programming-for-dummies

358
Understanding Hash Tables
Avoiding collisions with double hashing
Another way to avoid collisions is to use double hashing:
1. The hash function calculates a hash value for each key.
2. If a collision occurs, the computer calculates a second hash value.
Essentially, you wind up with a hash table within another hash table, as
shown in Figure 3-10.
1
2
With double-hashing, the first hash table points to
a second hash table, which points to the data.
3
4
5
1
6
2
7
3
Figure 3-10:
Double
hashing
creates
miniature
hash tables
within a
larger hash
table.
8
Hash table #1
Hash table #2
4
5
6
7
8
Dick
Ross
Sally
Evans
Double hashing can reduce the number of duplicate hash values (collisions),
but here are a couple of drawbacks:
✦ A collision can occur even after the double hashing.
✦ Double hashing is a more complicated solution than chaining.
The more complex a program, the greater the chances of something
going wrong.