Problem

Consider the effect of introducing lookahead to the design of the corrected third refinement of the sequential master file 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.