1. Programming November 13, 2001

2. Administrivia From santaclaus@northpole.org Thu Nov 8 12:05:31 2001 Date: Thu, 8 Nov 2001 12:04:36 -0500 (EST) From: santaclaus@northpole.org no goodies for you just a lump of coal

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4. Where we are Built a machine Built an operating system to control the machine Now, want to run programs under the operating system

5. What is programming We did machine language –Single instructions that tell the hardware what to do –Primitive Arithmetic, simple branching, communication with memory We built state machines –States using memory –Transitions modeling tasks

6. Problem solving  We’ve seen Truth tables Logic gates States and transitions in a state machine Machine language  Now, higher level programming language

7. To build a computer program Figure out what you want to do –Understand the rules that guide the process you are automating Make sure that your rules are complete Translate the rules into the computer language –Build structures to hold your data –Build tools to manipulate them Make sure that the program does what the rules say

8. Figuring out the rules For traffic lights, –We stored data that told us the current color of lights –We read input from sensors –We had rules that told us whether to change state –We had rules that told us how to change state

9. Light A Traffic Light Behavior IF A=1 AND B=0 Always IF A=0 AND B=1 Otherwise Light B Otherwise Always

10. Turn Memory Into Variables Store data to tell current color of lights –Dim LightA, LightB as Integer ‘ 0 for red, 1 for yellow, 2 for green Read input from sensors –Dim SensorA, SensorB as Integer ‘ tell if cars are waiting

11. Turn Rules Into Statements Decide whether to change state –If LightA = 2 And SensorA = 0 And SensorB = 1 Then ‘ here we want to specify that the colors change –If LightB = 2 And SensorA = 1 And SensorB = 0 Then ‘ again, we want to specify that the colors change

12. Build shell of program Dim LightA, LightB as Integer Dim SensorA, SensorB as Integer If LightA = 2 And SensorA = 0 And SensorB = 1 Then ChangeGreenToYellow(LightA) ChangeYellowToRed(LightA) ChangeRedToGreen(LightB) If LightB = 2 And SensorA = 1 And SensorB = 0 Then ChangeGreenToYellow(LightB) ChangeYellowToRed(LightB) ChangeRedToGreen(LightA)

13. Some Rules Statements have to be in blocks How does the computer know that it is If LightA = 2 And SensorA = 0 And SensorB = 1 Then ChangeGreenToYellow(LightA) ChangeYellowToRed(LightA) ChangeRedToGreen(LightB) And Not If LightA = 2 And SensorA = 0 And SensorB = 1 Then ChangeGreenToYellow(LightA) ChangeYellowToRed(LightA) ChangeRedToGreen(LightB)

14. Some Rules Statements have to be in blocks If LightA = 2 And SensorA = 0 And SensorB = 1 Then ChangeGreenToYellow(LightA) ChangeYellowToRed(LightA) ChangeRedToGreen(LightB) End If

15. More Rules We have to tell the program to loop Do While condition action Loop

16. More Rules We have to tell the program to loop Do While StillWantToControlTraffic RunMyTrafficControlProgram Loop

17. Procedures Must fill in functions to change lights Private Sub ChangeGreenToYellow(Light As Integer) Light = 1 End Sub Private Sub ChangeYellowToRed(Light As Integer) Light = 2 End Sub Private Sub ChangeRedToGreen(Light As Integer) Light = 0 End Sub

18. Could build Procedure of Procedures ChangeGreenToYellow(LightA) ChangeYellowToRed(LightA) ChangeRedToGreen(LightB) Could become the command ChangeLights(LightA,LightB) Private Sub ChangeLights(Light1 As Integer, Light2 As Integer) ChangeGreenToYellow(Light1) ChangeYellowToRed(Light1) ChangeRedToGreen(Light2) End Sub

19. Using the procedure ChangeLights(LightB,LightA) then does ChangeGreenToYellow(LightB) ChangeYellowToRed(LightB) ChangeRedToGreen(LightA)

