Part (a)

Write the fillBlock method that fills a 2D array with characters from a string.

Solution

private void fillBlock(String str)
{
    int index = 0;
    for (int r = 0; r < numRows; r++)
    {
        for (int c = 0; c < numCols; c++)
        {
            if (index < str.length())
            {
                letterBlock[r][c] = str.substring(index, index + 1);
            }
            else
            {
                letterBlock[r][c] = "A";
            }
            index++;
        }
    }
}

Part (b)

Write the encryptMessage method that encrypts using column-based reading.

Solution

public String encryptMessage(String message)
{
    String result = "";
    fillBlock(message);
    
    for (int c = 0; c < numCols; c++)
    {
        for (int r = 0; r < numRows; r++)
        {
            result += letterBlock[r][c];
        }
    }
    return result;
}

Key Concepts

Common Mistakes