我在 python 中实现了 boyer moore 算法,我需要计算子字符串在字符串中出现的次数。
我的字符串存储在一个向量中:
string = ['A', 'B', 'B', 'C', 'F', 'D', 'B', 'B']
针也是一个向量:
needle = ['B', 'B']
我的问题是:
我实现的算法只返回第一次出现针的索引。在上面的示例中,它返回1
,但是,正如我之前所说,我需要每次计算针是否出现在数组中,并期望它返回 2;
示例和预期回报
1
haystack = ['A', 'B', 'B', 'C', 'F', 'D', 'B', 'B']
needle = ['B', 'B']
expected return = 2
2
haystack = ['A', 'F', 'T', 'C', 'B', 'F', 'T', 'a', 'F', 'T']
needles = ['F', 'T']
expected return = 3
我的代码i 在search
函数中,我尝试用 更改return i
(索引)counter += 1
来计算匹配的针数,但是,如果我这样做,它会给我以下错误:
Traceback (most recent call last):
File "booyer-moore.py", line 103, in <module>
main(files)
File "booyer-moore.py", line 98, in main
index = search(haystack, needle)
File "booyer-moore.py", line 18, in search
i += max(offset_table[len(needle) - 1 - j], character_table.get(haystack[i]))
IndexError: list index out of range
def search(haystack, needle):
counter = 0
if len(needle) == 0:
return 0
character_table = createCharacterTable(needle)
offset_table = createOffsetTable(needle)
i = len(needle) - 1
while i < len(haystack):
j = len(needle) - 1
while needle[j] == haystack[i]:
if j == 0:
return i
# counter += 1
i -= 1
j -= 1
i += max(offset_table[len(needle) - 1 - j], character_table.get(haystack[i]))
return counter
def createCharacterTable(needle):
table = {}
for i in range(len(needle) - 1):
table[needle[i]] = len(needle) - 1 - i
return table
def createOffsetTable(needle):
table = []
lastPrefixPosition = len(needle)
for i in reversed(range(len(needle))):
if isPrefix(needle, i + 1):
lastPrefixPosition = i + 1
table.append(lastPrefixPosition - i + len(needle) + 1)
for i in range(len(needle) - 1):
slen = suffixLength(needle, i)
table[slen] = len(needle) - 1 - i + slen
return table
def isPrefix(needle, position):
j = 0
for i in range(position, len(needle)):
if needle[i] != needle[j]:
return False
j += 1
return True
def suffixLength(needle, position):
length = 0
j = len(needle) - 1
for i in reversed(range(position + 1)):
if needle[i] == needle[j]:
length += 1
else:
break
j -= 1
return length