The fourth question on the 2017 AP Computer Science Exam tasked you with processing a 2d `int`

matrix and using the values to fill in a 2d matrix with references of an arbitrary class, `Position`

.

### Part A

In the first part you were to implement a method called `findPosition`

that went through a 2d array of `int`

values and return a new `Position`

reference based on where a value is found.

```
public static Position findPosition( int num, int[][] intArr ) {
for ( int r = 0; r < intArr.length; r++ ) {
for ( int c = 0; c < intArr[ r ].length; c++ ) {
if ( intArr[ r ][ c ] == num ) {
return new Position( r, c );
}
}
}
return null;
}
```

In general, if you’re given an array to process on the AP exam you’re going to want to iterate through each value. In this case we need a nested loop to go through rows and columns.

We’ll then compare the value at each cell with the value `num`

we’re looking for. When it’s found we want to create and return a new `Position`

object based on the row and column where `num`

was found.

If we get all the way through `intArr`

then we’ll return `null`

.

### Part B

Part B expects you to call the `findPosition`

method that you created in Part A to build a 2d matrix of `Position`

objects.

```
public static Position[][] getSuccessorArray( int[][] intArr ) {
Position[][] out = new Position[ intArr.length ][ intArr[ 0 ].length ];
for ( int r = 0; r < intArr.length; r++ ) {
for ( int c = 0; c < intArr[ r ].length; c++ ) {
out[ r ][ c ] = findPosition(intArr[r][c] + 1, intArr);
}
}
return out;
}
```

For this we’re going to create a new `Position`

matrix that’s the same size as the source matrix `intArr`

. We’ll return `out`

at the end.

And like Part A we’ve got a 2d array to process. So we’ll go through each cell using nested loops.

The problem description tells you that at each element in the matrix you should fill in a `Position`

object matching the row and column where you could find the value that’s one more than the cell you’re at. The PDF from College Board for this problem has a diagram that would help way more than I can do with text.

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