`LightBoard`

is #4 from the from the 2019 AP Computer Science A Free Response problems.

https://apcentral.collegeboard.org/pdf/ap19-frq-computer-science-a.pdf?course=ap-computer-science-a

## Part (a) `LightBoard`

constructor

```
public LightBoard(int numRows, int numCols)
{
lights = new boolean[numRows][numCols];
for(int r = 0; r < lights.length; r++)
for(int c = 0; c < lights[0].length; c++)
if(Math.random() <= 0.4)
lights[r][c] = true;
}
```

Missing the initialization of `lights`

(the first line) is a common mistake. A constructor is responsible for setting the initial value of each instance variable. In the case of an array or list, making the array/list object is separate from assigning/adding values.

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

see Generating numbers with Math.random().

## Part (b) `evaluateLight`

method

```
public boolean evaluateLight(int row, int col)
{
int onInColumn = 0;
for(int r = 0; r < lights.length; r++)
if(lights[r][col])
onInColumn++;
if(lights[row][col])
{
if(onInColumn % 2 == 0)
return false;
}
else
{
if(onInColumn % 3 == 0)
return true;
}
return lights[row][col];
}
```

## 2019 AP CS Exam Free Response Solutions

- APCalendar Free Response Solution
- StepTracker Free Response Solution
- Delimiters Free Response Solution

## Additional 2D array resources

- Intro to 2D arrays
- 2D array exercises
- Mazer FR
- Droppy FR
- Flight FR
- MatrixManipulator exercise
- DeterminantFinder exercise

## Help & comments

Get help from AP CS Tutor Brandon Horn