# Memoization

### From HaskellWiki

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'''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. |
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− | 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. |
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<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.

Tryslow_fib 30

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.

Trymemoized_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 ..] !!)