Slide 7.2: Variable-length record addition demonstrations (cont.)

Slide 7.1: Variable-length record addition demonstrations
Slide 7.3: Variable-length record addition demonstrations (cont.)
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Variable-Length Record Addition Demos (Cont.)

The system may immediately transform the downloaded file avail.txt into the following file avail1.txt:

-1    531  
76    12    File Structures: An Object-Oriented Approach with C++|0201874016|94.80|360|
46    100   Learning WML & WMLScript|1565929470|17.48|12|
52    158   XML in a Nutshell, 2nd Edition|0596002920|39.95|39|
36    222   Java and XSLT|0596001436|26.37|890|
82    270   WAP Servlets: Developing Dynamic Web Content With Java and WML|047139307|32.99|4|
60    364   WAP Development with WML and WMLScript|0672319462|18.99|56|
83    436   Advances in Security and Payment Methods for Mobile Commerce|1591403456|89.95|182|
76    531   M Commerce: Technologies, Services, and Business Models|0471135852|23.09|5|

where

Each of the head/last/offsets takes five bytes.
The first record is head+' '+last+'\n' where head is the head (offset) of the avail list and last is the offset of the last record.
For the remaining records, each record begins with length+' '+offset+' ' where
- the length is the actual record length including '\n' but not including the length+' '+offset+' ' , and
- the offset is the disk offset right before the length.

After processing for a while, the file may become the following file avail2.txt:

222   531  
76    12    File Structures: An Object-Oriented Approach with C++|0201874016|94.80|360|
46    100   *|-1     WML & WMLScript|1565929470|17.48|12|
52    158   XML in a Nutshell, 2nd Edition|0596002920|39.95|39|
36    222   *|436    XSLT|0596001436|26.37|890|
82    270   WAP Servlets: Developing Dynamic Web Content With Java and WML|047139307|32.99|4|
60    364   WAP Development with WML and WMLScript|0672319462|18.99|56|
83    436   *|100    in Security and Payment Methods for Mobile Commerce|1591403456|89.95|182|
76    531   M Commerce: Technologies, Services, and Business Models|0471135852|23.09|5|

where the asterisk * means the record is deleted. These file structures were discussed in the previous slides.

^†Note that the above file structures are designed by the instructor. Various other file structures are available or can be designed.