In some applications, an alternative to lookahead can be achieved by cleverly ordering the transactions. For example, the original problem caused by a modification followed by a deletion of the same master fi le record could have been solved by processing a deletion before a modification. This would have resulted in the master fi le being written correctly and an error message appearing in the exception report. Investigate whether there is an ordering of the transactions that can solve all the difficulties listed in Problems 5.2 through 5.4.

Problems 5.2:

Check whether your answer to Problem 5.1 can correctly handle a modification transaction followed by a deletion transaction, both transactions being applied to the same master fi le record. If not, modify your answer.

Problems 5.3:

Check whether your answer to Problem 5.1 also can correctly handle an insertion followed by a modification followed by a deletion, all applied to the same master fi le record. If not, modify your answer.

Problems 5.4:

Check whether your answer to Problem 5.1 can also handle correctly n insertions, modifications, or deletions, n > 2, all applied to the same master file record. If not, modify your answer.

Problem 5.1:

Consider the effect of introducing lookahead to the design of the corrected third refinement of the sequential master fi le update problem. That is, before processing a transaction the next transaction must be read. If both transactions apply to the same master fi le record, then the decision regarding the processing of the current transaction depends on the type of the next transaction. Draw up a 3 × 3 table with the rows labeled by the type of the current transaction and the columns labeled by the type of the next transaction and fill in the action to be taken in each instance. For example, two successive insertions of the same record clearly are an error. But two modifications may be perfectly valid; for example, a subscriber can change address more than once in a given month. Now develop a flowchart for the third refinement that incorporates lookahead.