20. The program Private Sub ChangeGreenToYellow(Light As Integer) Light = 1 End Sub Private Sub ChangeYellowToRed(Light As Integer) Light = 2 End Sub Private Sub ChangeRedToGreen(Light As Integer) Light = 0 End Sub Private Sub ChangeLights(Light1 As Integer, Light2 As Integer) ChangeGreenToYellow(Light1) ChangeYellowToRed(Light1) ChangeRedToGreen(Light2) End Sub

21. The program (cont.) Dim LightA, LightB as Integer Dim SensorA, SensorB as Integer If LightA = 2 And SensorA = 0 And SensorB = 1 Then ChangeLights(LightA,LightB) End If If LightB = 2 And SensorA = 1 And SensorB = 0 Then ChangeLights(LightB,LightA)

22. Make it happen forever Dim LightA, LightB as Integer Dim SensorA, SensorB as Integer Dim StillWantToControlTraffic as Integer StillWantToControlTraffic = 1 Do While StillWantToControlTraffic If LightA = 2 And SensorA = 0 And SensorB = 1 Then ChangeLights(LightA,LightB) End If If LightB = 2 And SensorA = 1 And SensorB = 0 Then ChangeLights(LightB,LightA) End If Loop

23. What could go wrong? Program could get confused –Check for consistency Replace Private Sub ChangeGreenToYellow(Light As Integer) Light = 1 End Sub With Private Sub ChangeGreenToYellow(Light As Integer) If (Light = 0) Then Light = 1 Else ReportInconsistency() End If End Sub

24. Building a bigger program Could write this as a subroutine –Private sub ControlTrafficLight(light1,light2,sensor1,sensor2) Could reuse the subroutine to do a whole string of lights. But how would we keep track of hundreds of lights?

25. Arrays Build arrays –LightNS[1], LightNS[2], LightNS[3], … –LightEW[1]. LightEW[2]. LightEW[3], … –SensorNS[1], SensorNS[2], SensorNS[3], … –SensorEW[1], SensorEW[2], SensorEW[3], …

26. Arrays (con’t). Keep track of things in array ControlTrafficLight(lightNS[i],lightEW[i],sensorNS[i],sensorEW[i]) Control all the traffic lights in Manhattan For i = 1 To 100 ControlTrafficLight(lightNS[i],lightEW[i],sensorNS[i],sensorEW[i]) Next i But, –lights may want to communicate –Light at an intersection carries more information

27. Object oriented programming Figure out characteristics of your data –objects Figure out operations you will want to perform –Methods Modern idea in programming.

28. Objects Traffic light at intersection involves –Lights in each direction Call them red, yellow and green and not 0,1,2 –Sensors in each direction –Timing (rate of change in each direction) Timings needn’t be the same –Neighboring Lights May affect change as much as sensors

29. Methods Method of querying color of light Method of changing color of light Method of scheduling a color change later …

30. What happens to the program? Compiled or interpreted –Eventually it gets translated into machine language If compiled –Can store executable and run again If interpreted –Interpret each time it is executed

31. What does the compiler do? Identifies variables (need space in RAM) –Uses stores and loads to get values to registers Parses commands –Turns each command into a string of machine language commands Sets things up for execution

32. History of Programming Languages Fortran (1954) for scientific Cobol (1959) for business Algol (1958) more universal Fortran Lisp (1958) string/concept oriented APL (1960) formula oriented

33. History of Programming Languages PL/1 (1964) from Algol + Fortran Basic (1964) for everyone to use Simula (1967) combines with Algol to yield Smalltalk (1969) – object oriented BCPL  B  C (1971) Algol  Pascal (1971)  Modula 1,2,3,

34. History of Programming Languages C++ (1983) C with object oriented features –Often C is still used Awk (1978)  Perl (1987) report generators –Web programming language Java (1991) object oriented and portable –Web applets, devices Visual Basic(1991) macros and programs –Core of Microsoft systems

35. What makes a good language Does the task you want Keeps you from making mistakes Supports debugging when you need it Has a strong tool kit

36. Next class Look at a bigger system Some internals of how the compiler works Some notable bugs How do we know if we’ve gotten it right?