SWT - Diagrammatics

Lecture 2/4 - Diagramming in Computer Science27-April-2000

Review

• Definitions,• Historical Facts,

– Maps, Geometry, Topological Diagrams, Science and Diagrams.

• Advantages of Diagramming,• Diagram use across fields.

Overview

• Diagram Distinctions • Diagram Taxonomies• Diagram use in Computer Science

– Venn, Flowcharts, NSD’s, Structure, Dataflow, ERD’s, Cell and Arrows, State, Petri nets.

• Logic Gate Diagrams• How to operate Theseus under CM

Diagram Distinctions• Diagrams portray associations:

– metric, – topological and – symbolic

• In computer systems, above the hardware level, Euclidean space is unimportant

• Much more common in software diagrams are associations in topological space.

Diagram Distinctions• Within topological diagrams, associations can happen in

three principal ways:– Adjoinment

– Linkage

– Containment

A B

A B

A B

Diagram Taxonomies• A Taxonomy is useful only to the ones that use it.• A simple diagrammatic taxonomy can be based on the

diagrammatic domain.• Several researchers have focused on a variety of

diagrammatic aspects and have proposed respective classifications.

• Efforts to categorise diagrams have created a large set of taxonomies.

• Alan. Blackwel * has proposed a taxonomy of taxonomies :* http://www.mrc-cbu.cam.ac.uk/projects/twd/mypapers/TwD98.html

A taxonomy of Diagram Taxonomies• Dimensions of categorisation :

– 1. The representation :• the graphic domain & structure

– 2. The message :• the information domain

– 3. The relation between the representation and the message :• Pictorial correspondence

– 4. The process of interpreting and modifying representations :• Information processing & tools

– 5. The context - convention :• Cultural conventions

– 6. The mental representation• Interpersonal variation

Venn Diagrams• Are related to circuits and logic gates• Elements use containment to depict information• Standard mathematical functions like “a set of”,

“a genuine subset of” etc are depicted• Easy to compare Venn Diagrams because of

their visual representation

Venn Diagrams• The two following Venn diagrams show that the next two

functions are equal:– NOT(A OR B)– (NOT(A)) AND (NOT(B))

• You can write a small java program to verify this if you want!

NOT(A OR B) (NOT(A)) AND (NOT(B))

NOT A

NOT B

non shaded part is equal to double-shaded part

A

B

Flowcharts• They are topological, graph-based constructions that are

often filled with program text.• The control logic of the program is shown through simple

branches and loops.• They are usually generated by analysts as a specification

to programmers, who then convert the charts into source code.

• However, for large systems, they can get messy, spanning in many pages as decisions have many branches.

• Goldstine claims he created the first flowchart for computers in 1947, while working with Von Neumann.

Flowchartsstart

format disk

formatanother?

get formatting parameters

end

yes

noI/O

Terminal

DisplayIf

Process

Nassi-Shneiderman (NS) Diagrams• Hierarchy is shown using

enclosure and adjacency• Decisions are shown by splitting the

lines into smaller, parallel boxes• Loops are shown by enclosing a small

box into a box labelled with the condition of the loop

• However, the early termination of loops (e.g. break) and multiple conditionals present problems for NS diagrams

Nassi-Shneiderman (NS) Diagramss1

if i1true false

if i2

while w1

while w2

true falses2

s3s4 s5

s6

s7

s8

Structure Diagrams• They are hierarchical, modular

break downs of a program• Between tree levels, links indicate what

kind of information travels between levels

• They are usually represented by trees• They are a part of the “structural analysis” activity, in

which a system is partitioned in a top-down manner• However, a multitude of labelled edges and nodes

conveying lots of information can reduce readability

Structure Diagramscalculate payroll

get employeeand pay record

calculate net pay

printpay cheque

calculate deductions

calculate tax

records

record name

employee name

salarypayment

employee name

payment

date & time

overtime

tax deductions

employee category expenses

Data-flow Diagrams

• Are oriented to flow-type operations• Objects of data are shown in relationship to procedures• No decision logic is shown• The diagrams are most often used to model the flow of

