Topics - Basic arrays
Coding Guidelines
- Give identifiers semantic meaning and make them easy to read (examples numStudents, grossPay, etc).
- Keep identifiers to a reasonably short length.
- Use upper case for constants. Use title case (first letter is upper case) for classes. Use lower case with uppercase word separators for all other identifiers (variables, methods, objects).
- Use tabs or spaces to indent code within blocks (code surrounded by braces). This includes classes, methods, and code associated with ifs, switches and loops. Be consistent with the number of spaces or tabs that you use to indent.
- Use white space to make your program more readable.
- Use comments after the ending brace of classes, methods, and blocks to identify to which block it belongs.
Assignment/Lab Documentation
At the beginning of each programming assignment you must have a comment block with the following information:
/*-------------------------------------------------------------------------
// AUTHOR: your name
// FILENAME: title of the source file
// SPECIFICATION: description of the program
// FOR: CSE 110- Lab #8
// TIME SPENT: how long it took you to complete the assignment
//-----------------------------------------------------------*/
Getting Started
Create a class called Lab8. Use the same setup for setting up your class and main method as you did for the previous assignments. Be sure to name your file Lab8.java.
Additionally, make another file called Arrays.java. This file will be an object, so simply start it off by declaring an Arrays class. You can copy the following skeleton and fill in the appropriate code below each of the comments:
public class Arrays {
// Instance Variables
// Constructors
// findMin
// findMax
// calcSum
// calcAverage
// toString
}
Hints
- (Page 250) An array will be used in this program.
- (Page 258) Section 6.3 contains some array algorithms used in this lab.
- The lecture and code example videos are also good references for this lab.
Task Overview
Your task for this lab is to create a class called Arrays with some array processing methods. This class will maintain an array and the number of elements present in it. Additionally, methods will be available to display the current min and max elements along with the average of all of them. Finally, a toString() method will be available to cleanly display all the array elements.
Finally, you will write a simple driver class to test out the above Arrays class.
Part 1: Instance Variables for Arrays
The first thing to do for the Arrays class is to set up its instance variables. Declare the following (private) instance variables:
- An int array called array ? this will be the array we will be writing methods for.
- An int called count - this represents the number of valid elements in the array.
Part 2: Constructors for Arrays
The Arrays class will have two constructors. The first constructor takes the maximum size of the array as input as a parameter and initializes the array instance variable appropriately. It also sets count to size. Finally, it will initialize all of the array elements to some values between 0 and 10, inclusive. To create this constructor, follow these steps:
- Import java.util.Random to make use of the random number generator.
- Create a constructor with the following header: public Arrays(int size)
- Initialize your array variable and set its size to size (see the chart on page 252 for reference on initializing arrays). Be very careful that you are setting the value of your array instance variable, as opposed to creating a new variable called array.
- Set the value of the count variable to size because we will be populating the entire array.
- Copy the following code to the constructor in order to generate random values between 0 and 10, inclusive:
Random rand = new Random();
for (int i = 0; i < count; i++) { array[i] = (rand.nextInt(10));
}
Next, create another constructor with the following header: public Arrays(int[] arr). This constructor will initialize the class by using the passed arr argument in order to fill its instance variables. The following things need to be done inside of this constructor:
- Set the array variable equal to arr.
- Set the count variable equal to the length of the array.
Part 3: Displaying the Output
findMin()
The first method of this class will search the array for the minimum element. Copy the following code for the findMin method. Note how the count instance variable is used instead of array.length. This is just in case the entire array is not being used (it will be in our case, though).
public int findMin() {
int min = array[0]; // Set min to the first element for (int i = 1; i < count; i++) {
// Reassign min if there is a smaller element if (array[i] < min) {
min = array[i];
}
}
return min; // Return the smallest element
}
findMax()
Using the above code as reference, write a method which finds the maximum element within the array. You can refer to page 259 if you are stuck.
calcSum()
The calcSum() method will be a private method (for clarity, it is the only private method in this class). It will be used later on within the class as a helper method, but never outside of the class. This method will return the sum of all of the elements in the array. You can use the following steps as guidelines for how to complete this method, if you choose:
- Use the following as the header: private int calcSum()
- Declare an int variable for the sum - initialize it to 0
- Write a for loop to iterate through all the elements in the array (remember to use the count instance variable instead of the length of the array).
- Add the value of each element to the sum.
- Return the sum after the for loop You can refer to page 259 if you are stuck.
calcAverage()
This method will return the average of all of the elements in the array. Use the following for the header of this method: public double calcAverage(). IMPORTANT: Use the calcSum() private method in your computation of the average for full credit. Hints:
- You will need to make use of the total elements in the array.
- You will need to cast the result to a double at the appropriate time to achieve an accurate average.
toString()
The toString() method is called whenever an object is passed into a print statement. This particular
toString() method will print the following, assuming the array consists of elements {1, 2, 3, 4}:
[ 1, 2, 3, 4 ]
Copy the following code to use for this method:
public String toString() { String output = "[ ";
for (int i = 0; i < count; i++) { output += array[i];
if (i != count - 1) {
output += ", ";
}
}
return output + " ]";
}
Part 4: Test Class for Arrays
At this point, the Arrays class is completed. The next step is to create a driver class to test it. This is the Lab8.java file that you created at the beginning of the lab. Copy the following code into the main method of the Lab8.java file. This code tests the first constructor of the Arrays class along with all its methods.
// Create an Arrays object using the first constructor Arrays arr1 = new Arrays(5);
// Print the contents of the array in arr1 System.out.println(arr1);
// Call findMin, findMax, and calcAverage on arr1 and print their values System.out.println("Min: " + arr1.findMin());
System.out.println("Max: " + arr1.findMax()); System.out.println("Average: " + arr1.calcAverage()); System.out.println();
The next step is to add code which tests the second constructor of the Arrays class. To do this, complete the following tasks by adding code to the end of the code you just copied into the main method.
- Create an int array of length 3 - explicitly set its values to any 3 ints by using an array initialization list. See the chart on the bottom of page 252 for a reference on how to set an array with initial values.
- Create an Arrays object using the second constructor. Note that this involves passing the array
variable you just created. Call this object arr2.
- Print the arr2 object by passing it into a println statement.
- Print the min, max, and average of the arr2 object, as what was done with the arr1 object.
Sample Output
Below is an example of what your output should roughly look like when this lab is completed. Please note that your values will almost certainly be different, depending both on the random number generator and the values of the array you created in Lab8.java. The following run initialized its array in the arr2 object to contain {1, 2, 3}.
Sample Run:
[2, 1, 8, 4, 4]
Min: 1
Max: 8
Average: 3.8
[1, 2, 3]
Min: 1
Max: 3
Average: 2.0