Java Programming. Computer Science 112

Transcription

1 Java Programming Computer Science 112

2 Yay Programming! How did the RPS program go? Did you notice the part where I pointed at the answer on the board that we did together? Biggest problems in NumbersAndMath Incorrect math 60*60*24*365 Missing directions Fortnights - I will always give you the answer, because there is no interpreting, unlike English Zip the src folder. C'mon. Matters in real life; automated grading Have I mentioned the Fiver Plan?

3 The Fiver Plan Sometimes you just can't get to a homework Family thing Work thing Hangover thing But the cumulative nature of learning programming means that you can prove retroactively that you do actually know the material. So if you ever get a 10 on a program, all past zeroes that are encompassed by that program become 5's.

4 Recap: If The diamond symbol in the flow charts is an "if" statement. A decision to be made. Convert the English version written out in to an "if clause" using && for and for or () for parentheses to control the order it resolves.

5 Example - Snooze Alarm again Start Clock waits for right time Alarm Rings Hit snooze button Get up yet? No Yes Get up Get ready for work End

6 It's a diamond, so it's an if... right? If one of the branches goes back up, it makes a loop. Same exact process to convert the decision in to code though. Still a "decision clause" "While I'm not ready to get up, hit snooze and wait for the alarm" while(!treadytogetup ) } // Hit snooze // Wait // Alarm rings // Check sleepiness level

7 Yay while loops! While I'm not at the door, take a step While I am not full, eat some pie While I owe you money, run while( tmoneyiowe > 0 ) // Run away }

8 When does while end? while( something true right now ) // I run forever. So something in here // better be connected to that while-clause } Otherwise this is an INFINITE LOOP, and it is BAD.

9 Infinite loops are what? They are BAD See previous slide.

10 How to prevent infinite loops? while( tmoneyiowe > 0 ) // Run away } Running away will never change the money owed, so this loop will run forever.

11 How to prevent infinite loops. while( tmoneyiowe > 0 ) // Pay a dollar instead } tmoneyiowe is going to go down now, so we'll finish eventually.

12 Examples while( tmoneyiowe > 0 ) thismoney = thismoney + 1; tmoneyiowe = tmoneyiowe - 1; } int tlinesprinted = 0; while( tlinesprinted < 5 ) System.out.println("I like pie."); tlinesprinted++; // This ++ is a shortcut for "add 1 to this". }

13 do-while Neat subtle variation "while" checks first and then does stuff "do-while" does stuff and then checks do // Give him a dollar } while (tmoneyiowe > 0); // Wrong pick if I owe 0. While makes more sense do // Print something to screen } while ( tlinesprinted < tlinesiwant ); // I know I want to print at least one

14 Grab that old worksheet Paper assignments are pretty much always going to be for things like this. Let's take those loop flowcharts and make them in to code. Roll the screen up now, me.

15 for I knoweth the number of the loop A while loop is used when stuff is going to happen and you don't know when it will end Terminating might depend on user input Might depend on an outside timer If you know exactly how many times you want to do something, that is a "for loop" It has the worst syntax ev-ar Write this next bit down I'm sorry

16 An idiom for for-loops for( int i = 0; i < tcount; i++ ) } Part three will connect to part two to prevent infinite loops in the same way as a while for( thing to do once at the start; same as a while clause - "while this is true"; what to do after the end of each loop)

17 Please don't just always do that Nothing magical about examples or idioms This slide added for that one kid last year who thought for loops had to have "i" in them for( int tpeoplecounted = 0; tpeoplecounted < 5; tpeoplecounted++ ) If initial thing is complicated, it can be outside int tpieslices = sum total of all pie eaten by all students last year; // maybe other stuff happens too for( ; tpieslices >= 0; tpieslices-- ) // Oooh, the first part is blank.

18 That one bug Eclipse keeps causing Don't put a semicolon after a while or for or if! Eclipse is just too excited to help....but you do after a do. if( tmonkeys > 5 ); //BAD for( int i = 0; i < 10; i++ ); // BAD } while( tpie >= 0 ); // BAD } do } while ( tpants == true ); // good

19 Advanced magic You have for loops and while loops and feel all powerful Take a moment to feel powerful Two advanced commands when working with loops continue = "Start the next loop right now." break = "Finish this whole loop right now." If a while loop is horribly complicated and has four ways to end you can rely on "break" as a way out

20 break and continue // Manage a list of extortion victims for( int i = 0; i < tcount; i++ ) if( this guy is broke ) continue; // No need to process this guy take his money; if( I have enough money ) break; // No need to process anyone else }

21 The console menu program "Show a menu and do stuff until they want to quit" do // Show Menu (Pick 1 or 2, or 3 to quit) // If choice 1 do stuff // If choice 2 do stuff } while ( they don't pick 3 ); Like in all programming, there are many ways to do this.

22 Double loopatron Show Menu Try again 1, 2, or 3? N Y 1 Do 1's stuff Y 2 Do 2's stuff Say goodbye I am worth copying down. I may come up later. Taking notes is fun!

23 Except "show menu" is tricky Two ways the user can mess up Enter a number that isn't a choice Enter something that isn't a number True of all user input so I'll say them again Valid but incorrect Invalid Tooootally on the test

24 Valid but incorrect Enter R P or S I enter H Enter your salary I enter -70 You followed instructions, but your input didn't make sense

25 Invalid Enter R P or S I enter 76 Enter your salary I enter "Idaho" I was expecting a certain type of input and you tried to break me.

26 Parse Yon Input I have explained the perils of nextint for getting numbers from the user. I consider Scanner nerd-functional. It technically works, but it doesn't account for how actual people use things I am going to recommend always using nextline, and then checking to see if it is a number. Law of "If it works it works" is equal to law of "If it crashes you lose points" though, so you can risk following your heart.

27 Parlez-vous Integer? String tliteralpick = tkeyboard.nextline(); int tintegerpick = Integer.parseInt(tLiteralPick); But if they enter "H", parseint will fail! Crashing is the worst thing ever. Never. Crash. "try" to parse the String or you get

28 Try-throw-catch "Please "try" to do this. If you fail, "throw" me an exception and I'll "catch" it. try } tintegerpick = Integer.parseInt(tLiteralPick); catch( NumberFormatException e )// The type is from last slide } System.out.println("That isn't even a number"); tintegerpick = -1;

29 try program }? Yay! I'll never crash? You'll just end suddenly and abruptly Doesn't count as fixed Always have your try around as few lines as possible In a complicated program you don't want to end up inside ten of them at the same time Never be happy about hitting it It should NOT be used like an if. Never intentionally hit it. It is used to guard against failings in code you can't control, like parseint. Detailed details are way later in semester.

30 Handholding time Make a program with a menu It needs some good choices and a quit choice. This program, make all choices numbers. For each choice, just print a string. Don't care what. If you change around the basic double loopatron, that is okay. But you must use as few lines as possible. No crazy triple Polish notation.

31 We have to go deeper One of your choices is now a menu itself. Ha ha. If you have been neat and organized, this is seriously a copypaste. Ctrl+A Ctrl+I are your friend. It fixes your ugly for you. Platform/Compiler dependant. Looking for "reformat" or "indent" command. It straightens out all the lines correctly. You can work on homework or go home. Please stop not turning anything in. I give 6 points for even trying.