Mastering Arrays in Java Programming: Learn, Implement, and Practice

1. For Friday • Read 10.3-10.5 • No quiz • Program 6 due

2. Program 6 • Any questions?

3. Problem 2 • Write a program that finds the smallest of several integers. Assume that input will end when a sentinel value of –999 is read. Do not count –999 as one of the integers to consider.

4. Problem 3 • Write a program that sums a sequence of integers. Assume that the first integer read specifies the number of values remaining to be entered. Your program should read only one value at a time. A typical input sequence might be 5 100 200 300 400 500

5. Practice Problem • Write Java code to ask for a student’s exam score out of 100. Your program is then to match the exam score to a letter grade and print the grade to the screen. The letter grade is to be calculated as follows: 90 and above A 80-89 B 70-79 C 60-69 D below 60 F

6. Arrays • Sometimes we need to process lots of similar elements • We could create a variable for each element • But this can lead to very unwieldy code • Instead, we use arrays

7. What is an Array? • A sequence of elements, all of the same type. • An array has a name just like other variables. The entire array is referred to by this name. • An array is a special kind of object.

8. Examples?

9. Creating Arrays • How do we declare an array of Student objects? • How do we create an array of Student objects? • How do we put a Student object into the array?

10. Accessing a Particular Element

11. Processing the Whole Array

12. Problem 1 • Write code to print all of the Students in our array, assuming that the Student class includes an appropriate toString method.

13. Finding the One You Want

14. Problem 2 • Write a method to locate and print a particular Student, given the Student’s name. Assume that the Student class has a getName method.

15. Primitive Array Declaration • To specify that a variable is an array, we include square brackets, [], in the declaration. • int [] scores; • char [] gradeArr; • The square brackets can come after the variable name, as they do in some other languages: • int scores[]; • char gradeArr[];

16. Review • Since arrays are objects, we create them using what keyword?

17. Array Creation • In the creation, we have to specify the type and size of the array:scores = new int[5];gradeArr = new char[10];price = new double[20]; • Once the array is created, the size of the array cannot be changed.

18. Array Creation continued • We often use named constants or variables for the size in an array declaration:final int SIZE = 10;final int MAX_ELEMS = 15;int [] arr = new int[SIZE];double[] flArr = new double[MAX_ELEMS];

19. Accessing Individual Elements • Subscripts (or indices) always start at 0, so an array with 5 elements has one at 0, one at 1, one at 2, one at 3, and one at 4. • We access a particular array element by using the array name followed by the index in square brackets: score[0] arr[9]

20. Using Array Elements • All of the following are valid:score[0] = 4;score[0] += 7;score[1] = score[0] -2;score[2] = score[1] + 5 * score[0]; score[j] = score[j + 1]; • Note: index can be any integral expression.

21. Getting Data into Arrays score[0] = 30; grade[3] = ‘A’; price[2] = 10.39;

22. Array Initialization • We can put initial values into an array when we create it. • We must list all of the values:int [] num = {58, 43, 60, 21, 38};

23. Array Practice • Create an array to hold the tax for up to 10 different sales • Create an array to hold the final letter grades for a class with up to 40 students • Create an array of integers which holds the final average for those 40 students • Create an array of characters with initial values ‘a’, ‘d’, ‘y’, and ‘w’ • Assign TAX_RATE * price to the first item in your first array

24. Problem 1 • Write Java code to read values from the keyboard to fill the array scores. Input should stop when a negative number is entered. The maximum size of the array is in a constant ARR_SIZE.

25. Problem 2 • Write Java code to add up the first num_elements values in the array myVals and store the sum in the variable mySum.