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8 Nov

8 Nov. Backtracking Structure clashes & program decomposition. Backtracking. In the forms of iteration and selection used in JSP, the condition test precedes execution of a part: we use read-ahead rule to handle serial input files if pattern is a single character or record

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8 Nov

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  1. 8 Nov Backtracking Structure clashes & program decomposition

  2. Backtracking In the forms of iteration and selection used in JSP, the condition test precedes execution of a part: • we use read-ahead rule to handle serial input files if pattern is a single character or record • we use multiple read ahead rule if pattern is some fixed number, n, of characters (or records) • use backtracking if there is no fixed number, n

  3. Backtracking Problem: A cardfile of punched cards is sorted into ascending sequence of values of a key which appears in each card. • 1st card for each group with a common key value is a header card • Other records are detail cards. • Each detail card carries an integer amount Sample CardFile • It is required to produce a report showing the totals of amount for all keys

  4. Corresponding data structures

  5. Basic Program Structure

  6. Backtracking Modified problem: file has a record-type field: • 'header‘ - indicates group-id of the set of records that follow • 'detail' - indicates that record belongs to a group and contains data • other – if not of type ‘header’ or ‘detail’, record is an error • Program should detect the following errors: • header missing • detail cards have some other identifier (not ‘detail’) • Groups containing errors should be listed as an errorlist (not totaled)

  7. Structure of Error List & Program

  8. Backtracking Recognition problem: We cannot recognize a good group with any predetermined fixed read ahead. We tackle the recognition problem using ‘backtracking’ in 3 stages: Stage a: Assume (posit) that a group will be good (ignore the recognition difficulty) and design the full program accordingly. Stage a Stage b: Verify each data along the way – if any data contradicts the assumption of good data, issue a quit statement that branches to the second selection (badgroup) Replace ‘select’ and ‘or’ with ‘posit’ and ‘admit’; check for header and detail errors and issue quit statements Stage b Stage c: Modify the program text resulting from stage b. to ensure that side-effects are repealed where necessary • note state of file and buffer (immediately following posit) • restore state of file and buffer (immediately following admit) Comment: By breaking the problem into distinct stages, we isolate difficulties and can focus on correct design of each partial solution

  9. Backtracking control structure • Backtracking control structure • Javal implentation of backTracking

  10. Exception Handling in Java Backtracking • structure diagram and structure text • Java implementation Try (стараться)..catch (поймать)..finally(окончательно) in JSP • Java Exception Handling in JSP Example: TryException • NumberFormatException and ArrayOutOfBoundsException are unchecked (несдержанный) 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. HelloRead2

  11. Sample Cardfile H 0001 D 0001 012 D 0001 020 H 0002 D 0002 015 H 0003 D 0003 025 D 0003 -015

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