# Unit 4A - Free Response - Sound B

Each coding exercise has its own folder. Open its `.java` file and press **Run** to try it. **Run Tests** checks every graded exercise in this lesson at once.

The following is a free response question from 2011. It was question 1 on the exam. You can see all the free response questions from past exams at [Past AP CSA Exams](https://www.savemyexams.com/ap/computer-science/college-board/a/past-papers/).

**Question 1.** Digital sounds can be represented as an array of integer values. For this question, you will write two unrelated methods of the *Sound* class.

A partial declaration of the `Sound` class is shown below.

```java
public class Sound
{
 /** the array of values in this sound; guaranteed not to be null */
 private int[] samples;

 /** Changes those values in this sound that have an amplitude
  *  greater than limit */
  *  Values greater than limit are changed to limit.
  *  @param limit the amplitude limit
  *         Precondition: limit >= 0
  *  @return the number of values in this sound that this
  *    method changed
  */
 public int limitAmplitude(int limit)
 { /* to be implemented in part (a) */ }

 /** Removes all silence from the beginning of this sound.
  *  Silence is represented by a value of 0.
  *  Precondition: samples contains at least one nonzero value
  *  Postcondition: the length of samples reflects the
  *                  removal of starting silence
  */
 public void trimSilenceFromBeginning()
 { /* to be implemented in part (b) */ }

 // There may be instance variables, constructors, and methods
 // that are not shown.
}
```

**Part b.** Recorded sound often begins with silence. Silence in a sound is represented by a value of 0.

Write the method `trimSilenceFromBeginning` that removes the silence from the beginning of a sound. To remove starting silence, a new array of values is created that contains the same values as the original `samples` array in the same order but without the leading zeros. The instance variable `samples` is updated to refer to the new array. For example, suppose the instance variable `samples` refers to the following array.

![Sound Table3](images/soundTable3.png)

After `trimSilenceFromBeginning` has been called, the instance variable `samples` will refer to the following array.

![Sound Table4](images/soundTable4.png)

## How to Solve This

1. You will need to loop through each element in the array until you reach a non-zero element. You will also need to keep track of the number of leading zeros.
2. Remember that you must replace the samples array with a new array without the leading zeros. How do you create an array of a particular size?

> Check your understanding: Which loop would be best for this problem? — answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/soundB.html#fr_soundb_1

> Check your understanding: Which is the correct code for creating an integer array variable named `samples2` and setting it to refer to an array of integers of size `count`? — answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/soundB.html#fr_soundb_2

## Mixed Up Code

> Check your understanding: The method `trimSilenceFromBeginning` below contains correct code for one solution to this problem, but it is mixed up. — answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/soundB.html#SoundB

## Try and Solve Part B

### Exercise 1

FRQ Sound B: Finish writing the method `trimSilenceFromBeginning` that removes the silence from the beginning of a sound. To remove starting silence, a new array of values is created that contains the same values as the original `samples` array in the same order but without the leading zeros. The instance variable `samples` is updated to refer to the new array.

> Open the `01-FRQSoundB` folder and edit `Sound.java`. Press **Run** to try it; **Run Tests** checks it (2 tests).