RouteCipher free response answer 2011

The RouteCipher problem from the 2011 AP Computer Science Exam is typical of free response problems that test 2 dimensional arrays. The problem requires you to traverse the array in row major order, which is most easily accomplished with nested for loops.

RouteCipher is #4 from the from the 2011 AP Computer Science A Free Response problems.

https://secure-media.collegeboard.org/apc/ap11_frq_comp_sci_a.pdf

Part (a) fillBlock method

private void fillBlock(String str)
{
    int strIndex = 0;

    for (int r = 0; r < numRows; r++)
    {
        for (int c = 0; c < numCols; c++)
        {
            if (strIndex >= str.length())
                letterBlock[r][c] = "A";
            else
                letterBlock[r][c] = str.substring(strIndex, strIndex + 1);

            strIndex++;
        }
    }
}

See 2D array exercises for additional traversals and other challenges.

Part (a) fillBlock method (alternate solution)

private void fillBlock(String str)
{
    for (int i = 0; i < numRows * numCols; i++)
    {
        if (i >= str.length())
            letterBlock[i / numCols][i % numCols] = "A";
        else
            letterBlock[i / numCols][i % numCols] = str.substring(i, i + 1);
    }
}

Part (b) encryptMessage method

public String encryptMessage(String message)
{
    String cipherText = "";
    int mIndex = 0;

    while(mIndex < message.length())
    {
        fillBlock(message.substring(mIndex));  // ignores extra characters
        cipherText += encryptBlock();
        mIndex += numRows * numCols;
    }

    return cipherText;
}

Part (b) encryptMessage method (alternate solution 1)

public String encryptMessage(String message)
{
    String cipherText = "";
    
    String mRemaining = message;

    while (mRemaining.length() > 0)
    {
        fillBlock(mRemaining); // ignores extra characters
        cipherText += encryptBlock();
        if(mRemaining.length() > numRows * numCols)
            mRemaining = mRemaining.substring(numRows * numCols);
        else
            mRemaining = "";
    }

    return cipherText;
}

Part (b) encryptMessage method (alternate solution 2)

public String encryptMessage(String message)
{
    String cipherText = "";

    for(int i = 0; i < message.length(); i += numRows * numCols)
    {
        fillBlock(message.substring(i));
        cipherText += encryptBlock();
    }

    return cipherText;
}

This is the same as the first solution except with a for loop.

2011 AP CS Exam Free Response Solutions

Additional resources for 2D arrays

Help & comments

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