Jimmy Tan - 9 days ago 4
Java Question

# Confusing about calculating the probe sequence in double hashing

I know that in Double Hashing,

``````h1(key) = key mod 11
h2(key) = 7 - (key mod 7)
``````

The
`h1`
represents starting at location h1(key),
`h2`
represents the size of the step taken.

But I do not know how to solve for the probe sequence.

For example, if key is
`14`
.

Can you explain to me why the answer is
`3,10,6,2,9,5,1,8,4,0`
.

In your example, the size of the table is 11 (positions numbered 0 to 10). The size of the step is the number to add to the current position to get the next position (modulus the size of the table).

``````h1 = 14 mod 11 = 3
h2 = 7 - (14 mod 7) = 7 - 0 = 7
``````

So, the first position, call it `p`, is 3, as given by `h1`. Each subsequent position, `p'` is given by --

``````p' = (p + h2) mod table_size
``````

for this example,

``````p' = (p + 7) mod 11
``````

so, the second position is --

``````(3 + 7) mod 11 = 10 mod 11 = 10
``````

and the third is --

``````(10 + 7) mod 11 = 17 mod 11 = 6
``````

and so on.