The `HorseBarn`

problem from the 2012 AP Computer Science Exam is typical of free response problems that test arrays.

`HorseBarn`

is #3 from the 2012 AP Computer Science A Free Response.

https://secure-media.collegeboard.org/apc/ap_frq_computerscience_12.pdf

## Part (a) `findHorseSpace`

method

```
public int findHorseSpace(String name)
{
for(int i = 0; i < spaces.length; i++)
{
Horse h = spaces[i];
if(h != null && name.equals(h.getName()))
return i;
}
return -1;
}
```

## Part (b) `consolidate`

method

```
public void consolidate()
{
int nextSpace = 0;
for(int i = 0; i < spaces.length; i++)
{
if(spaces[i] != null)
{
spaces[nextSpace] = spaces[i];
nextSpace++;
}
}
for(int i = nextSpace; i < spaces.length; i++)
spaces[i] = null;
}
```

See Consolidate an array for an explanation of this algorithm and additional related resources.

## Part (b) `consolidate`

method (alternate solution 1)

```
public void consolidate()
{
int nextSpace = 0;
for (int i = 0; i < spaces.length; i++)
{
if (spaces[i] != null)
{
spaces[nextSpace] = spaces[i];
if(i != nextSpace)
spaces[i] = null;
nextSpace++;
}
}
}
```

It is possible to set each space from which a horse was moved to `null`

during the initial traversal. The condition `i != nextSpace`

ensures that each horse that remained in its original position is not set to `null`

.

## Part (b) `consolidate`

method (alternate solution 2)

```
public void consolidate()
{
Horse[] newSpaces = new Horse[spaces.length];
int newIndex = 0;
for(int oldIndex = 0; oldIndex < spaces.length; oldIndex++)
{
if(spaces[oldIndex] != null)
{
newSpaces[newIndex] = spaces[oldIndex];
newIndex++;
}
}
spaces = newSpaces;
}
```

This approach makes a new array instead of moving the elements within the existing array. The algorithm is the same. This approach does not require a second loop, since all unused positions in the new array will already be `null`

. The instance variable `spaces`

must be updated to refer to the new array.

This approach would not work if the array to be modified was a parameter (instead of an instance variable).

## Part (b) `consolidate`

method (alternate solution 3)

```
public void consolidate()
{
for(int i = 0; i < spaces.length; i++)
{
if(spaces[i] == null)
{
int nextHorse = i + 1;
while(nextHorse < spaces.length && spaces[nextHorse] == null)
nextHorse++;
if(nextHorse < spaces.length)
{
spaces[i] = spaces[nextHorse];
spaces[nextHorse] = null;
}
}
}
}
```

The problem can also be solved by identifying each `null`

position then finding the `Horse`

that belongs there, if any. This approach is unnecessarily complex and error prone.

## 2012 AP CS Exam Free Response Solutions

- ClimbingClub Free Response Solution
- RetroBug Free Response Solution
- GrayImage Free Response Solution

## Help & comments

Get help from AP CS Tutor Brandon Horn