1. Arrays and Strings

2. Why? Consider a class of 30 students, each has a score for hw1  Do we want to have 30 variables with different names?  Would it be easier if we have only one name for all 30 scores? We can create structures that are composed of multiple items.

3. Arrays A structure that allows multiple items of the same type Declaring an array of 30 integers  Allocating space in the memory int[] hw1Scores = new int[30]; Declaring an array of 30 doubles double[] gpa = new double[30]; Generally type[] name = new type[size];

4. Accessing an element in an array Each element is identified by an “index”  First element has an index of 0  Last element has an index of (size – 1), eg. 29 in hw1Scores 0 1 2 3 4 5 … 28 29 6 th element in hw1Scores  hw1Scores[5] 89 78 2377 … 0 567897

5. Use and modify an array element hw1Scores[5] = 70; hw1Scores[5] = hw1Scores[5] + 2; System.out.print(hw1Scores[5]);

6. Adding all the scores to find average

7. int sum = hw1Scores[0]; for (int i = 1; i < 30; i++) { sum = sum + hw1Scores[i]; } double average = sum / 30.0;

8. Adding all the scores to find average int sum = hw1Scores[0]; for (int i = 1; i < hw1Scores.length; i++) { sum = sum + hw1Scores[i]; } double average = sum / (double)hw1Scores.length;

9. 2-Dimensional Arrays Similar to a table (matrix), with rows and columns 30 students (rows), 10 scores (columns)  Declaring 30 x 10 integer array int[][] scores = new int[30][10]; Generally type[][] name = new type[rows][columns];

10. Accessing 2D Array Elements Similar to 1D arrays  Index starts at 0 0 1 2 3 4 5 6 7 …. 0 1 2 3 4 5 … ?

11. Accessing 2D Array Elements Similar to 1D arrays  Index starts at 0 0 1 2 3 4 5 6 7 …. 0 1 2 3 4 5 … scores[4][5] -- ?th student, ?th hw ?

12. Average of the hw2 scores? What are the indexes?  Scores[?][?] 0 1 2 3 4 5 6 7 …. 0 1 2 3 4 5 …

13. hw2 average int sum = scores[0][1]; for (int row = 1; row < scores.length; row++) { sum = sum + scores[row][1]; } double average = sum / (double)scores.length;

14. Average score for the 4 th student? What are the indexes?  Scores[?][?] 0 1 2 3 4 5 6 7 …. 0 1 2 3 4 5 …

15. Average of the 4 th student int sum = scores[3][0]; for (int col = 1; col < scores[3].length; col++) { sum = sum + scores[3][col]; } double average = sum / (double)scores[3].length;

16. Average of the 4 th student int sum = scores[3][0]; for (int col = 1; col < scores[0].length; col++) { sum = sum + scores[3][col]; } double average = sum / (double)scores[0].length; Same number of columns in all rows

17. Strings Multiple characters Delimited by double quotes  ”Hello, my name is Mickey Mouse.” Declaration (allocating space in memory)  String studentName; // note capital S Initialization  String studentName = ”John Smith”;

18. String Operaters +  Concatenation—combining two strings  String name = firstName + ” ” + lastName;

19. String Operations/Methods equals()  yields boolean (true/false)  generally more appropriate than ==  studentName.equals(personName) equalsIgnoreCase()  Not case sensitive  String name1 = ”John”, name2 = ”john”;  name1.equalsIgnoreCase(name2) // yields true charAt(i)  yields a character at index i  name1.charAt(3) // yields n indexOf(str)  yields the index of where str starts  name1.indexOf(”oh”) // yields 1