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Provided Code

public class Data
{
    public static final int MAX = /* value not shown */;
    private int[][] grid;

    /** Fills all elements of grid with randomly generated values.
     *  Each value: between 1 and MAX inclusive, divisible by 10, NOT divisible by 100.
     *  Precondition: grid is not null, grid has at least one element. */
    public void repopulate() { /* to be implemented in part (a) */ }

    /** Returns the number of columns in grid that are in increasing order.
     *  A column is in increasing order if each row's element >= previous row's element.
     *  Precondition: grid is not null, grid has at least one element. */
    public int countIncreasingCols() { /* to be implemented in part (b) */ }
}

Part A — 4 Points

Write repopulate, which fills every element of grid with a randomly generated value between 1 and MAX inclusive, divisible by 10, and not divisible by 100. All valid values must have equal probability of being chosen.

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public void repopulate()
{
    for (int r = 0; r < grid.length; r++)
    {
        for (int c = 0; c < grid[0].length; c++)
        {
            int val;
            do
            {
                val = (int)(Math.random() * MAX) + 1;
            } while (val % 10 != 0 || val % 100 == 0);
            grid[r][c] = val;
        }
    }
}

Part B — 5 Points

Write countIncreasingCols, which returns the number of columns in grid where each element in each row after the first is >= the element in the previous row.

Write Your Solution

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public int countIncreasingCols()
{
    int count = 0;
    for (int c = 0; c < grid[0].length; c++)
    {
        boolean increasing = true;
        for (int r = 1; r < grid.length; r++)
        {
            if (grid[r][c] < grid[r-1][c])
            {
                increasing = false;
            }
        }
        if (increasing)
        {
            count++;
        }
    }
    return count;
}

Common Mistakes to Avoid

for (int r = 0; r < grid.length; r++) {
    if (grid[r][c] < grid[r-1][c])  // ArrayIndexOutOfBounds when r=0!

for (int r = 1; r < grid.length; r++) {  // start at 1
    if (grid[r][c] < grid[r-1][c])

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