如何将sum的值存储在新数组b中

Question

public class HW9P1
{
   public static void main(String[] args)
   {
      /*int[][] a1 = { { 4, -2, 10 }, 
                     { 19, 3, -5 } };
      int[] b1 = coloumnsum(a1);*/

      int[][] a2 = { { 7, -10 }, 
                     { 4, 13 }, 
                     { 1, 0 } };
      int[] b2 = coloumnsum(a2);

      int i,j;
      for (i=0; i<b2.length; i++)
      {

         System.out.print(b2[i] + " ");


      }

  }
   public static int[] coloumnsum (int[][] a)
   {
      int[] b = new int[a[0].length];
      int i,j, sum= 7;

         for (j=0; j<a[0].length; j++)
         {
            for (i=0; i<a.length; i++)
            {
               sum = sum + a[i][j];


            }
         }
         return b;

   }
}

I have this code so far, which gives me the sum from the columns.Takes and 2d array and return 1d array. when i run this code, i get two 00 For ex {2,4} {1,9} sum should be {3,13}

Answer 1

You aren't storing your sum back into the array b . You actually don't need sum , just add the values directly in the array. Like,

public static int[] coloumnsum(int[][] a) {
    int[] b = new int[a[0].length];
    for (int j = 0; j < a[0].length; j++) {
        for (int i = 0; i < a.length; i++) {
            b[j] += a[i][j];
        }
    }
    return b;
}

When I make that change your current code produces (as expected)

12 3

Answer 2

1. I don't know why you're starting the sum at 7 if all you want is to sum the columns of the 2D array.
2. you're not assigning anything to accumulator array b .

below illustrates the correct approach to sum the columns:

public static int[] coloumnsum (int[][] a)
{
       int[] b = new int[a[0].length];
       for (int i = 0; i < a[0].length; i++){
           for (int j = 0; j < a.length; j++)
               b[i] += a[j][i];                   
       }        
       return b;  
}