Arrays and Strings

Arrays and Strings

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.

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];

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 • 6th element in hw1Scores • hw1Scores[5] 89 23 77 56 78 97 78 … 0

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

Adding all the scores to find average

Adding all the scores to find average int sum = hw1Scores[0]; for (inti = 1; i < 30; i++) { sum = sum + hw1Scores[i]; } double average = sum / 30.0;

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

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];

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 …

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 , ?thhw

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

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;

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

Average of the 4th 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;

Average of the 4th 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

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”;

String Operaters • + • Concatenation—combining two strings • String name = firstName + ” ” + lastName;

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

Arrays and Strings