Calculating catalan numbers with memoization

Question

I am trying to use memoization in order to calculate catalan numbers, but it just does not seem to work, what do I need to change?

def catalan_mem(n, mem = None):
    if n==0:
        return 1
    if mem==None:
        mem={}
    sum=0
    if n not in mem:
        for i in range(0, n):
            sum = sum + catalan_mem(i) * catalan_mem(n-i-1)
        mem[n]=sum
    return mem[n]

pass `mem` as parameter to the recursive calls – user2390182 Dec 01 '15 at 14:21 — user2390182, Dec 01 '15 at 14:21

score 0 · Answer 1 · answered Dec 01 '15 at 14:23

0

You need to pass the cache to your recursive calls:

sum = sum + catalan_mem(i, mem) * catalan_mem(n-i-1, mem)

Otherwise, every method call creates a new empty cache.

answered Dec 01 '15 at 14:23

user2390182

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Calculating catalan numbers with memoization

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