { 
      matriz[ j ][ k ] = count;
      switch( sector ) {
         case 3: j--;
            if( j == floor ) {
               sector = 0;
               top -= 1;
            }
            break;
         case 2: k--;
            if( k == floor ) {
               floor++;
               sector++;
            }
            break;
         case 1: j++;
            if( j == top ) {
               sector++;
            }
            break;
         case 0: k++;
            if( k == top ) {
               sector++;
            }
            break; 
      }
      count++;
   } 
}

We divide the operation into four stages:
A) advance in the columns
B) advance in the rows
C) back in the columns
D) back in rows
when it reaches the corresponding limit, it goes to the next sector, in sector "3" the upper limit is modified, and in "2" the lower limit.

Marce Puente
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