10 Nov 1.HW 3 Remember to submit a.jsp file: for a valid int for an int with errors that produces the java program 2.Structure clashes 3.HW 4: Backtracking Exercise

Data structure for a well-formed Integer

Possible errors We want the error messages to be useful. What are the different possible errors? 1.Start with non-sign, non-digit 2.Signed integer i.Non-digit immediately after sign ii.Non-digit in digit body 3.Unsigned integer i.Non-digit in digit body

Data structure for identifier with possible errors

Simple Programs The design method described above applies only to a class of "simple programs'. A 'simple program' has the following attributes: 1.The program has a fixed initial state; nothing is remembered from one execution to the next. 2.Program inputs and outputs are serial files. There may be more than one input and more than one output file. Each input and output file associated with the program is a tree structure and can be described with a grammar for regular expressions The input data structures define the domain of the program, the output data structures its range. Nothing is introduced into the program text which is not associated with the defined data structures. 3.The data structures are compatible, in the sense that they can be combined into a program structure as we done previously –The program structure derived from the data structures is sufficient for a workable program. –Elementary operations of the program language (possibly supplemented by more powerful or suitable operations resulting from bottom-up designing) are allocated to components of the program structure without introducing any further 'program logic'.

More complex programs: Structure Clashes Three types of structural clash: 1.Boundary clash (physical blocks on input don’t coincide with logical groups on output) 2.Ordering clash (wish report sorted by named, input is unsorted) 3.Interleaving clash (wish to produce report of user job times, inputs are interleaved)

Boundary clash example: An accounting program may have a boundary clash between an input file structured by months and an output file structured by weeks. from Jackson JSP In Perspective (2001)

Examples: Boundary Clashes Boundary clashes: The calendar consists of years, each year consisting of a number of days. In one structure the days may be grouped by months, but by weeks in another structure. There is a boundary clash here: the weeks and months can not be synchronised. A chapter of a printed book consists of text lines. In one structure the lines may be grouped by paragraphs, but in another structure by pages. There is a boundary clash because pages and paragraphs can not be synchronised. A file in a low-level file handling system consists of variable-length records, each consisting of between 2 and 2000 bytes. The records must be stored sequentially in fixed blocks of 512 bytes. There is a boundary clash here: the boundaries of the records can not be synchronised with the boundaries of the blocks.

Solution to Structural Clashes General view of problem Structure diagram Solution: program decomposition Program decomposition 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

Processing - general view: 1.Batch processing ( пакетная обработка данных) 2.Parallel processing 3.Quasi-parallel processing (program inversion) program inversion

Program Inversion инверсия программов We wish to design PA and PB independently of each other so that the structure of each is based on the data structure representation of its own problem environment. 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.

HW 4: Backtracking Exercise Warehouse (товарный склад) A warehouse records a transaction for every item received into or issued out of the warehouse: Transaction: 0-3 Item code 4‘+’ (in), ‘-’ (out) 9-10 initialization (контроль)

Backtracking Exercise: Warehouse (товарный склад) At the end of the day, the transaction file is sorted by item number, and a "Daily Net Movement Summary" showing the net movement of each item into or out of the warehouse is produced as shown below: Daily Summary A A Z End Summary

Backtracking Exercise: Warehouse (товарный склад) 1. However, parts are to be included on the report only if every shipment and order transaction has been properly verified, as indicated by a code in the initials field on each transaction record. 2.Parts with one or more unverified shipment or order transactions, indicated by a blank in the initials field, should be written on an error listing; they are not to be included in the report (no total line should be generated).

Backtracking exercise We don't know how many shipment and order transactions exist for any part number. –Thus, we don't know how many records we have to read in order to recognize whether we have a part all of whose shipments and orders have been verified. –Need to use backtracking Develop solution in 3 stages, creating a.jsp file for: –Stage 1 (ignoring the recognition difficulty –Stage 2 (using posit and admit) –Stage 3 (taking care of side effects) –Elaborated program structure