Convert two dimensional array to one dimensional array in Java

Question

I'm trying to convert a two-dimensional array to a one-dimensional array in Java. I cannot use any additional data structures, ArrayLists, collections etc. for temporary storage. Only one array can be allocated - the one that will be returned from the method.

There are three tests with arrays inputed into the method to validate the program. Test one has a 2D array with 3 rows and columns, test two has a 2D array with 3 rows and 1 column, and test three is empty.

My current code is:

public static int[] twoDConvert(int[][] nums) {
    int index = 0;
    int[] combined = new int[nums.length*3];
    if (nums.length == 0) return combined;  
        for (int row = 0; row < nums.length; row++) {
            for (int col = 0; col < nums.length; col++) {
                combined[index] += nums[row][col];
                index++;
        }
    }
    return combined;
}

The first test works correctly, but for some reason the 2nd test throws ArrayIndexOutOfBoundsException (Index 1 out of bounds for length 1). This occurs no matter how large or small the combined array length is. How can I fix this?

try changing to for (int col = 0; col < nums[row].length; col++) { — Scary Wombat
– Scary Wombat, Commented Mar 10, 2020 at 0:40
That got it closer, it no longer throws an exception, test 1 and 3 passes, but test 2 still fails. — Goobla
– Goobla, Commented Mar 10, 2020 at 0:42
Change line 3 and 4 to these: if (nums.length == 0) return new int[0]; int[] combined = new int[nums.length * nums[0].length]; — Glenn Sandoval
– Glenn Sandoval, Commented Mar 10, 2020 at 0:52
@GlennSandoval that would only work if all rows had the same number of columns. You would have to create a helper method that returns the number of columns in each row. — Jason
– Jason, Commented Mar 10, 2020 at 0:58
@Jason sure, I was assuming that since that's the common case but you're right. I only wanted to give Goobla some heads up where the problem might be. — Glenn Sandoval
– Glenn Sandoval, Commented Mar 10, 2020 at 1:20

Jason · Accepted Answer · 2020-03-10 01:02:08Z

3

We create a single Integer array with a length determines by the size method. Size takes the 2-dimensional array and finds the sum of all columns in each row and returns that sum. We check if the length is 0, and if so, we return. We then loop through all rows, followed by a loop through all columns in that row. We then assign the value at that row and column to that next index in the array. Unfortunately we can't do any math (to my knowledge) to determine the index based of row/column since they could be variable sizes.

    public static int[] twoDConvert(int[][] nums) {
        int[] combined = new int[size(nums)];

        if (combined.length <= 0) {
            return combined;
        }
        int index = 0;

        for (int row = 0; row < nums.length; row++) {
            for (int column = 0; column < nums[row].length; column++) {
                combined[index++] = nums[row][column];
            }
        }
        return combined;
    }

    private static int size(int[][] values) {
        int size = 0;

        for (int index = 0; index < values.length; index++) {
            size += values[index].length;
        }
        return size;
    }

edited Mar 10, 2020 at 1:02

answered Mar 10, 2020 at 0:53

Jason

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2 Comments

Jason Over a year ago

It literally only creates a single array, the array we're returning. I made sure of it. The size method doesn't create a new array, it loops through the existing 2D array and finds the sum of all columns in each row.

Goobla Over a year ago

I deleted that comment because I realized that a data collection wasn't created, only a variable holding a integer value, sorry for that.

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