Disciplined Software Engineering Lecture #9

Disciplined Software Engineering

Lecture #9•Watts S. Humphrey•Software Engineering Institute•Carnegie Mellon University•Pittsburgh, PA 15213

•Sponsored by the U.S. Department of Defense

Design Notation - Overview

•The importance of notation

•Boolean algebra

–Karnaugh maps

–useful symbols

–using Boolean functions

•Simplification

•Formal methods

The Importance of Notation

•Many software development defects are caused by misunderstandings and confusion.

•The notation used to represent program requirements and designs should be–precise and concise–convenient to use–commonly understood–suitable for representing a broad range of software functions

•Formal notations meet these criteria.

Class Properties- Mathematical Notation

English Symbol Example

Union, sum A or B, A+B

Intersection, product A and B, A*B, AB

Complement, not ' A', not A

All, universal 1

None, empty set 0

Boolean Algebra

•The algebra of classes

–defines and manipulates class relationships

–consistency - whether statements are self- consistent

–validity - whether statements represent reality

•Class representations

–Venn diagrams

–basic relationships

–the universal and empty sets

Venn DiagramsHumans

Men

Women

Humans

Non-adultWomen

AdultWomen

AdultMen

Non-adultMen

A’B’C’

Universal Class

AB’C’A’BC’

A’B’C

AB’CA’BC

ABC

ABC’

Properties of Classes - 1

•Reflexive: for every x, x<=x

•Idempotent: xx=x, x+x=x

•Antisymmetric: if x<=y and y<=x then x=y

•Transitive: if x<=y and y<=z then x<=z

•Commutative: xy=yx, x+y=y+x

•Associative: x(yz)=(xy)z, x+(y+z)=(x+y)+z

•Distributive: x(y+z)=xy+xz, x+yz=(x+y)(x+z)

Properties of Classes - 2

•Empty and universal classes:

• xx’=0, x+x’=1, 0<=x<=1

• x0=0, x+1=1, x+0=x, x1=x

•Consistency: x<=y, xy=x, x+y=y

• are equivalent

•Absorption: x(x+y)=x+xy=x

•DeMorgan’s theorem:

• (x’)’=x, (xy)’=x’+y’, (x+y)’=x’y’

An Example

•Suppose you had a switch that you wanted to be on under the following conditions

–A or B or C are on

–not when A and B are on

–not when A and C are on

•You could represent this as follows

•On = (A+B+C)*[(A*B)+(A*C)]’

Simplifying Boolean Expressions

•Consider the following guidelines when you simplify Boolean expressions

–if there are any primed parenthetical expressions, use DeMorgan’s theorem to remove them

–look for common terms that can be factored by the distributive law

–make substitutions to reduce complexity

–attempt to reduce expressions to a union of intersections

Simplification Example - 1•Use the light bulb example

• On = (A+B+C)*[(A*B)+(A*C)]’

•Apply DeMorgan’s theorem

• On = (A+B+C)*[(A’+B’)*(A’+C’)]

•Now, A’ is common to the last terms so

• On = (A+B+C)*[A’+(B’*C’)]

•Now, substitute x for B+C, and x’ for B’*C’

• On = (A+x)*(A’+x’)

Simplification Example - 2

•While this expression is fully simplified, it should be converted to a union of intersections

–On = (A+x)*(A’+x’)

–On = [(A+x)*A’]+[(A+x)*x’]

–On = [(A*A’)+(x*A’)]+[(A*x’)+(x*x’)]

–On = (x*A’)+(A*x’)

•Now, replacing x with B+C

–On = [(B+C)*A’]+[A*(B’*C’)]

–On = (A*B’*C’)+[A’*(B+C)]

Simplification Exercises

•Simplify the following expressions

•1. F = X+(X’*Y)

•2. F = (X+Y)+[(X*Z)+Y]

•3. F = (X’*Y’*Z’)+(X*Y*Z’)+(X*Y’*Z’)

•4. F = [X’*(Y+Z’)]’*(X+Y’+Z)*(X’*Y’*Z’)’

Exercise 1 Answer

•Starting with F = X+(X’*Y)

•Use the distributive law

• F = (X+X’)*(X+Y)

•Now, with the property of the universal class

• X+X’ = 1 AND 1*(X+Y) = X+Y

•So the answer is

• F = X+Y

Exercise 2 Answer•Starting with F = (X+Y)+[(X*Z)+Y],

•Apply the distributive law to give

• F = (X+Y)+(X+Y)*(Z+Y)

