programming-for-dummies

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Sequential Search
Block searching
Another technique to speed up sequential searching on sorted data is jump,
or block searching. Rather than search one item at a time, this method jumps
over a fixed number of items (such as five) and then examines the last item:
✦ If this last item is greater than the value the algorithm is trying to
find, the algorithm starts searching backward.
✦ If this last item is less than the value the algorithm is trying to find,
the algorithm jumps another block forward, as shown in Figure 2-3.
Figure 2-3:
Jump, or
block,
searching
can speed
up a
sequential
search on
sorted data.
18 46 89 93 102 146 198
Searching for the number 102 takes four
jumps in a normal sequential search.
18 46 89 93 102 146 198
Searching for the number 102 can only
take two jumps in a block search.
The block searching algorithm works like this:
1. Jump ahead a fixed number of items (a block).
2. Compare the last value of the block:
• If this value is less than the data to find, search sequentially within
the block.
• Otherwise, jump to the end of a new block and repeat Step 2.
The basic idea behind block searching is to skip ahead through a sorted list
of data and then slow down when it gets closer to that data. This is like looking
through a telephone book for the name Winston Smith by skipping every
ten pages until you reach the S section and then searching sequentially until
you find the name Smith and finally the name Winston Smith.
Block searching can work only with sorted data. If data isn’t sorted, block
searching can’t work at all.