[R] on how to make a skip-table

[gildororonar at mail-on.us]

Quoting "Zhang Weiwu" <zhangweiwu at realss.com>:

> Jim Holtman asked me to elaborate the problem:
>
>     It is a common problem in reading sparse variable-lenght record data
>     file.  Records are stored in file one next to another. The length of
>     each record is known in advance, but a lot of them records are invalid,
>     and should be skipped to make efficient use of memory.
>
>     Ideally the datafile-reading routine should receive a skip-table. Before
>     reading each wanted/valid record, it seeks forward for the distance
>     given in the skip-table. The problem is how to obtain such a skip table.


Ideally, in C, yes, you need to calculate the steps, because you use a  
pointer.

Jim Holtman asks what you intend to do, I guess because he sees the  
problem may a different solution in R.

What you need is not a skip-table, but a offset table, and you skip  
from the begining to the offset directly, not from the previous  
position.

e.g. instead of skip-1, skip-3, skip-5, skip-7
you should do: offset-1, offset-4, offset-9, offset-15

Consider the skip-table is about the reverse of cumsum, and is harder  
to achieve, you should start with offset-table.


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