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    <p>Paul's answer might be the best, but here's what I came up with over my lunch break (untested, admittedly, but still a fun exercise in thinking). Not sure of the quickness/optimization of my algorithm. I'd personally suggest looking at something like MongoDB, a NoSQL database, since it seems it may lend itself nicely to solving this kind of problem (what with map/reduce and all)</p> <pre><code># assuming events is a dictionary of id keyed to item bought... user = {} for (cust_id, item) in events: if not cust_id in users: user[cust_id] = set() user[cust_id].add(item) # now we have a dictionary of cust_ids keyed to a set of every item # they've ever bought (given that repeats don't matter) # now we construct a dict of items keyed to a dictionary of other items # which are in turn keyed to num times present items = {} def insertOrIter(d, k, v): if k in d: d[k] += v else: d[k] = v for key in user: # keep track of items bought with each other itemsbyuser = [] for item in user[key]: # make sure the item with dict is set up if not item in items: items[item] = {} # as we see each item, add to it others and others to it for other in itemsbyuser: insertOrIter(items[other], item, 1) insertOrIter(items[item], other, 1) itemsbyuser.append(item) # now, unless i've screwed up my logic, we have a dictionary of items keyed # to a dictionary of other items keyed to how many times they've been # bought with the first item. *whew* # If you want something more (potentially) useful, we just turn that around to be a # dictionary of items keyed to a list of tuples of (times seen, other item) and # you're good to go. useful = {} for i in items: temp = [] for other in items[i]: temp[].append((items[i][other], other)) useful[i] = sorted(temp, reverse=True) # Now you should have a dictionary of items keyed to tuples of # (number times bought with item, other item) sorted in descending order of # number of times bought together </code></pre>
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    1. POOptimize algorithm for creating a list of items rated together, in Python
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    1. COEmbrace the defaultdict! Never again check on the existence of a dict key before updating its value - just access the key and let the defaultdict initialize it if it isn't there. The easiest code to maintain is the code that doesn't exist.
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