CP122 CS I. Chapter 7a: Java Arrays

Transcription

1 CP122 CS I Chapter 7a: Java Arrays

2 Better Online Ticket Sales (BOTS) act clears congress Tech News!

3 Tech News! Better Online Ticket Sales (BOTS) act clears congress My Friend Cayla: Data collection privacy issues

4 Hackerz Tip of the Day: User Authentication Username / Password

5 Hackerz Tip of the Day: User Authentication Password Password Hash Hashing function

6 Hackerz Tip of the Day: User Authentication secret SDLKJVOUAY Hashing function

7 Hackerz Tip of the Day: User Authentication SDLKJVOUAY secret?

8 Hackerz Tip of the Day: User Authentication aardvark abacus secret ALKJLIDSFL URYJDFOLC SDLKJVOUAY

9 Hackerz Tip of the Day: User Authentication aardvark abacus secret ALKJLIDSFL URYJDFOLC SDLKJVOUAY A match!

10 Hackerz Tip of the Day: User Authentication Obtain hashed password, p for each word in dictionary: if hash(word) == p: print password is + word

11 Chapter 7a: Arrays How can we store a bunch of data in a list? What operations can be performed on Java arrays?

12 Building a Football Roster Suppose we wanted to write a program to store the salary of every player on a football team How would we do that? (FootballSalaries.java)

13 Isn't there a cleaner way to write FootballSalaries.java?

14 Java Arrays Creating a new array long[] salaries = new long[53];

15 Java Arrays Creating a new array long[] salaries = new long[53]; Data type of elements in the array.

16 Java Arrays Creating a new array long[] salaries = new long[53]; Empty square brackets denotes an array. Without the brackets, the variable would look like a normal long.

17 Java Arrays Creating a new array long[] salaries = new long[53]; Legal variable name. Same rules as regular variables.

18 Java Arrays Creating a new array long[] salaries = new long[53]; Style note: use of plural names is a good thing to remind us that there are multiple elements.

19 Java Arrays Creating a new array long[] salaries = new long[53]; Use new keyword just like you would when creating a new object.

20 Java Arrays Creating a new array long[] salaries = new long[53]; Repeated data type of elements in the array.

21 Java Arrays Creating a new array long[] salaries = new long[53]; Another set of square brackets with an integer value in between. This value represents the size of the array (number of elements).

22 Java Arrays Creating a new array long[] salaries = new long[53]; The array's size is fixed/static. You cannot change the size of the array after it has been created.

23 Java Arrays Initializing an array long[] salaries = { , };

24 Java Arrays Initializing an array long[] salaries = { , }; Instead of using new we can use curly braces and a comma-separated list of initial values.

25 Java Arrays Accessing an array element salaries[0];

26 Java Arrays Accessing an array element salaries[0]; Start with the name of the array.

27 Java Arrays Accessing an array element salaries[0]; Then use square brackets and an integer. This integer is the index of the element you want to access.

28 Java Arrays Accessing an array element salaries[0]; Arrays use an indexing scheme that starts at 0.

29 Java Arrays Accessing an array element salaries[0]; The first element in the array has index 0. This is sometimes called the zeroth element

30 Java Arrays Accessing an array element salaries[?]; The last element in an array of size n...?

31 Java Arrays Accessing an array element salaries[n-1]; The last element in an array of size n...? n-1

32 Java Arrays Accessing an array element System.out.println(salaries[0]);

33 Java Arrays Accessing an array element System.out.println(salaries[2000]); What if the size of the array is 10?

34 Java Arrays Changing the value an array element salaries[0] = ;

35 Java Arrays Changing the value an array element salaries[0] = ; Left-hand side of assignment operator is the same as array access.

36 Java Arrays Changing the value an array element salaries[0] = ; Right-hand size is new value. Can be any legal expression.

37 Java Arrays Loop Access for(int i = 0; i < salaries.length; i++) { System.out.println(salaries[i]); }

38 Java Arrays Loop Access for(int i = 0; i < salaries.length; i++) { System.out.println(salaries[i]); } For loop starts at 0 (corresponding with the 0 th element in the array).

39 Java Arrays Loop Access for(int i = 0; i < salaries.length; i++) { System.out.println(salaries[i]); } For loop continues while i is less than the length/size of the array.

40 Java Arrays Loop Access for(int i = 0; i < salaries.length; i++) { System.out.println(salaries[i]); } What is the value for i during the last execution of the loop?

41 Java Arrays Loop Access for(int i = 0; i < salaries.length; i++) { System.out.println(salaries[i]); } We can use the loop counter, i, as an index into our array.

42 Java Arrays in Memory Address Value Array Access 0x myarray[0] 0x myarray[1] 0x myarray[2] 0x myarray[3] 0x myarray[4]

43 FootballSalaries.java with arrays