22 Nov

1. Comments on HW32. Java I/O – Exception handling

3. Inversion4. JSD

Pascal identifier

Errors in Pascal Identifier

A compiler for INTEGERsWhat do you think about it?

INTEGER

Possible errors

1. Integer does not start with a sign or digit

2. Sign followed by non-digit

3. At least one digit does not follow the sign of a signed INTEGER

4. Non-digit follows 1st digit of an INTEGER

Errors in INTEGER

Design for Human Error

Base data structure that allows for human errors on correct data structure – Allows intelligible messages to explain error in terms of

a deviation from the known structure– Data structure to accommodate errors should contain

all possible errors

Cognitive principle:– We make sense of the unknown / unrecognizable in

reference to what we know / recognize

A note on Java Text I/O

• HelloFileRead2• HelloFileRead2a

If a checked (задержанный) exception throws an exception, you must either catch it or else declare in the method header that the exception is thrown.

Scanner methods catch an exception generated for an empty file

Structure Clashes:Modified Net Movement Problem

Report

Title line Report body End line

Report line*

Report file

Daily transaction

file

Block*

Record count Block body

*

Input file

Transaction record

Suppose the input file is blocked, with each block containing a record count followed by a number of records.

Modified Net Movement Problem:The essence of the difficulty

The program must have an operation that is executed once per block and an operation that is executed once per group

So there must be both a block component and a group component

But we cannot have a single program structure with process block and process group components.

We have a boundary clash--the boundaries of blocks are not synchronized with the boundaries of groups.

Solution to Boundary Clash:Decomposition

Daily Net Movement Report

Input file PA

Inter- mediate

filePB

Input, intermediate and report files, together with their correspondences

Daily transaction

file

Block *

Record count

Block body

*

Daily transaction

file

Transaction record

Transaction record

Input file Intermediate Intermediate Report file

*

Daily transaction

file

*

*Transaction

record

Part group

Daily net movement

report

Title lineReport body End line

Total line *

Program PA Program PB

file file

Comments on JSP Solution

• Correctness– PA and PB can be designed using JSP. The programs we obtain

are distinct. – A serial file forms a boundary between any pair of programs.

We don't have to think "dynamically". • For example, we don't need to ask, "What if a group extends over

several blocks?" or • "What if a group has no data records?“

– We know our programs are correct, because we can think in terms of static data structures

• Inefficient– By introducing an intermediate file, we have roughly doubled the

execution time (in comparison with a program that produced a report without an intermediate file).

– We will learn a little later how to optimize our design by a simple program transformation, program inversion.

General Solution for Structural Clashes

The resolution of the structure clash is effected by decomposing the program P into two programs, PA and PB as shown below:

A PA I PB X

Correspondences between A, I, XThe data structures of the serial input file, A, the

intermediate file, I, and the output file, X are shown below:

* *

*

*

*

A

B

C Z

Y

X

C

I

•There is no structure clash between A and I, so the structure of PA can be constructed without difficulty; the same is true of I and X and PB. • Whereas in general, the description of a file as an iteration of records is rarely sufficient--though always true--in designing a program, in the case of resolving a structure clash, this description is invaluable. •The intermediate file, I, contains the largest component common to both A and X--an idea analogous to the "greatest common divisor" of two integers.

Analysis of PA and PB

Program decomposition allows us to focus on a correct analysis of PA and PB.

The processing for the system diagram

IPA PB

can be accomplished in a number of ways:

1. Batch processing2. Parallel processing (co-routines)3. Inversion (quasi-parallel)

Processing of PA and PB using (a) batch processing, (b) parallel

processing and (c,d) program inversion

IPA PB

A PA

X

PA produces a record and invokes the subroutine PBI which uses it to produce X.

PBI

We say that PBI is inverted with respect to its input file.

PB produces X, invoking the subroutine PAI to obtain the next record.

PAI

PB

A

X

We say that PAI is inverted with respect to its output file.

(a) (b)

(c)

(d)

call PA get

PA:

put call PB

put call PB

put call PB

PB

get(process)

call PA get

(process)

(process)

Program Inversion (инверсия программ)

• Program inversion is a purely mechanical transformation of the independent programs, PA and PB, into a main program and subroutine

• The subroutine has a single ENTRY point and stores its return address within its state vector so that it is resumable.

– 3 states of the subroutine: (i) open the file for rading/writing; (ii) process a record; (iii) close file

Concept of state vector (вектор состояния)

A program consists of:

1. the program text

2. local variables

3. text pointer to next instruction to be executed

state vector

Program Inversion

Significance of Inversion

Uses of inversion

The roots of Object-based analysis in Jackson’s entities

When item transactions are sorted by item # into groups, we recognize each item as an entity (or object). To produce a report showing net movement of each item, weMust process each item group as an iteration of transaction records. It is wrong toProcess the file as a selection of records as below:

Jackson System Development (JSD)

Ideas underlying JSD1. The real-world as modeled: a simulation (analogical model)2. Each entity is JSD is object. JSD is object based3. System is modeled as a network of communicating sequential processes4. JSD specification is in principle executable, and can be transformed into

executable code (using inversion).

JSD method consists of 6 steps: (Student Loan System)1. Entity / Actions 2. Entity Structure3 Initial Model and System Diagram4 Function5. System Timing6 Implementation

Student Loan System(Ñèñòåì äëÿ обработка студенческéõ заемîâ)

• A university gives loans to students. Before getting a loan, there is an evaluation process after which agreement is always reached. A TE transaction records each step of the evaluation process, and a TA transaction records the overall loan agreement. A student can take any number of loans, but only one can be active at any time. Each loan is initiated by a TI transaction. Then, the student repays the loan with a series of repayments. Each repayment transaction is recorded by a TR transaction. Finally, a loan is terminated by a TT transaction.

• Two output functions are desired: (1) an inquiry function that prints out the loan balance for any student, and (2) a repayment acknowledgment

sent to each student after payment is received by the university.

Entity/action step

• Characteristics of entities and actions

• Make candidate list of entities and actions

• Finalize list of entities and actions

Student Loan SystemConsider the following description of a student loan system:

A university gives loans to students. Before getting a loan, there is an evaluation process after which agreement is always reached. A TE transaction records each step of the evaluation process, and a TA transaction records the overall loan agreement. A student can take any number of loans, but only one can be active at any time. Each loan is initiated by a TI transaction. Then, the student repays the loan with a series of repayments. Each repayment transaction is recorded by a TR transaction. Finally, a loan is terminated by a TT transaction.

Two output functions are desired: (1) an inquiry function that prints out the loan balance for any student, and (2) a repayment acknowledgment sent to each student after the university receives payment.

Entity Structure Diagram

Modeling real-world entity as process stream

System diagramNetwork of communicating sequential processes

Using JSP to Implement Functions

System Implementation using Inversion

Implementation of Scheduler using JSP