•Now, factor out the common X+Y term

• F = (X+Y)*[1+(Z+Y)]

•By the property of the universal class, 1+Z+Y=1

•so the answer is

• F = X+Y

Exercise 3 Answer

•Starting with F = (X’*Y’*Z’)+(X*Y*Z’)+(X*Y’*Z’)

•Factor out the common Z’ term

• F = Z’*(X’Y’+XY+XY’)

•Factoring out the common X term

• F = Z’*[X’Y’+X(Y+Y’)] = Z’*(X’Y’+X)

•Now, with the distributive law

• F = Z’*(X’+X)*(Y’+X) = Z’*(Y’+X)

Exercise 4 Answer•Starting with F = [X’*(Y+Z’)]’*(X+Y’+Z)*(X’*Y’*Z’)’

•First apply DeMorgan’s theorem, giving

• F = (X+Y’Z)*(X+Y’+Z)*(X+Y+Z)

•Factoring out the common X term gives

• F = X+Y’Z*(Y’+Z)*(Y+Z)

•Factoring out the common Z term

• F = X+Y’Z*(Z+Y’Y) = X+Y’Z

Karnaugh Maps - 1

•For expressions of 6 variables or less, Karnaugh maps can aid the simplification process.

•Karnaugh maps are a structured way to represent all possible combinations of the variables in an expression.

•Factoring is then done by associating related terms to achieve simplification.

Karnaugh Maps - 2

•Boolean expressions can be represented in minterm form.

•The minterms are the products of the variables in all possible combinations.

•The minterms for X and Y are:• X’*Y’, X*Y’, X’*Y, XY

•There are 2**n minterms for n variables.

Karnaugh Maps - 3

X

YZ

00 01 11 10

0

1

XY’

X X

00 01 11 10

0

1

X’Z

X X

X

YZ

00 01 11 10

0

1

X X

X

YZ

X X

XY’+X’Z

A 4-Variable Karnaugh Map

WX00 01 11 10

00

01 X X

11

10

X X

YZ

X

X

A 5 Variable Karnaugh Map

110 111 101 100VW

000 001 011 010

00

01

11

10

XYZ

Map Factoring - 1

•When two adjacent cells both have 1s, then those cells can be factored, eliminating the variable that is different for the two cells.

•For example, the 0100 and 0101 cells can be factored, eliminating the variable z.

•This is shown on the following map.

Map Factoring - 2

WX00 01 11 10

00

01 X X

11

10

X X

YZ

X

X

Map Factoring - 3

•Similarly, when four adjacent cells have 1s, then those cells can be factored, eliminating the two variables that are different for the four cells.

•For example, the 0101, 0111, 1101, and 1111 cells can be factored, eliminating the variables w and y.


Map Factoring - 4

WX00 01 11 10

00

01 X X

11

10

X X

YZ

X

X

Map Factoring - 5

•When all three of the possible factorings are made, the resulting expression is

•F = w’yz + wxy’ + xz


Map Factoring - 6

WX00 01 11 10

00

01 X X

11

10

X X

YZ

X

X

W’YZ+W’XY’+XZ

Simplification Exercises

•Using Karnaugh Maps, simplify the following expressions

•1. F = X+(X’*Y)

•2. F = (X+Y)+[(X*Z)+Y]

•3. F = (X’*Y’*Z’)+(X*Y*Z’)+(X*Y’*Z’)

•4. F = [X’*(Y+Z’)]’*(X+Y’+Z)*(X’*Y’*Z’)’

Exercise 1 Answer00 01 11 10

0

1 1

0 0 1 1

1 1 1

X

YZThe function is F = X+(X’*Y)

00 01 11 10

0

1 1

0 0 1 1

1 1 1

X

YZFactoring this gives

F = X+Y


0

1 1

0 0 1 1

1 1 1

X

YZThe function is

F = (X+Y)+[(X*Z)+Y] = X+Y+XZ+Y = X+Y+XZ

00 01 11 10

0

1 1

0 0 1 1

1 1 1

X


F = X+Y


0

1 0

1 0 0 0

1 0 1

X

YZThe function is

F = (X’*Y’*Z’)+(X*Y*Z’)+(X*Y’*Z’) = X’Y’Z’+XYZ’+XY’Z’

00 01 11 10

0

1 0

1 0 0 0

1 0 1

X


F = Y’Z’+XZ’ = Z’(Y’+X)

Exercise 4 Answer - 1

00 01 11 10

0

1 1

0 1 0 0

1 1 1

X

YZ

The function is

F = [X’*(Y+Z’)]’*(X+Y’+Z)*(X’*Y’*Z’)’ = (X+Y’Z)*(X+Y’+Z)*(X+Y+Z)

This can be covered in 3 maps.

