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Transcript of Programming
Programming
AdministriviaFrom [email protected] Thu Nov 8 12:05:31 2001Date: Thu, 8 Nov 2001 12:04:36 -0500 (EST)From: [email protected]
no goodies for youjust a lump of coal
& TFrom [email protected] Thu Nov 8 12:05:31 2001Return-Path: <[email protected]>Received: from Princeton.EDU (postoffice.Princeton.EDU [128.112.129.120]) by upright.CS.Princeton.EDU (8.11.6/8.11.6) with ESMTP id fA8H5QQ29528 for <[email protected]>; Thu, 8 Nov 2001 12:05:31 -0500 (EST)Received: from yuma.Princeton.EDU (yuma.Princeton.EDU [128.112.128.89]) by Princeton.EDU (8.9.3/8.9.3) with SMTP id MAA29199 for dpd; Thu, 8 Nov 2001 12:04:36 -0500 (EST)Date: Thu, 8 Nov 2001 12:04:36 -0500 (EST)From: [email protected]: <[email protected]>Status: RO
no goodies for youjust a lump of coal
Where we are
• Built a machine
• Built an operating system to control the machine
• Now, want to run programs under the operating system
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
Problem solving
We’ve seenTruth tables
Logic gates
States and transitions in a state machine
Machine language
Now, higher level programming language
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
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
Light
ATraffic Light Behavior
IF A=1
AND B=0
Always
IF A=0
AND B=1
Otherwise
Light BOtherwise
Always
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
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
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 ThenChangeGreenToYellow(LightB)ChangeYellowToRed(LightB)ChangeRedToGreen(LightA)
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 ThenChangeGreenToYellow(LightA)ChangeYellowToRed(LightA)ChangeRedToGreen(LightB)
• And NotIf LightA = 2 And SensorA = 0 And SensorB = 1 Then
ChangeGreenToYellow(LightA)
ChangeYellowToRed(LightA)ChangeRedToGreen(LightB)
Some Rules
• Statements have to be in blocksIf LightA = 2 And SensorA = 0 And SensorB = 1 Then
ChangeGreenToYellow(LightA)
ChangeYellowToRed(LightA)
ChangeRedToGreen(LightB)
End If
More Rules
• We have to tell the program to loopDo While condition
action Loop
More Rules
• We have to tell the program to loopDo While StillWantToControlTraffic
RunMyTrafficControlProgramLoop
Procedures
• Must fill in functions to change lightsPrivate Sub ChangeGreenToYellow(Light As Integer)
Light = 1End SubPrivate Sub ChangeYellowToRed(Light As Integer)
Light = 2End SubPrivate Sub ChangeRedToGreen(Light As Integer)
Light = 0End Sub
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
Using the procedure
ChangeLights(LightB,LightA) then does
ChangeGreenToYellow(LightB)
ChangeYellowToRed(LightB)
ChangeRedToGreen(LightA)
The program
Private Sub ChangeGreenToYellow(Light As Integer)Light = 1
End SubPrivate Sub ChangeYellowToRed(Light As Integer)
Light = 2End SubPrivate Sub ChangeRedToGreen(Light As Integer)
Light = 0End SubPrivate Sub ChangeLights(Light1 As Integer, Light2 As Integer)
ChangeGreenToYellow(Light1)
ChangeYellowToRed(Light1)ChangeRedToGreen(Light2)
End Sub
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)
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
What could go wrong?
• Program could get confused– Check for consistency
• ReplacePrivate Sub ChangeGreenToYellow(Light As Integer)
Light = 1End Sub
• WithPrivate Sub ChangeGreenToYellow(Light As Integer)
If (Light = 0) ThenLight = 1
ElseReportInconsistency()
End IfEnd Sub
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?
Show the array structure
• Build array• Keep track of things in array• Control all the traffic lights in Manhattan• But items in array have to track more info
– Time for lights each way– Location– Maybe dependencies on other lights
• So need better ways of storing data• Modern programming
– Data are objects– There are methods for operating on data
Having built up to datatypes
• What are the relative merits of different languages• Visual vs. line oriented• Debugging• What the language gives you• Some history of programming languages
– And tools
– What is it about perl, awk, …
• What does the compiler do?
• Let’s look at a bigger system
• Maybe the registrar’s or PeopleSoft
• Then joys and pitfalls of modern programming
• How do we know if we’ve gotten it right?