问题
I have the following situation:
for x1 in range(x1, x2):
for x2 in range(x3, x4):
for x3 ...
...
f(x1, x2, x3, ...)
How to convert this to a mechanism in which I only tell python to make n nested loops where the variable name is x1, x2, x3, x4, ...? I don't want to write every possibility manually of course, since there might be very many dimensions.
回答1:
What you want to do is iterate over a product. Use itertools.product.
import itertools
ranges = [range(x1, x2), range(x3, x4), ...]
for xs in itertools.product(*ranges):
f(*xs)
Example
import itertools
ranges = [range(0, 2), range(1, 3), range(2, 4)]
for xs in itertools.product(*ranges):
print(*xs)
Output
0 1 2
0 1 3
0 2 2
0 2 3
1 1 2
1 1 3
1 2 2
1 2 3
回答2:
Recommended: itertools
itertools is an awesome package for everything related to iteration:
from itertools import product
x1 = 3; x2 = 4
x3 = 0; x4 = 2
x5 = 42; x6 = 42
for x, y, z in product(range(x1, x2), range(x3, x4), range(x4, x5)):
print(x, y, z)
gives
3 0 2
3 0 3
3 0 4
3 0 5
3 0 6
3 0 7
3 0 8
3 0 9
3 0 10
3 0 11
3 0 12
3 0 13
3 0 14
3 0 15
3 0 16
3 0 17
3 0 18
3 0 19
3 0 20
3 0 21
3 0 22
3 0 23
3 0 24
3 0 25
3 0 26
3 0 27
3 0 28
3 0 29
3 0 30
3 0 31
3 0 32
3 0 33
3 0 34
3 0 35
3 0 36
3 0 37
3 0 38
3 0 39
3 0 40
3 0 41
3 1 2
3 1 3
3 1 4
3 1 5
3 1 6
3 1 7
3 1 8
3 1 9
3 1 10
3 1 11
3 1 12
3 1 13
3 1 14
3 1 15
3 1 16
3 1 17
3 1 18
3 1 19
3 1 20
3 1 21
3 1 22
3 1 23
3 1 24
3 1 25
3 1 26
3 1 27
3 1 28
3 1 29
3 1 30
3 1 31
3 1 32
3 1 33
3 1 34
3 1 35
3 1 36
3 1 37
3 1 38
3 1 39
3 1 40
3 1 41
Create a cross-product function yourself
def product(*args):
pools = map(tuple, args)
result = [[]]
for pool in pools:
result = [x + [y] for x in result for y in pool]
for prod in result:
yield tuple(prod)
回答3:
You could also use numpy.ndindex to achieve what you are looking for:
import numpy
for y1, y2, y3 in numpy.ndindex(x2, x4, ...):
...
Thats especially useful when you are already using numpy for something else in your script. You would have to add the 'starting point' (i.e. x1, x2, ...) to each dimension though (if it's not 0).
回答4:
Use multiprocessing!
import multiprocessing
from itertools import product
results = []
def callback(return_value):
# this stores results
results.append(return_value)
if __name__=="__main__" :
pool = multiprocessing.Pool(4)
args = product(range1, range2, range)
for x, y, z in args:
pool.apply_async(f,(x,y,z),call_back=callback)
pool.close()
pool.join()
Now you can run your function while also taking advantage of the full capacity of your machine!
来源：https://stackoverflow.com/questions/50860578/how-to-make-n-dimensional-nested-for-loops-in-python