The `TokenPass`

problem from the 2013 AP Computer Science Exam is typical of free response problems that test arrays.

`TokenPass`

is #2 from the from the 2013 AP Computer Science A Free Response problems.

https://secure-media.collegeboard.org/digitalServices/pdf/ap/apcentral/ap13_frq_comp_sci.pdf

## Part (a) `TokenPass`

constructor

```
public TokenPass(int playerCount)
{
board = new int[playerCount];
for(int i = 0; i < board.length; i++)
board[i] = (int) (Math.random() * 10) + 1;
currentPlayer = (int) (Math.random() * board.length);
}
```

A regular for loop through `board`

is necessary for initialization of each element. A foreach loop won’t work since changes will not affect the array itself.

`Math.random`

returns a value in the range of `0.0 <= board[i] < 1.0`

. The steps to convert this to the desired range of `1 <= board[i] <= 10`

are below.

Statement | Range |
---|---|

`board[i] = Math.random();` |
`0.0 <= board[i] < 1.0` |

`board[i] = Math.random() * 10;` |
`0.0 <= board[i] < 10.0` |

`board[i] = (int) (Math.random() * 10);` |
`0 <= board[i] <= 9` |

`board[i] = (int) (Math.random() * 10) + 1;` |
`1 <= board[i] <= 10` |

`currentPlayer`

must be set to a random valid index in `board`

. This is a very common task that can be accomplished by multiplying the return value of `Math.random`

by the length/size of the array/list and casting the result to an `int`

.

For more on working with `Math.random()`

see Generating numbers with Math.random().

## Part (b) `distributeCurrentPlayerTokens`

method

```
public void distributeCurrentPlayerTokens()
{
int tokens = board[currentPlayer];
board[currentPlayer] = 0;
int targetIndex = currentPlayer + 1;
while(tokens > 0)
{
if(targetIndex == board.length)
targetIndex = 0;
board[targetIndex]++;
tokens--;
targetIndex++;
}
}
```

This problem requires traversal of an array with modification of each element, which precludes a foreach loop. This solution loops while there is at least one token remaining to distribute. A for loop that runs for the number of tokens is also possible.

There are 2 common approaches to ensuring that `targetIndex`

remains a valid index in `board`

. The solution above explicitly checks for validity and resets `targetIndex`

to 0 when it exceeds the array bounds.

An alternative is to use modular division (`%`

) to constrain `targetIndex`

. See Division operations for more discussion of using `%`

to constrain values.

It is important that the original value of `board[currentPlayer]`

is used as the number of tokens to distribute. `board[currentPlayer]`

can be modified during token distriution.

## Part (b) `distributeCurrentPlayerTokens`

method (alternate solution)

```
public void distributeCurrentPlayerTokens()
{
int tokens = board[currentPlayer];
board[currentPlayer] = 0;
int targetIndex = currentPlayer + 1;
for(int t = 1; t <= tokens; t++)
{
board[targetIndex % board.length]++;
targetIndex++;
}
}
```

## 2013 AP CS Exam Free Response Solutions

- MusicDownloads Free Response Solution
- GridWorldUtilities & JumpingCritter Free Response Solution
- Skyview Free Response Solution

## Help & comments

Get help from AP CS Tutor Brandon Horn