Apriori算法的Python实现

永不落伍

　　Apriori算法是数据挖掘中频发模式挖掘的鼻祖，从60年代就开始流行，其算法思想也十分简单朴素，首先挖掘出长度为1的频繁模式，然后k=2
　　将这些频繁模式合并组成长度为k的频繁模式，算出它们的频繁次数，而且要保证其所有k-1长度的子集也是频繁的，值得注意的是，为了避免重复，合并的时候，只合并那些前k-2个字符都相同，而k-1的字符一边是少于另一边的。
　　以下是算法的Python实现：

__author__ = 'linfuyuan'
min_frequency = int(raw_input('please input min_frequency:'))
file_name = raw_input('please input the transaction file:')
transactions = []

def has_infrequent_subset(candidate, Lk):
for i in range(len(candidate)):
subset = candidate[:-1]
subset.sort()
if not ''.join(subset) in Lk:
return False
lastitem = candidate.pop()
candidate.insert(0, lastitem)
return True

def countFrequency(candidate, transactions):
count = 0
for transaction in transactions:
if transaction.issuperset(candidate):
count += 1
return count

with open(file_name) as f:
for line in f.readlines():
line = line.strip()
tokens = line.split(',')
if len(tokens) > 0:
transaction = set(tokens)
transactions.append(transaction)
currentFrequencySet = {}
for transaction in transactions:
for item in transaction:
time = currentFrequencySet.get(item, 0)
currentFrequencySet[item] = time + 1
Lk = set()
for (itemset, count) in currentFrequencySet.items():
if count >= min_frequency:
Lk.add(itemset)
print ', '.join(Lk)
while len(Lk) > 0:
newLk = set()
for itemset1 in Lk:
for itemset2 in Lk:
cancombine = True
for i in range(len(itemset1)):
if i < len(itemset1) - 1:
cancombine = itemset1 == itemset2
if not cancombine:
break
else:
cancombine = itemset1 < itemset2
if not cancombine:
break
if cancombine:
newitemset = []
for char in itemset1:
newitemset.append(char)
newitemset.append(itemset2[-1])
if has_infrequent_subset(newitemset, Lk) and countFrequency(newitemset, transactions) >= min_frequency:
newLk.add(''.join(newitemset))
print ', '.join(newLk)
Lk = newLk