Well, let's see. You read the 9 track tape in the forward direction till you get a record of EBCDIC blanks, then skip to the end of file and do a read backwards (the operation does exist!) on the tape back to the original record. :-) :-)

Sorry, that was with record oriented files. Now days most file systems are continuous streams of characters, and the record boundaries are defined by context. The normal operation to read from a file does so in the forward direction and advances the pointer by that same amount. This is the problem. if you desire to read a file in the reverse direction, you must do so with forward semanticts[sp?]. This means you must "backup" a record, read the record just backed over, then "backup" again. It isn't easy, but it can be done. Others have indicated there is a Perl module to help you in this, and if done correctly ("tie" to a file handle comes to mind) it might even seem like a normal operation.

That being said, reading in the reverse direction isn't commonly done, as there are usually better ways to do the same thing. Reading the records into a hash, and operating on them that way is probably a "better idea". Also consider the original dataset. It might be better formatted at the source to eliminate the need to do the fancy dance with the data. A good look at "why" you need to access the data in this manner might pay off better in the long run.

Good luck!

In reply to Re: read a file in both directions by herby1620
in thread read a file in both directions by Anonymous Monk

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