00 01 11 10

0

1 1

0 1 1 1

1 1 1

X

YZ

X+Y’Z

00 01 11 10

0

1 1

1 1 1 0

1 1 1

X

YZ

X+Y’+ZX+Y+Z

Exercise 4 Answer - 2

00 01 11 10

0

1 1

0 1 0 0

1 1 1

X

YZThe total function is the productof these maps, or a map with 1swhere all 3 maps have 1s and 0selsewhere. This is

00 01 11 10

0

1 1

0 1 0 0

1 1 1

X

YZFactoring this map gives the answer

F = X+Y’Z

Factoring Zeros

•When factoring 1s, the 1s on a map are covered. This gives the minterm form of the expression.

•The same map can be factored by covering the 0s and then applying DeMorgan’s Theorem.

•This gives the maxterm form of the expression, or the intersection of unions.

Example of Zeros FactoringThe following map could be factored by covering 1s as follows, giving

F = X+Y+Z

00 01 11 10

0

1 1

0 1 1 1

1 1 1

X

YZ

00 01 11 10

0

1 1

0 1 1 1

1 1 1

X

YZ

Covering the 0s, however, gives

F’ = X’Y’Z’

Applying DeMorgan’sTheorem gives the answer

F = X+Y+Z

Another Factoring ExampleThe following map could be factored by covering 1s as follows, giving theminterm result

F = XY’+X’Y+Z

00 01 11 10

0

1 1

0 1 1 1

1 1 0

X

YZ

00 01 11 10

0

1 1

0 1 1 1

1 1 0

X

YZ

Covering the 0s, however, gives

F’ = X’Y’Z’+XYZ’

Applying DeMorgan’sTheorem gives the answer inmaxterm form

F = (X+Y+Z)*(X’+Y’+Z) = (X+Y)*(X’+Y’)+Z

Factoring Out Zeros

•Sometimes, a 1s factoring will be complicated by one or two badly placed 0s. This can be handled as follows

•Factor the map as if the inconvenient 0s were 1s.

•Then, treating each of these factors as a small map, factor out the 0s.

Factoring Out 0s Example - 1

WX00 01 11 10

00

01 1 1

11

10

1 0

YZ

0

1

1

0 011

0

1

110

Except for two inconvenient0s, this map has a verysimple factoring

F = Y’+W’Y = Y’+W’

Because of the 0 atW’X’Y’Z’ and the 0at W’XYZ, however,we must factor out0s.

Factoring Out 0s Example - 1

WX00 01 11 10

00

01 1 1

11

10

1 0

YZ

0

1

1

0 011

0

1

110

The first factor, Y’,creates the mini mapin the variables W, X,and Z. On this map,W’X’Z’ is factored out:

F1 = Y’*(W’X’Z’)’ = Y’*(W+X+Z)

This factoring is essentiallyY’ and not W’X’Z’.

The second factoring is

F2 = W’Y*(XZ)’ = W’Y*(X’+Z’)

So the function is

F = Y’*(W+X+Z)+W’Y*(X‘+Z’)

Factoring With More Variables

•Karnaugh maps are possible with up to 6 variables. Beyond that, special tricks are needed.

•If the expression involves 6 or fewer variables in most of the terms with an occasional additional variable, an approach much like factoring out 0s can be used.

•This method is called exception factoring and can be used with any number of variables.

Exception Factoring - 1

•To factor the following function on a 4-variable Karnaugh map

• F = AX’Y’+WXY’+W’Y’Z+XY’Z+W’XY’Z’+WX’Y’

•1. Construct the function, using the variables

• W, X, Y, and Z.

•2. For the term with the A, instead of 1, enter an

• A in that square.

•3. In factoring out zeros, treat A as A+0.


WX 00 01 11 10

00

01 1 1

11

10

1 0

YZ

0

0

1

0 011

0

0

010

First, construct the map, usingonly the terms containing W,X, Y, and Z.

F1 = WXY’+W’Y’Z+XY’Z+W’XY’Z’+WX’Y’

WX 00 01 11 10

00

01 0 0

11

10

0 0

YZ

0

0

0

0 0AA

0

0

0AA

Next, enter the A term. Put an Awherever there are 0s and nothingwhere there is a 1.

