Personal tools

Memoization

From HaskellWiki

(Difference between revisions)
Jump to: navigation, search
(Int type for list indexes)
(link to The Fibonacci sequence)
Line 3: Line 3:
 
'''Memoization''' is a technique for storing values of a function instead of recomputing them each time the function is called.
 
'''Memoization''' is a technique for storing values of a function instead of recomputing them each time the function is called.
   
A classic example is the recursive computation of [[Fibonacci number]]s.
+
A classic example is the recursive computation of [[The Fibonacci sequence|Fibonacci numbers]].
   
The immediate implementation of Fibonacci numbers without memoization is horribly slow.
+
The naive implementation of Fibonacci numbers without memoization is horribly slow.
 
Try <hask>slow_fib 30</hask>, not too much higher than that and it hangs.
 
Try <hask>slow_fib 30</hask>, not too much higher than that and it hangs.
 
<haskell>
 
<haskell>

Revision as of 20:31, 5 August 2007


Memoization is a technique for storing values of a function instead of recomputing them each time the function is called.

A classic example is the recursive computation of Fibonacci numbers.

The naive implementation of Fibonacci numbers without memoization is horribly slow.

Try
slow_fib 30
, not too much higher than that and it hangs.
slow_fib :: Int -> Integer
slow_fib 0 = 0
slow_fib 1 = 1
slow_fib n = slow_fib (n-2) + slow_fib (n-1)

The memoized version is much faster.

Try
memoized_fib 10000
.
memoized_fib :: Int -> Integer
memoized_fib =
   let fib 0 = 0
       fib 1 = 1
       fib n = memoized_fib (n-2) + memoized_fib (n-1)
   in  (map fib [0 ..] !!)


See also