Writing a Program in a High-level Language

• 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 the structures

• Make sure that the program does what the rules say

Elements of Programming

• We looked at 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– A “hardwired” program

Elements of Programming

• We’ve seen– Truth 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 the structures

• 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, Inputs and Outputs 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 = 0 And LightB = 2 And SensorA = 1 And

SensorB = 0 Then

• here we want to specify that the colors change

– If LightA = 2 And LightB= 0 And SensorA = 0 And SensorB = 1 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 LightB = 0 And SensorA = 0 And SensorB = 1 Then

ChangeGreenToYellow(LightA)ChangeYellowToRed(LightA)ChangeRedToGreen(LightB)

If LightA = 0 and 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 the instructions are

If LightA = 2 And LightB = 0 And SensorA = 0 And SensorB = 1 ThenChangeGreenToYellow(LightA)ChangeYellowToRed(LightA)ChangeRedToGreen(LightB)

• And not …If LightA = 2 And LightB = 0 And SensorA = 0 And SensorB = 1 Then

ChangeGreenToYellow(LightA)

ChangeYellowToRed(LightA)ChangeRedToGreen(LightB)

Some Rules

• Statements have to be in blocksIf LightA = 2 And Light B = 0 And SensorA = 0 And

SensorB = 1 Then

ChangeGreenToYellow(LightA)

ChangeYellowToRed(LightA)

ChangeRedToGreen(LightB)

End If

More rules: Loops

• Don’t want just a single state change

• Program should run ‘forever,’until we want it to stop

• Can do this with a “while” loop, which is governed by a termination/continuation condition

More Rules

• We have to tell the program to loopDo While condition

action Loop

More Rules

• We have to tell the program to loop

Do While StillWantToControlTraffic RunMyTrafficControlProgram

Loop

Procedures

• We said: ChangeGreenToYellow (LightA)• Could also say: ChangeGreenToYellow(LightB)• Must write a procedure to change lights

Private Sub ChangeGreenToYellow(Light As Integer)

Light = 1

End Sub

• When called with an argument, such as LightA, substitutes it for “Light” parameter internally– So effect is to set LightA to 1

Procedures

• SimilarlyPrivate Sub ChangeYellowToRed(Light As Integer)

Light = 2

End Sub

Private Sub ChangeRedToGreen(Light As Integer)

Light = 0

End Sub

• Procedure parameters and runtime arguments

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)

i.e. Light1 is LightB, and Light2 is LightA

Similarly, ChangeLights(LightA, LightC) does

ChangeGreenToYellow(LightA)ChangeYellowToRed(LightA)ChangeRedToGreen(LightC)

The Program: Procedure Definitions

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 LightB = 0 And SensorA = 0 And SensorB = 1 Then

ChangeLights(LightA,LightB)

End If

If LightA = 0 And 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 LightB = 0 And SensorA = 0 And SensorB = 1 Then

ChangeLights(LightA,LightB)

End If

If LightA = 0 And 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?

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], …

Arrays (con’t).

• Access particular lights/sensors in array:ControlTrafficLight(lightNS[i],lightEW[i],sensorNS[i],sensorEW[i])

• Control 100 traffic lights:For i = 1 To 100

ControlTrafficLight(lightNS[i],lightEW[i],sensorNS[i],sensorEW[i]) Next i

• But:– Lights may want to communicate

– Lights affected by neighboring lights, not just sensors

Object oriented programming

• Figure out characteristics of your data– Objects

• Figure out operations you’ll want to perform – Methods

• A modern programming style

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– Neighboring Lights

• May affect change as much as sensors

– …

Methods

• Method of querying color of light

• Method of changing color of light

• Method of scheduling a color change later

• …

What happens to the program?

• It is either compiled or interpreted– Eventually it gets translated into machine

language

• If compiled– Can store executable and run again

• If interpreted– Interpret from original language each time it is

executed

What does the compiler do?

• Identifies variables– These need space in RAM– Uses stores and loads to get values to registers

• Parses commands– Turns each program statement into a string of

machine language commands

• Sets things up for execution

Steps in compilation

• Lexical analysis– Identify all keywords– Identify all operators– Identify all variables– Make everything into tokens

• Parsing– Turn the tokens into operations– Build a computation tree

• Code generation– Generate machine code

Lexical analysis

• Keywords– If … Then .. End If– If .. Then .. Else … End If– Do While … … Loop– Private sub ….– End sub– Dim … as Integer

• Operators= (in 2 contexts)

If (Light = 0)Light = 1

+ - * /- also in 2 contexts (unary or binary)

Simplified code fragment

Dim low As Integer, high As Integer Dim fahrenheit As Double, celsius As

Double For fahrenheit = low to high

celsius = 5 / 9 * (fahrenheit - 32) print fahrenheit, celsius Next fahrenheit

End

Code fragment (lex’ed)

Dim low As Integer, high As IntegerDim fahrenheit As Double, celsius As DoubleFor fahrenheit = low to high

celsius = 5 / 9 * (fahrenheit - 32) print fahrenheit , celsius Next fahrenheit

End

Keywords, variables, constants, operators, functions, separators

Code fragment (cont.)

Dim <tag1> As Integer, <tag2> As IntegerDim <tag3> As Double, <tag4> As DoubleFor <tag3> = <tag1> to <tag2>

<tag4> = 5 / 9 * (<tag3> - 32) print <tag3> , <tag4> Next <tag3>

End

Replace variables by tags these are really locations in RAM

How things are defined (i.e. the types of the variables) determines:how much RAM they needhow operations on them work

Code fragment (cont.)

