减少Java中的多维数组

Question

I'm having trouble understanding how to start the first index of array from the bottom to to the top of a multidimensional array. Here's what I've tried to initialize my array from bottom to top (in 2d array table format):

  for(int row = arrayName.length - 1; row > 0; row--) {
     for(int col = 0; col < arrayName.length; col++) {
        arrayName[row][col] = ' ';
     }
  }      

or

for(int row = arrayName.length - 1; row > 0; row--) {
  for(int col = arrayName.length - 1; col < 0; col--) {
     arrayName[row][col] = ' ';
  }
}      

i mean..when i run the program, the array always store my values from top to bottom, the opposite of what i wanted it to do. Please help! thanks in advance.

Answer 1

Matrices and Arrays don't really have any inherent sense of direction. Their direction is entirely determined by the order in which you choose to display the information. Depending on how you structure your loop, the contents of the array will be outputted differently. If you want to store elements from the "bottom" to the "top," what that actually means is that your print out loop should be structured in an opposite direction from your assignment loop. So for example (assuming an NxN array),

for (int i = 0; i < arr.length; i++) {
    for (int j = 0; j < arr.length; j++) {
        arr[i][j] = <some value here>;
    }
}

Say for simplicity's sake you think of vertex (0,0) being in the top left corner, where columns increase as you move right and rows increase as you move down. Then to print from the bottom up, you would want to start from the last row and move to the first row.

for (int i = arr.length - 1; i >= 0; i--) { // Note the >= 0, which was incorrect
                                            // in the code you posted
    for (int j = 0; j < arr.length; j++) {
        System.out.print(arr[i][j]+" ");
    }
    System.out.println();
}

That would start from the bottom row and print out its contents from the first to last column, then move up to the second to last row, etc. all the way up to the first (top) row. So there is no inherent directionality in arrays at all, but because we are printing in the opposite order from the one used in the value assignment, the values will appear to be printed from the "bottom up." If you wanted to also invert the columns, printing the last one first, you could do this:

for (int i = arr.length - 1; i >= 0; i--) {
    for (int j = arr.length - 1; j >= 0; j--) {
        System.out.print(arr[i][j]+" ");
    }
    System.out.println();
}

I hope that explanation was clear. Best of luck~!

PS. As there is no inherent directionality in arrays, you should not necessarily be storing the values in the direction you want to print them. You should store the values in the way that makes the most sense to you and in the way that makes them easy to manage. Then whenever you need to print the values, you can print them however you like.

Answer 2

Try this code, but I see that you are filling the array with empty spaces, how can you realize that your code is not working good try for example filling numbers to test your code.

for(int row = arrayName.length -1; row >= 0; row--){
    for(int col = arrayName.length - 1; row >= 0; col--){
       arrayName[row][col] = col;
    }
}