Binary Files Ch 11.3

Transcription

1 Binary Files Ch 11.3

2 Announcements Please only use one Scanner variable in homework (makes our lives easier) Last lab next week, please show up: - Extra credit points - Short lab

3 Random file access So far, we have been... - starting at the beginning of the file - can only move forward (one object at a time) Random file access allows us to skip different places in a file, along with the capability to both read and write

4 Random file access Opening: The second argument to the constructor is your file permissions (write/read): r = read only w = write only rw = read and write

5 Random file access Random access is done by storing a file pointer (much like an array index, starting at 0) The file pointer starts at the beginning of the file (be careful not to write over old data) New methods with random access: seek() - moves to that spot in the file length() - length of the file getfilepointer() - where you are in the file

6 Random file access Unfortunately, RandomAccessFile class does not have readobject() and writeobject() The file pointer is annoying because it is the actual bytes, instead of index For example: an int is four bytes, so if you write 2 ints, the first one starts at spot 0 and the second one at spot 4 (See: RandomAccess.java)

7 Recursion Ch 9.1

8 - recursion Highlights

9 Recursion No fancy blue words or classes this chapter Recursion is simply calling a method from inside itself This copy will re-run the method on any new arguments or information (See: BadRecursion.java)

10 Recursion If you forget your stopping case, you will not get an infinite loop but crash the program This is because every function call takes up more memory, so you constantly ask for more memory Eventually the memory (stack) cannot store anymore

11 Recursion

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13 Recursion basics Good recursion must have 2 parts: - A recursive call on a smaller problem - An ending case (see: In order to use recursion, you must be able to identify a subproblem that is very similar to the original problem Each step must get you closer to the solution

14 Recursion: In words A child couldn't sleep, so her mother told a story about a little frog, who couldn't sleep, so the frog's mother told a story about a little bear, who couldn't sleep, so bear's mother told a story about a little weasel...who fell asleep....and the little bear fell asleep;...and the little frog fell asleep;...and the child fell asleep. (See: Story.java)

15 Recursion: Fibonacci The Fibonacci numbers are defined as: F(n) = F(n-1) + F(n-2) In other words, you add the previous two to get the next This is recursion! Computing F(n) involves solving smaller problems of F(n-1) (See: FibonacciRecursion.java)

16 Recursion basics For recursion, you can basically assume your function works as you want it to (even though you have not written it) If you have the ending case and reduction step correct, then it will! Fake it until you make it

17 Recursion: Family tree Person Descendant

18 Recursion: Basic example Remember, code starts in main and runs from top to bottom in sequence (normally) When you call a function you go execute all the function's code is run before going back to the original code Code order is important in recursion! (See: StringRecursion.java)

19 Recursion: Root finding Find a root of: (see: RootFind.java) Method: 1. Find one positive y and 1 neg. y 2. Find midpoint (of x values) 3. update y-pos/neg

20 Recursion: Dictionary search Open the dictionary to the middle - If the word is not on that page, reopen in the middle of the unsearched side (See: DictionarySearch.java)

21 Recursive delete The File class can (regarding delete)... - Delete only a single file at a time - A directory must be empty in order to be deleted Write a program that would delete all files and all directories (See: RecursiveDelete.java)

22 Miscellaneous notes Try googling recursion and click on the spelling suggestion Recursion is very powerful and used in many advanced algorithms It will give you a headache for a while... =(