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
89
78
23
77
56
78
…
0
97
 6th element in hw1Scores
 hw1Scores[5]
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 (int i = 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 (int i = 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
0
1
2
3
4
5
…
5
?
6
7
….
Accessing 2D Array Elements
 Similar to 1D arrays
 Index starts at 0
0 1
2 3
4
0
1
2
3
4
5
…
5
6
?
scores[4][5] -- ?th student , ?th hw
7
….
Average of the hw2 scores?
 What are the indexes?
 Scores[?][?]
0 1
2 3
4
5
0
1
2
3
4
5
…
6
7
….
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
0
1
2
3
4
5
…
6
7
….
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