如何在Java中从二维数组中删除占位符值？

Question

I have an int[ ][ ] a that contains both positive and negative integers. I want to copy only the positive integers into another array, int[ ][ ] result. Here's my attempt:

for( int r = 0; r < a.length; r ++ )
        {
            int aC = 0;
            for( int resC = 0; resC < result[r].length; resC++ )
            {
                while (a[r][aC] < 0)
                {
                    aC++;
                }
                result[r][resC] = a[r][aC];
                aC++;
            }
        }

When I run the program it crashes on line 6 quoting a java.lang.ArrayIndexOutOfBoundsException: 3 error. When I run the debugger I can see that it works so long as r = 0.

I feel like I'm missing something blindingly obvious but after staring at it for an hour I still can't find the problem.

Answer 1

Why have 3 loops? Just loop your 2 dimensions and copy positive values:

for (int r = 0; r < a.length; r++)
    for (int c = 0; c < a[r].length; c++)
        if (a[r][c] > 0)
            result[r][c] = a[r][c];

---

Unless of course you want to condense the values (question didn't say, but code attempt seems to indicate). Or did you also want to truncate?

  Input              Copy            Condensed        Truncated
-1  1   2           0  1  2           1  2  0           1  2
-1  -1  3    ==>    0  0  3    ==>    3  0  0    ==>    3
4   -1  5           4  0  5           4  5  0           4  5

Here is a version with truncation:

int[][] result = new int[a.length][];
for (int r = 0; r < a.length; r++) {
    int count = 0;
    for (int c = 0; c < a[r].length; c++)
        if (a[r][c] > 0)
            count++;
    result[r] = new int[count];
    for (int c = 0, cRes = 0; c < a[r].length; c++)
        if (a[r][c] > 0)
            result[r][cRes++] = a[r][c];
}

Answer 2

You have no bounds checking on

a[r][aC]

so

while (a[r][aC] < 0)

will fail.

Also as you will now have uneven sized arrays, it would be better to to add to an ArrayList, and then convert to an array afterwards if necessary.

Something like

ArrayList <Integer> posValues = new ArrayList <Integer> ();
for( int r = 0; r < a.length; r ++ )
{
        for( int c = 0; c < a[r].length; c++ )
        {
            if (a[r][c] > 0)
            {
                posValues.add (new Integer (a[r][c]));
            }
        }
 }

Answer 3

When we reach the inner for loop, we don't have any guarantees about result[r] , and so r may very well be out of bounds. Instead of this:

  for (int r = 0; r < a.length; r++) { int aC = 0; for (int resC = 0; resC < result[r].length; resC++) { 

The code will be safer like this:

    for (int r = 0; r < a.length; r++) {
        int aC = 0;
        for (int resC = 0; resC < a[r].length; resC++) {

Because, we know that a[r] will be within bounds.

But this is not nearly enough. The innermost while loop is also likely to try to access an index out of bounds, in fact it's inevitable when you have at least one negative value. Actually you need to invert the looping logic there, instead of iterating over indexes in result, it will make more sense to iterate over indexes in the source. Like this:

    for (int r = 0; r < a.length; r++) {
        int resC = 0;
        for (int aC = 0; aC < a[r].length; aC++) {
            if (a[r][aC] < 0) {
                continue;
            }
            result[r][resC++] = a[r][aC];
        }
    }

Finally, with variables renamed, it becomes a lot easier to understand what the code is trying to do:

    for (int row = 0; row < source.length; row++) {
        int targetCol = 0;
        for (int col = 0; col < source[row].length; col++) {
            if (source[row][col] < 0) {
                continue;
            }
            target[row][targetCol++] = source[row][col];
        }
    }