F2 = AX’Y’


WX 00 01 11 10

00

01 1 1

11

10

1 0

YZ

0

0

1

0 011

0

0

01A

First, factor out the 1s, assumingthe A was a 1

F1 = Y’

Next, factor out the A term, treatingit as A+0

F = Y’*[A+(W’X’Z’)’] = Y’*(A+W+X+Z)

Other Useful Symbols

•The following basic mathematical symbols are also useful in defining programming functions.

• - means “is a member of the set”

• - means “is not a member of the set”

• - means “for all members of the set”

• - means “there exists a member of the • set”

Notation Examples

•To update a file when dataset D is a member of the file

•(D File) :: Update

•To add a number x(i), when every number i is an even integer

• i when (i even) :: add x(i)

•To clear a register when any number n is negative

• i when (i < 0) :: Clear

Function Completeness

•A set of functions is complete if it covers all possible conditions.

•For a complete set of functions, the Karnaugh map for the sum (or union) of the functions covers all map positions.

•There may be multiply-covered map positions.

Function Orthogonality

•A set of functions is orthogonal if no two functions cover the same conditions.

•This means that the product (intersection) of any function with any other function is zero.

• i, j where (i j) => F(i)*F(j) = 0

•

•The map may not be completely covered.

Complete and Orthogonal

•A set of functions is complete and orthogonal if

–the functions cover all elements

–no two functions cover the same elements

Complete and Orthogonal Functionsf, g, h are orthogonal

00 01 11 10

0

1 f

00 01 11 10

0

1

f g

f, g, h are complete

00 01 11 10

0

1

f g

f, g, h are complete and orthogonal

h g

f g gh fh

g fh fg h g f

f h

fh

Formal Methods - 1•Formal methods for software design have been developed based on mathematical concepts

•The premise of formal methods is that

–computer programs can be treated as mathematical statements

–the principles of mathematics thus apply

–the design of a correct program can then be treated as a theorem derivation

–the use of mathematically rigorous methods should thus produce correct programs.

Formal Methods - 2

•Formal methods typically use a program precondition, a program postcondition, and a program invariant.

•The program then transforms the precondition to the postcondition while preserving the invariant.

•To prove program correctness

–prove that the program transforms the preconditions to the postconditions

–and preserves the invariants

Formal Methods - 3

•To formally develop a new program

–start with a postcondition

–derive the precondition

–determine the invariants

•

•Define a program that transforms the precondition to the postcondition and preserves the invariant.

•If the program involves iteration, each cycle should progress toward loop termination.

The Pros of Formal Notations

•They are precise.

•They can compactly represent complicated functions.

•They form a foundation for learning formal methods.

The Cons of Formal Notations

•They take time to learn.

•They take more time to build the familiarity needed to use them for design.

•Your associates may not understand them.

Formal Methods in This Course

•Adherents claim formal methods provide a rigorous way to develop quality software. Sceptics argue that they are not cost effective.

•While formal methods show promise, their general use will require

–user training

–users who make fewer errors with formal proofs than they do in software design

•Formal methods are not treated further in this course.

Formal Method Suggestions - 1

•If you are familiar with or trained in formal methods

–use them with your PSP

–measure your quality and productivity

–compare your results to others who use such methods

–compare your results to your results when you do not use formal methods

–decide if these methods are effective for you

•Tell others what you find.

Formal Method Suggestions - 2

•If you are not familiar with formal methods, consider them

–ask users to show you their data

–take a course or study the references

–try them for yourself

•

•Measure your quality and productivity before and after and decide if they are effective for you.

•Tell others what you find.

Assignment #9•Read appendix B in the text and work through the examples

•Using PSP2.1, write program 8A to sort a linked list of N records into ascending order on a real number field in the record Allow the user to select the field to be used for sorting after the list has been created. User may want to sort same list on different fields Should not have to reenter to do so.

•Read the program specifications in Appendix D and the PSP2.1 description in Appendix C. Follow the assignment specifications in Appendix C.

Messages to Remember from Lecture 9 - 1

•1. A poor design notation is a source of error.

•2. By using a defined and rigorous notation,

• you can improve the quality of your designs

• and eliminate defects.

Messages to Remember from Lecture 9 - 2

•3. Select and use an appropriate notation from

• among those available.

•4. Try formal methods in your software

• development and see if they work for you.

Disciplined Software Engineering Lecture #9

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