data• However, they usually get large and complex and

multiple-page spanning happens nearly always

Dataflow DiagramsSALES DEPT

ORDER FILE

CUSTOMER FILE

GETCUSTOMER

RECORD

CHECKCUSTOMER

CREDIT ORDER INFORMATION

CREATE NEWCUSTOMER

RECORD

CUSTOMER RECORD

NEW CUSTOMER INFORMATION

CUSTOMER FILE

NEW CUSTOMER RECORD

COLLECT ALL ORDERS FOR

CUSTOMER

INVALID CUSTOMER

CUSTOMERINFORMATION

CUSTOMER ORDER RECORDS

Entity-Relationship (ER) Diagrams

• The representation of data is often accomplished using diagrams

• ER diagrams are usually used to depict databases• Extremely simple – three types of nodes• Entity and Relations form a graph and can have

associated attributes and cardinality

Attribute Entity Relationship

Entity-Relationship (ER) Diagrams

DepartmentEmployee Manager

CompanyPerson

Manages

IsA IsA

Works-forName

SSN Salary

Job title

NameAddress

Phone nr

Name

HasMany

Software Level Charts• At a higher level, the functions of a system are often

thought as layers• These diagrams work only on simple access schemes• However, more complex

schemes will result in a complex graph that cannot be represented with adjoining regions

Application

Unix Operating System

Unix API

X-lib DB API

Motif

Cell and Arrow Diagrams• In a combination of adjoinment and link-based

conventions, data structures are often depicted as adjacent memory locations linked by pointers

• This is usually used for teaching purposes or for program documentation

• In programming, pointer manipulation of linked lists is shown as diagrams of Cell and Arrows

Cell and Arrow Diagrams

start

element1

element2

elementN …

start

Insertion of a new element (element2) into a linked list

temp

NULL

element1 elementN

NULL

(1)

(2)

(3)

State Transition Diagrams• Well known in computer science as originating from the

study of finite automata• Are used for modelling a variety of event-based CS

domains including parsing, user interface design, and circuit design

• At the application level, they represent transaction flows, appliance controls, marketing scripts etc

• With the exception of special symbols and terminal nodes, all nodes are treated the same.

State Transition DiagramsRecognise if the pattern “bc” exists in string “aabaaabbabca”

s1 s2 s3b

ac

a

b

c

Petri Nets• Are closely related to data flow graphs• The main distinction is that the graphs are bipartite,

made up of a set of places and transitions• Useful for concurrent, asynchronous, distributed,

parallel, and nondeterministic systems.• Each type of node can be further subdivided into

subtypesannihilator

generator

trivial transition

splitting

collection

terminal place

initial place

trivial place

branching

junction

Petri NetsD available

Request D Request D

D Ready

FinishedWith D and P

D Ready

FinishedWith D and P

P available

ProcessProcess ReleaseD and P

ReleaseD and P

P ReadyP Ready

Request PRequest P

Logic Gates• Logic gates are the components of logic circuits• There are three main gates: AND, OR and NOT• An AND gate returns “true” (or 1) if both of its inputs are

“true”• An OR gate returns “true” (or 1) if either of its inputs is

“true”• A NOT gate returns the opposite of its input

AND OR NOT

A B Q0 0 00 1 01 0 01 1 1

A B Q0 0 00 1 11 0 11 1 1

A Q0 11 0

Logic Circuits• Logic circuits are used in electronic devices.• Formed by combining many logic gates• More complex logic circuits are assembled from

simpler ones which in turn are assembled from gates

A B Q0 0 00 1 11 0 11 1 0

Combining gates Together• A N input gate can be constructed by placing N

gates in a special configuration :

xyz

xyz

o

o

xyz o

xyz

o

Other Logic Gates

• Include: nand

nor

xnor

xor

buffer

A B Q

0 0 1

0 1 0

1 0 0

1 1 0

A B Q

0 0 1

0 1 1

1 0 1

1 1 0

A B Q

0 0 0

0 1 1

1 0 1

1 1 0

A B Q

0 0 1

0 1 0

1 0 0

1 1 1

Analog Circuit Diagrams

• Represent electronic design schemata• Strict Notation - Graph-Network topology• No direction

The end of lecture 2