For <tag3> = <tag1> to <tag2> <tag4> = 5 / 9 * (<tag3> - 32) print <tag3> , <tag4> Next <tag3>

The instructions in the loop must be unwound<tag3> = <tag1><tag4> = 5 / 9 * (<tag3> - 32) print <tag3> , <tag4> <tag3> = <tag3> + 1If <tag3> <= <tag2> go back

<tag3> = <tag1><tag4> = 5 / 9 * (<tag3> -

32) print <tag3> , <tag4> <tag3> = <tag3> + 1If <tag3> <= <tag2> go

back

The unwound loop can be translated into machine language

Store 32 in R3Store 5/9 in R4Store 1 in R5Load <tag1> into R1L1: Store R1 into <tag3>Load <tag3> into R2Subtract R3 from R2 and store in R2Multiply R4 by R2 and store in R2

Store R2 in <tag4>Print R1,R2Add R5 to R1 and store in R1Store R5 in <tag3>Load <tag2> into R6Subtract R6 from R5 and store in R5Go back to L1 if R6 > 0

How is the unwinding etc done: Parsing

• Language is defined by a grammar

• Grammar is defined by production rules

• Parsing is done by unwinding

• Grammars and rules for parsing languages are complex, so let’s look at something simpler: A grammar for generating numbers (instead of computer programs)

How do we specify a grammar?

• 2 aspects to a language– Symbols– Rewriting rules (also called productions)

• Simple language for generating numbers– Symbols

• Non-terminals– <number>, <digits>, <sign>, <digit>

• Terminals– + - . 1 2 3 4 5 6 7 8 9

Simple rewriting rules

• <number> <sign> <digit><digits> . <digits>• <sign> + | -• <digits> <digit><digits> | <digit> | • digit

An example• <number> <sign> <digit><digits> . <digits>• <sign> + | -• <digits> <digit><digits> | <digit> | • digit

<number> <sign><digit><digits>.<digits> <sign><digit><digit><digits>.<digits> <sign><digit><digit>.<digits> <sign><digit><digit>.<digit><digits> <sign><digit><digit>.<digit><digit> …+98.65

Alternative rules

• <number> <sign> <digit><digits> . <digits>• <sign> + | -• <digits> <digit><digits> | <digit> | • digit

• <number> <sign><digits>.<digits>| <sign><digits>

• <sign> +|-

• <digits> <digit><digits> | <digit>

• <digit> 0|1|2|3|4|5|6|7|8|9

Parsing

• <number> <sign><digits>.<digits>| <sign><digits>

• <sign> +|-

• <digits> <digit><digits> | <digit>

• <digit> 0|1|2|3|4|5|6|7|8|9

What rules were applied to get 123.45?

What about real languages?

• The complete grammar for C – about 400 lines long– 58 tokens (based on keywords)– 65 basic productions (each with many options)

– Only a few complex situations

Some programming language issues

• Type declarations– If you Dim something as an integer and then try

to make it hold a double, what should happen?

• Verification– How do you tell if your specification is right?– How do you tell if your program meets your

specification?

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

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,

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

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

Big number bug

On June 4, 1996 an unmanned Ariane 5 rocket launched by the European Space Agency exploded just forty seconds after its lift-off from Kourou, French Guiana. The rocket was on its first voyage, after a decade of development costing $7 billion. The destroyed rocket and its cargo were valued at $500 million. A board of inquiry investigated the causes of the explosion and in two weeks issued a report. It turned out that the cause of the failure was a software error in the inertial reference system. Specifically, a 64 bit floating point number relating to the horizontal velocity of the rocket with respect to the platform was converted to a 16 bit signed integer. The number was larger than 32,768, the largest integer that could be stored in a 16 bit signed integer, and so the conversion failed.

Pentium II bug

• Software bug encoded in hardware• Division algorithm uses a lookup table of

1066 entries• Only 1061 of the entries are downloaded to

the PLA (programmed logic array from which the data are used)

• Intel had to recall all versions of the chip

Syntax “typo” bugs

• NASA Mariner 1 , Venus probe (1992)

• Intended to be the first US spacecraft to visit another planet, it was destroyed by a range officer on 22 July 1962 when it behaved erratically four minutes after launch. – Essentially a period instead of a comma in a

FORTRAN DO-Loop

Control flow bug

• AT&T long distance service fails for nine hours(Wrong BREAK statement in C code)

• January 15, 1990:• 70 million of 138 million long distance customers

in the US lost long distance service. • Cost to ATT was between $ 75 Million and $100

Million (plus the loss of good will).

• E-mail buffer overflow (1998)• Several E-mail systems suffer from a "buffer

overflow error", when extremely long e-mail addresses are received.  The internal buffers receiving the addresses do not check for length and allow their buffers to overflow causing the applications to crash.  Hostile hackers use this fault to trick the computer into running a malicious program in its place.

Data structure management bug

Summary• Programming is difficult

– Have to thoroughly understand the task– Have to anticipate all possibilities– Code is written at a fairly primitive level– Impossible to anticipate what users might do

• Programming languages allow the user to use tools to build code

• But everything still has bugs• The cost of a bug can be very large

• There is no Moore’s Law for software.

Where are we

• We’ve built a computer

• We’ve built programs– And looked under the hood

What’s next• Algorithms• Networking: The Internet, Email, the Web Operating Systems (Mon,

Apr 7)• Sound and Graphics (Wed, Apr 9 and Mon, Apr 14, resp.)• Distributed Systems (Wed, Apr 16)

– Sharing files, sharing cycles, distributed computing

• Complexity theory– Undecidable problems, unsolvable (in practice) problems

• Applications of hard problems• Social impacts

– Digital rights management– Access to information (Digital Divide)– Artificial intelligence