Finding whether a string starts with one of a list's variable-length prefixes

Question

I need to find out whether a name starts with any of a list\'s prefixes and then remove it, like:

if name[:2] in [\"i_\", \"c_\", \"m_\", \"l_\", \"d_\", \"t         


        

Answer 1

When it comes to search and efficiency always thinks of indexing techniques to improve your algorithms. If you have a long list of prefixes you can use an in-memory index by simple indexing the prefixes by the first character into a dict.

This solution is only worth if you had a long list of prefixes and performance becomes an issue.

pref = ["i_", "c_", "m_", "l_", "d_", "t_", "e_", "b_"]

#indexing prefixes in a dict. Do this only once.
d = dict()
for x in pref:
        if not x[0] in d:
                d[x[0]] = list()
        d[x[0]].append(x)


name = "c_abcdf"

#lookup in d to only check elements with the same first character.
result = filter(lambda x: name.startswith(x),\
                        [] if name[0] not in d else d[name[0]])
print result