Programming with C#: Understanding Data Structuring and Expression Evaluation

1. Unit 8 • Introduced two of the three key aspects in any programming language • structuring of data • statements and control flow • Final key area • data processing • how are new values created from old? • Using additional features of Visual Studio • to understand clearly how all this fits together Learning to Program with C# - 8

2. Key aspect of object-orientation • structuring of the problem solution according to the data inherent in the model • in the whole rocket project, the smallest data values are • arena size, obstacles, obstacle size, obstacle position, rocket position, thrust level, velocity, distance travelled in last time period, acceleration due to gravity, rocket attitude, sky colour, ground colour • object model allows these values to be stored in clumps – e.g. rocket values together • along with the operations over those values – the methods Learning to Program with C# - 8

3. But this is computing • The whole point is that we compute new values from old • It is all these individual little values that must be updated for the model to change over time, and be interesting • the rocket velocity for example changes over time, and as adjustments are made through the flight controls • Time to see how this computing takes place Learning to Program with C# - 8

4. Expressions • In order to compute new values during the execution of a program • we must write down the description of that computation when we write the program • these descriptions are called expressions, and get combined together with the textual descriptions for statements etc. • it is very valuable to keep track of these different grammatical categories • Expressions look quite mathematical • unsurprisingly, since they are a form of algebra • they can operate over numbers, but also the wide range of other values/objects available in the programming language Learning to Program with C# - 8

5. Of what do expressions consist of? • Three primary components may appear: • Identifiers • these are names for stored values • we won't know exactly what the value is until the program runs • Literal values • these are values known about at the time we write the expression e.g. 1, 7.3, true • Operators • these are the actual functions that take existing values and produce new values • familiar examples are + and – from mathematics • they have just the same meaning here • many others also Learning to Program with C# - 8

6. One crucial new statement • The assignment statement • once new values are computed, we need to store them often, for use later • a new value is assigned to a storage location • the location has a name – an identifier • the data members of an object are storage locations • First the expression is evaluated to get our new value, then this is stored into the location identified by the location identifier • The = sign is NOT a test for equality, but signifies assignment LocationIdentifier=Expression; Learning to Program with C# - 8

7. Let's go on an expression hunt… • In the AdvancedRocketry solution • use Object Browser to get to the code for the EnginesOn/EnginesOff methods of Rocket • manipulate theEngineThrust location • setting it to an initialvalue, or to zero,respectively • the expressions are assimple as they come – just a single literal value • remember, expression is the bit on the right of the = public void EnginesOn() { // 10.0 is the initial value EngineThrust = 10.0; } public void EnginesOff() { EngineThrust = 0.0; } Learning to Program with C# - 8

8. Now find IncThrustLevel… public void IncThrustLevel( int level ) { EngineThrust = EngineThrust + (double)level; } • Still EngineThrust that is being updated • but by a more complicated expression • EngineThrust and level are being added together – the + operator combines them • in passing, level is an integer – so it is being converted to a double or floating point number, before being added • How is this expression evaluated? • first, the identifiers are replaced by whatever values they currently have • ie, the current value stored as the rocket's thrust level • and the value passed to the method on the call that has caused this line to be executed – this value is stored under the identifier level as specified in the first line of the method Learning to Program with C# - 8

9. Assistance from Visual Studio • Complexity is increasing • Where do the identifiers come from? What do they mean? • VS can help • move the pointer over an identifier • the location and type of the identifier should appear • Here, EngineThrust is shown to be a location of type double – that is a floating point value, and it is a member of Rocket (from Rocket.EngineThrust) • A short description may also be given • depends how thorough the programmer who created this was • Try this on the body of the TurnLeft method Learning to Program with C# - 8

10. Further assistance • Visual Studio can also show you the values that these locations contain during execution • using its debugger • you can step through the instructions of a program, seeing how values change and how the output develops • First attempt at this • Move the pointer into the left margin of the IncThrustLevel method, click the mouse • a purple circle should appear and the line go purple • this is a breakpoint – when the program runs, it will halt every time execution reaches this point Learning to Program with C# - 8

11. This is the Debug Toolbar • Display it, through View menu, if it isn't visible • The elements are as follows, respectively • Start – gets the program running (same as the menu option) • if you're stopped in a program, this will simply continue execution • Pause and Stop and Restart – self-explanatory • Show next statement – jumps to the next statement to be executed • Step Into • if you've stopped at a method call, this will take you one statement INTO the code of the method, so you can see what's happening • Step Over • steps to the next statement in sequence, executing the one you're at • Step Out • jumps out to the call of the method that you're stopped in, executing any statements between the stopping point and the end of the method Learning to Program with C# - 8

12. Try it out • Resize Visual Studio so that you will be able to see the rocket flying at the same time as you can see the VS window • With your breakpoint set, click Start • The animation window will partially display • and then VS will halt at the breakpoint Learning to Program with C# - 8

13. First, let's examine values • Remember, this is happening on the first call to IncThrustLevel • check where you expect that is, in the AutoFly method of the RocketController you are currently using in ExecuteMission • Bring up QuickWatch from Debug menu • type in, for example, EngineThrust followed by Enter • You will see details about the current value of EngineThrust appearing in the window • Try level also • Are you getting the values you expectednt ? • I got 10 for EngineThrust – the default value it is set to when the engines are switched on • and 60 for level, the amount of the first call to IncThrustLevel Learning to Program with C# - 8

14. Now let's try the step commands • These allow you to see statement by statement as the program executes • first, press Step Over a couple of times • this should take you out of IncThrustLevel, back to the point it was called from • Is this what you expected? • continue pressing Step Over until the Land(); statement is highlighted • notice that the Animation window is updated each time you pass a call to Coast, and that it takes a little time for a Coast statement to be completed Learning to Program with C# - 8

15. Exploring Land(); • Choose Step Into this time • you are taken to the code for Land • Now choose Step Over repeatedly • when you get to the while loop, you see the behaviour you should expect • cycling between the test and the statement, as the rocket is brought back to vertical • you can even see Theta changing • choose 'Autos' in Windows in Debug menu • click + next to ARocket, the continue pressing +s until a whole list of data is displayed • look for Theta in there – it'll have some radian value • now go back to pressing Step Over • you should see the value of Theta changing • you'll probably get bored of this after a while • pick Stop from the tool bar then Learning to Program with C# - 8

16. Back to expressions • This is a start • a full treatment is beyond this course • A few important operators • the . (full stop) is the member access operator • used to access methods and data values of an object • e.g. ARocket.EnginesOn accesses the EnginesOn method • the round brackets ( … ) after a method are the method call operator • any necessary parameters should be supplied within the brackets Learning to Program with C# - 8

17. Method calls… • Are sometimes statements • when it is marked as void • e.g. move pointer over the method names in the body of the TakeOff method – they are all marked initially as void, and are all being used as statements • and at other times, they're expressions • when they deliver a value • when they are marked with a type/class name • e.g. move pointer over IsTilting in the Land method • it is marked as bool indicating that it returns a value of boolean type when it is evaluated Learning to Program with C# - 8

18. Summary • Amazingly, this limited treatment covers most of the expressions coming up in the Rocketry project • explore around and see them in action • use the debugger to move around the code as it executes • the best way to really get on board how these instructions work • make small changes and see if they do what you expect • Everything we've done so far has involved mostly looking at existing code • essential first step in programming is understanding what the language can do • Next, we'll start building some small programs from scratch Learning to Program with C# - 8