Get the cartesian product of a series of lists?
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22-08-2019 - |
Question
How can I get the Cartesian product (every possible combination of values) from a group of lists?
Input:
somelists = [
[1, 2, 3],
['a', 'b'],
[4, 5]
]
Desired output:
[(1, 'a', 4), (1, 'a', 5), (1, 'b', 4), (1, 'b', 5), (2, 'a', 4), (2, 'a', 5) ...]
Solution
In Python 2.6+
import itertools
for element in itertools.product(*somelists):
print(element)
Documentation: Python 3 - itertools.product
OTHER TIPS
import itertools
>>> for i in itertools.product([1,2,3],['a','b'],[4,5]):
... print i
...
(1, 'a', 4)
(1, 'a', 5)
(1, 'b', 4)
(1, 'b', 5)
(2, 'a', 4)
(2, 'a', 5)
(2, 'b', 4)
(2, 'b', 5)
(3, 'a', 4)
(3, 'a', 5)
(3, 'b', 4)
(3, 'b', 5)
>>>
For Python 2.5 and older:
>>> [(a, b, c) for a in [1,2,3] for b in ['a','b'] for c in [4,5]]
[(1, 'a', 4), (1, 'a', 5), (1, 'b', 4), (1, 'b', 5), (2, 'a', 4),
(2, 'a', 5), (2, 'b', 4), (2, 'b', 5), (3, 'a', 4), (3, 'a', 5),
(3, 'b', 4), (3, 'b', 5)]
Here's a recursive version of product()
(just an illustration):
def product(*args):
if not args:
return iter(((),)) # yield tuple()
return (items + (item,)
for items in product(*args[:-1]) for item in args[-1])
Example:
>>> list(product([1,2,3], ['a','b'], [4,5]))
[(1, 'a', 4), (1, 'a', 5), (1, 'b', 4), (1, 'b', 5), (2, 'a', 4),
(2, 'a', 5), (2, 'b', 4), (2, 'b', 5), (3, 'a', 4), (3, 'a', 5),
(3, 'b', 4), (3, 'b', 5)]
>>> list(product([1,2,3]))
[(1,), (2,), (3,)]
>>> list(product([]))
[]
>>> list(product())
[()]
with itertools.product:
import itertools
result = list(itertools.product(*somelists))
I would use list comprehension :
somelists = [
[1, 2, 3],
['a', 'b'],
[4, 5]
]
cart_prod = [(a,b,c) for a in somelists[0] for b in somelists[1] for c in somelists[2]]
In Python 2.6 and above you can use 'itertools.product`. In older versions of Python you can use the following (almost -- see documentation) equivalent code from the documentation, at least as a starting point:
def product(*args, **kwds):
# product('ABCD', 'xy') --> Ax Ay Bx By Cx Cy Dx Dy
# product(range(2), repeat=3) --> 000 001 010 011 100 101 110 111
pools = map(tuple, args) * kwds.get('repeat', 1)
result = [[]]
for pool in pools:
result = [x+[y] for x in result for y in pool]
for prod in result:
yield tuple(prod)
The result of both is an iterator, so if you really need a list for furthert processing, use list(result)
.
Here is a recursive generator, which doesn't store any temporary lists
def product(ar_list):
if not ar_list:
yield ()
else:
for a in ar_list[0]:
for prod in product(ar_list[1:]):
yield (a,)+prod
print list(product([[1,2],[3,4],[5,6]]))
Output:
[(1, 3, 5), (1, 3, 6), (1, 4, 5), (1, 4, 6), (2, 3, 5), (2, 3, 6), (2, 4, 5), (2, 4, 6)]
Although there are many answers already, I would like to share some of my thoughts:
Iterative approach
def cartesian_iterative(pools):
result = [[]]
for pool in pools:
result = [x+[y] for x in result for y in pool]
return result
Recursive Approach
def cartesian_recursive(pools):
if len(pools) > 2:
pools[0] = product(pools[0], pools[1])
del pools[1]
return cartesian_recursive(pools)
else:
pools[0] = product(pools[0], pools[1])
del pools[1]
return pools
def product(x, y):
return [xx + [yy] if isinstance(xx, list) else [xx] + [yy] for xx in x for yy in y]
Lambda Approach
def cartesian_reduct(pools):
return reduce(lambda x,y: product(x,y) , pools)
Just to add a bit to what has already been said: if you use sympy, you can use symbols rather than strings which makes them mathematically useful.
import itertools
import sympy
x, y = sympy.symbols('x y')
somelist = [[x,y], [1,2,3], [4,5]]
somelist2 = [[1,2], [1,2,3], [4,5]]
for element in itertools.product(*somelist):
print element
About sympy.
A minor modification to the above recursive generator solution in variadic flavor:
def product_args(*args):
if args:
for a in args[0]:
for prod in product_args(*args[1:]) if args[1:] else ((),):
yield (a,) + prod
And of course a wrapper which makes it work exactly the same as that solution:
def product2(ar_list):
"""
>>> list(product(()))
[()]
>>> list(product2(()))
[]
"""
return product_args(*ar_list)
with one trade-off: it checks if recursion should break upon each outer loop, and one gain: no yield upon empty call, e.g.product(())
, which I suppose would be semantically more correct (see the doctest).
Regarding list comprehension: the mathematical definition applies to an arbitrary number of arguments, while list comprehension could only deal with a known number of them.
Recursive Approach:
def rec_cart(start, array, partial, results):
if len(partial) == len(array):
results.append(partial)
return
for element in array[start]:
rec_cart(start+1, array, partial+[element], results)
rec_res = []
some_lists = [[1, 2, 3], ['a', 'b'], [4, 5]]
rec_cart(0, some_lists, [], rec_res)
print(rec_res)
Iterative Approach:
def itr_cart(array):
results = [[]]
for i in range(len(array)):
temp = []
for res in results:
for element in array[i]:
temp.append(res+[element])
results = temp
return results
some_lists = [[1, 2, 3], ['a', 'b'], [4, 5]]
itr_res = itr_cart(some_lists)
print(itr_res)