Whats the best way to filter out some subsets from a generator. For example I have a string "1023" and want to produce all possible combinations of each of the digits. All combinations would be:
['1', '0', '2', '3']
['1', '0', '23']
['1', '02', '3']
['1', '023']
['10', '2', '3']
['10', '23']
['102', '3']
['1023']
I am not interested in a subset that contains a leading 0 on any of the items, so the valid ones are:
['1', '0', '2', '3']
['1', '0', '23']
['10', '2', '3']
['10', '23']
['102', '3']
['1023']
I have two questions.
1) If using a generator, whats the best way to filter out the ones with leading zeroes. Currently, I generate all combinations then loop through it afterwards and only continuing if the subset is valid. For simplicity I am only printing the subset in the sample code. Assuming the generator that was created is very long or if it constains a lot of invalid subsets, its almost a waste to loop through the entire generator. Is there a way to stop the generator when it sees an invalid item (one with leading zero) then filter it off 'allCombinations'
2) If the above doesn't exist, whats a better way to generate these combinations (disregarding combinations with leading zeroes).
Code using a generator:
import itertools
def isValid(subset): ## DIGITS WITH LEADING 0 IS NOT VALID
valid = True
for num in subset:
if num[0] == '0' and len(num) > 1:
valid = False
break
return valid
def get_combinations(source, comb):
res = ""
for x, action in zip(source, comb + (0,)):
res += x
if action == 0:
yield res
res = ""
digits = "1023"
allCombinations = [list(get_combinations(digits, c)) for c in itertools.product((0, 1), repeat=len(digits) - 1)]
for subset in allCombinations: ## LOOPS THROUGH THE ENTIRE GENERATOR
if isValid(subset):
print(subset)
Filtering for an easy and obvious condition like "no leading zeros", it can be more efficiently done at the combination building level.
def generate_pieces(input_string, predicate):
if input_string:
if predicate(input_string):
yield [input_string]
for item_size in range(1, len(input_string)+1):
item = input_string[:item_size]
if not predicate(item):
continue
rest = input_string[item_size:]
for rest_piece in generate_pieces(rest, predicate):
yield [item] + rest_piece
Generating every combination of cuts, so long it's not even funny:
>>> list(generate_pieces('10002', lambda x: True))
[['10002'], ['1', '0002'], ['1', '0', '002'], ['1', '0', '0', '02'], ['1', '0', '0', '0', '2'], ['1', '0', '00', '2'], ['1', '00', '02'], ['1', '00', '0', '2'], ['1', '000', '2'], ['10', '002'], ['10', '0', '02'], ['10', '0', '0', '2'], ['10', '00', '2'], ['100', '02'], ['100', '0', '2'], ['1000', '2']]
Only those where no fragment has leading zeros:
>>> list(generate_pieces('10002', lambda x: not x.startswith('0')))
[['10002'], ['1000', '2']]
Substrings that start with a zero were never considered for the recursive step.
One common solution is to try filtering just before using yield
. I have given you an example of filtering just before yield:
import itertools
def my_gen(my_string):
# Create combinations
for length in range(len(my_string)):
for my_tuple in itertools.combinations(my_string, length+1):
# This is the string you would like to output
output_string = "".join(my_tuple)
# filter here:
if output_string[0] != '0':
yield output_string
my_string = '1023'
print(list(my_gen(my_string)))
EDIT: Added in a generator comprehension alternative
import itertools
my_string = '1023'
my_gen = ("".join(my_tuple)[0] for length in range(len(my_string))
for my_tuple in itertools.combinations(my_string, length+1)
if "".join(my_tuple)[0] != '0')
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