Programming

1
Programming

• November 13, 2001

2
Administrivia

• From santaclaus_at_northpole.org Thu Nov 8
  120531 2001
• Date Thu, 8 Nov 2001 120436 -0500 (EST)
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• no goodies for you
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3
Administrivia

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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

• Weve 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
Traffic Light Behavior
Otherwise
IF A1 AND B0
Light A
Always
Always
IF A0 AND B1
Otherwise
Light B
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,Ligh
  tB)
• 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,sens
  or1,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
• LightNS1, LightNS2, LightNS3,
• LightEW1. LightEW2. LightEW3,
• SensorNS1, SensorNS2, SensorNS3,
• SensorEW1, SensorEW2, SensorEW3,

26
Arrays (cont).

• Keep track of things in array
• ControlTrafficLight(lightNSi,lightEWi,sensorNS
  i,sensorEWi)
• Control all the traffic lights in Manhattan
• For i 1 To 100
• ControlTrafficLight(lightNSi,lightEWi,sensorNS
  i,sensorEWi) 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 neednt 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 weve gotten it right?