Updating Sequential Files

Data that is formatted and written to a sequential file as shown in Section 17.4 cannot be modified without the risk of destroying other data in the file. For example, if the name "White" needs to be changed to "Worthington," the old name cannot be overwritten without corrupting the file. The record for White was written to the file as

300 White 0.00

If this record were rewritten beginning at the same location in the file using the longer name, the record would be

300 Worthington 0.00

The new record contains six more characters than the original record. Therefore, the characters beyond the second "o" in "Worthington" would overwrite the beginning of the next sequential record in the file. The problem is that, in the formatted input/output model using the stream insertion operator << and the stream extraction operator >>, fieldsand hence recordscan vary in size. For example, values 7, 14, 117, 2074, and 27383 are all ints, which store the same number of "raw data" bytes internally (typically four bytes on today's popular 32-bit machines). However, these integers become different-sized fields when output as formatted text (character sequences). Therefore, the formatted input/output model usually is not used to update records in place.

Such updating can be done awkwardly. For example, to make the preceding name change, the records before 300 White 0.00 in a sequential file could be copied to a new file, the updated record then would be written to the new file, and the records after 300 White 0.00 would be copied to the new file. This requires processing every record in the file to update one record. If many records are being updated in one pass of the file, though, this technique can be acceptable.





C++ How to Program
C++ How to Program (5th Edition)
ISBN: 0131857576
EAN: 2147483647
Year: 2004
Pages: 627
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