Question about a 2d array example

[silverglade]

Hi, I have a question about this little code example. Its always these little nested for loop tricks that mess me up! LOL. Hopefully I will get used to them. But below, I have the following questions about it. The code outputs a table of 0-9 in the top row, then the next row 10-19, etc, there are 5 rows total. My questions are commented on the code lines. Any help greatly appreciated. Thank you!

//********************************************************************
//  TwoDArray.java       Author: Lewis/Loftus
//
//  Demonstrates the use of a two-dimensional array.
//********************************************************************

public class TwoDArray
{
   //-----------------------------------------------------------------
   //  Creates a 2D array of integers, fills it with increasing
   //  integer values, then prints them out.
   //-----------------------------------------------------------------
   public static void main (String[] args)
   {
      int[][] table = new int[5][10];

      // Load the table with values
      for (int row=0; row < table.length; row++) // table array is 2D, how do get a length for it?
         for (int col=0; col < table[row].length; col++)//I don't understand table[row].length.
            table[row][col] = row * 10 + col; //I don't understand this.

      // Print the table
      for (int row=0; row < table.length; row++) 
      {
         for (int col=0; col < table[row].length; col++)
            System.out.print (table[row][col] + "\t");
         System.out.println();
      }
   }
}

[Toll]

Look at it as an array of arrays. So table[0] is itself an array, and any array methods can be used on it. That should hopefully make it easier to understand.

[silverglade]

Thank you Toll. Does table.length refer to 5 elements always in this example please?

[Toll]

Correct. table is an array with length 5; the only special thing about it is that the elements in the array are arrays as well.

[silverglade]

Great. Just one more question I think. I don't understand this code part.

table[row][col] = row * 10 + col;

It looks like it multiplies row times 10, so it would be counting by 10's or something, but the code outputs in regular sequence. 1 2 3 4 ....

[Toll]

Step through the loop one step at a time.

      for (int row=0; row < table.length; row++)
         for (int col=0; col < table[row].length; col++)
            table[row][col] = row * 10 + col;

The first time it sets the value, what are the values of row and col? What is the resulting value?

[silverglade]

I took a stab at it. ok. the value of row and table.length on the first iteration of the loop is while 0(row)is less than 5(table.length), then the inner for loop says while 0< ??(table[row].length,
then make table [0](row) and [0](col) = 0 * 10 + 0. So that is 0 from the right calculation, put into table [0][0]. So the first loop prints out zero. On the second iteration, the value of row is 1. The value of col is 1. so the next iteration should be table[1][1]= 1 * 10 + 1, which is the value 11. So at position 1, 1, print the value of 11. That doesn't make sense to me because I thought the second value printed is 2. Please any more advice greatly appreciated. thank you!

[Toll]

Not quite. The first time it assigns a value, row is 0 and col is 0. That far you've got it right. However, the inner loop runs inside the outer loop; thus, it loops from 0 to 9 (the length of the "inner" array) for every row. So the second iteration has row still as 0 and col as 1. After enough loops, row is 0 and col is 9, and the inner loop ends. The outer loop takes another step, and the inner loop starts over, meaning the values are now row=1 and col=0. Repeat until the outer loop is done.

[silverglade]

Thanks again Toll. Does that mean that the value of table[row].length means "loop this while col is less than 10?
EDIT: hahahahah! I GOT IT!!! thank you so much Toll. That little sob of a code (the little nested iterating for loops) almost ruined my day as they are the hardest to understand to me, go figure. THANK YOU!!! you made my afternoon! Derek