Introduction to Arrays an AP Computer Science A Topic
Consider the following. You are tracking the scores in a bowling game. In bowling, there are ten rounds. Results from individual rounds must be preserved, since sometimes the results of one round influence the results of the next, like in the case of bowling a strike (knocking down all 10 pins in one roll) or a spare (knocking down all pins in two rolls).
To solve this, we could use a series of variables. Suppose Round is a class to hold the values in each round of the bowling game:
Round round1;
Round round2;
...
Round round10;
As an alternative, you could store these values in one data structure. There are several types of data structures, in this tutorial, we will explore using an array. An array is a type of data structure that has a fixed length. All the data stored in an array are of the same type.
How Data is Stored?
Each value stored in an array is called an element. The size of the array is set when it is declared and created. This size does not change. So if more elements are needed a new array would need to be created, the current elements copied over, and the new elements added. Alternatively, you can use another stucture, like an ArrayList.
Similar to the characters in a String, each element in an array has an index. The first index is 0 and the last is the size of the array - 1.
Constructing an Array
When constructing an array object, we must specify the type of data that is stored. An array can store either primitive or reference data.
<type of data>[] nameOfArray = new <type of data>[<number of elements>];
An example of an array of rounds for bowling using the Round class would be as follows:
Round[] bowlingCard = new Round[10];
Initial Values of an Array
When we create an array, some initial values are stored in the array.
null - if the type of values are reference, null is stored in the elements to indicate that there is no object assigned to these elements. 0 is assigned to the elements if the type is int.0.0 is assigned to the elements if the type is double.\000 is assigned to the elements if the type is char.false is assigned to the elements if the type is boolean.
Your Turn
Let's try it in the Java Playground.
- Predict the values stored in
testArray.
- Run the code to determine if your prediction is correct. In the Java Playground, it will show you what the line of code does even though no output statement is written.
- Change the type of the array to each of the following, predict the code, and run the code to check your prediction.
Stringdoublecharboolean
Constructing an Array with Initial Values
We can also create an array and provide all the initial values. One way we can do this is by using an initializer list. This is good for when you have a shorter list of known values for an array. The values are listed in braces ({ }) and the array is assigned this list.
<type of data> [] nameOfArray = {<values separated by a comma>};
Your Turn
Let's try it in the Java Playground.
- Predict the values stored in
grades.
- Run the code to determine if your prediction is correct. In the Java Playground, it will show you what the line of code does even though no output statement is written.
- For each of the following, write the code, predict the output, and then run it to check your prediction:
- Add a new array called
answerOptions and assign it the values: a, b, c, d
- Add a new array called
weighIn and assign it the values: 10.7, 12.0, 12.2, 12.5, 13.2
Determine the Length of an Array
You can access the constant length to access the number of elements in an array.
int[] grades = {80, 70, 90, 95};
int numElements = grades.length;
Your Turn
Let's try it in the Java Playground.
- Predict the output.
- Run the code to determine if your prediction is correct.
- Change the initializer list by adding two more grades:
100 and 78
- Predict the output and then run the code to determine if your prediction is correct.
Accessing Elements of an Array
To access the elements of an array, use square brackets ([ ]) and include the index value in the square brackets. If we want to access the first element of an array of grades, it would look like this:
int[] grades = {80, 70, 90, 95};
int firstGrade = grades[0];
Your Turn
Let's try it in the Java Playground.
- Predict the output.
- Run the code to determine if your prediction is correct.
- Add a new variable
lastGrade and assign it the last value in the array. Use the length to determine the index value.
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