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class: center, middle, title title: Arrays

Arrays

Processing Related Values

Task: Send an email to each student

Dear {{name}},

You are awesome! Welcome to class.

Best,
Tech Elevator

Given a function that performs the actual send:

  • void sendEmail(String name)

Solution 1: variables for everyone

String ben = "Ben Haskell";
String jack = "Jack Skellington";
String leela = "Turanga Leela";

sendEmail(ben);
sendEmail(jack);
sendEmail(leela);

Solution 2: numbered variables

String student1 = "Ben Haskell";
String student2 = "Jack Skellington";
String student3 = "Turanga Leela";

sendEmail(student1);
sendEmail(student2);
sendEmail(student3);

Actual solution: Arrays

String[] names = new String[3];
names[0] = "Ben Haskell";
names[1] = "Jack Skellington";
names[2] = "Turanga Leela";

for (String name : names) {
  sendEmail(name);
}

Array basics

  • An array holds an ordered list of elements
  • Each element of an array has the same type
  • Arrays have a fixed length that cannot be changed

???

  • Other data structures allow you to change some of these constraints

Array variables

  • Declaring an array variable by appending [] to the name
String names[];
int scores[];
double sum, readings[];
  • Often initialized at the same time:
String names[] = new String[3];
int scores[] = new int[1000];
double sum, readings[] = new double[10];

???

  • can be interspersed with non-array

Accessing array elements

  • Arrays are indexed from 0
  • Put the index between square brackets after the variable name
String[] names = new String[3];

// Set values of elements:

// Note: 0, 1, 2 (not 1, 2, 3)
names[0] = "Ben Haskell";
names[1] = "Jack Skellington";
names[2] = "Turanga Leela";

// Index doesn't have to be a constant
for (int i = 0; i < names.length; i++) {
  sendEmail(names[i]);
}

???

  • also called "offset", since it's the distance from the start

Array initializer

  • Entire array can be initialized with special syntax
String[] names = new String[3];
names[0] = "Ben Haskell";
names[1] = "Jack Skellington";
names[2] = "Turanga Leela";

// Equivalent:
String[] names = {"Ben Haskell", "Jack Skellington", "Turanga Leela"};
int scores[] = new int[3];
scores[0] = 20;
scores[1] = 30;
scores[2] = 40;

// Equivalent:
int scores[] = {20, 30, 40};

Arrays are reference types

  • If you pass an array to a function, it can alter its contents
void modifyArray(String[] array, int i, String newValue) {
  array[i] = newValue;
}

// ...
String[] letters = {"Alpha", "Bravo", "Charlie"};
modifyArray(letters, 2, "Chaplin");
System.out.println(letters[2]);
// => Chaplin