问题
My problem is difficult to explain.
I want to create a function that contains nested for loops,
the amount of which is proportional to an argument passed to the function.
Here\'s a hypothetical example:
Function(2)
...would involve...
for x in range (y):
for x in range (y):
do_whatever()
Another example...
Function(6)
...would involve...
for x in range (y):
for x in range (y):
for x in range (y):
for x in range (y):
for x in range (y):
for x in range (y):
whatever()
The variables of the for loop (y) are NOT actually used in the nested code.
Your first thought might be to create ONE for loop, with a range that is to the power of the number argument...
THIS CAN NOT WORK because the product would be HUGE. I have instances required where there are 8 nested for loops.
The product is too large for a range in a for loop.
There are other arguments needed to be passed to the function, but I can handle that myself.
Here\'s the code (it creates the Snowflake Fractal)
from turtle import *
length = 800
speed(0)
def Mini(length):
for x in range (3):
forward(length)
right(60)
penup()
setpos(-500, 0)
pendown()
choice = input(\"Enter Complexity:\")
if choice == 1:
for x in range (3):
forward(length)
left(120)
elif choice == 2:
for x in range (3):
Mini(length/3)
left(120)
if choice == 3:
for x in range (6):
Mini(length/9)
right(60)
Mini(length/9)
left(120)
if choice == 4:
for y in range (6):
for x in range (2):
Mini(length/27)
right(60)
Mini(length/27)
left(120)
right(180)
for x in range (2):
Mini(length/27)
right(60)
Mini(length/27)
left(120)
if choice == 5:
for a in range (6):
for z in range (2):
for y in range (2):
for x in range (2):
Mini(length/81)
right(60)
Mini(length/81)
left(120)
right(180)
for x in range (2):
Mini(length/81)
right(60)
Mini(length/81)
left(120)
right(180)
right(180)
if choice == 6:
for c in range (6):
for b in range (2):
for a in range (2):
for z in range (2):
for y in range (2):
for x in range (2):
Mini(length/243)
right(60)
Mini(length/243)
left(120)
right(180)
for x in range (2):
Mini(length/243)
right(60)
Mini(length/243)
left(120)
right(180)
right(180)
right(180)
right(180)
if choice == 7:
for a in range (6):
for b in range(2):
for c in range (2):
for d in range (2):
for e in range (2):
for f in range (2):
for y in range (2):
for x in range (2):
Mini(length/729)
right(60)
Mini(length/729)
left(120)
right(180)
for x in range (2):
Mini(length/729)
right(60)
Mini(length/729)
left(120)
right(180)
right(180)
right(180)
right(180)
right(180)
right(180)
I\'d appreciate any help you can give me at all,
though if you suggest a different method (such as recursion),
please don\'t just paste the code; instead, suggests some ideas that could put me in the right direction.
(The algorithm is for a Specialist Math Assignment)
specs:
Python 2.7.1
Turtle
IDLE
Windows7
回答1:
This problem can be solved by recursion. I am just writing an algorithm here, since I believe this can be a general problem.
function Recurse (y, number)
if (number > 1)
Recurse ( y, number - 1 )
else
for x in range (y)
whatever()
回答2:
I'm not clear why you can't use the product of the bounds and do
for x in range(y exp n)
where n is the # of loops.... You say y exp n will be huge, but I'm sure python can handle it.
However, that being said, what about some sort of recursive algorithm?
def loop_rec(y, n):
if n >= 1:
for x in range(y):
loop_rec(y, n - 1)
else:
whatever()
回答3:
this can be done without recursion using itertools.product
import itertools
def function(n):
for x in itertools.product(range(n),repeat=n):
whatever()
回答4:
Recursion will be your best bet. Consider what it should do in the base case and in the recursive case.
Code left out, as per request.
回答5:
Here you go. Let ranges be your ranges, operate on result when you need to.
ranges=((1,4),(0,3),(3,6))
from operator import mul
operations=reduce(mul,(p[1]-p[0] for p in ranges))-1
result=[i[0] for i in ranges]
pos=len(ranges)-1
increments=0
print result
while increments < operations:
if result[pos]==ranges[pos][1]-1:
result[pos]=ranges[pos][0]
pos-=1
else:
result[pos]+=1
increments+=1
pos=len(ranges)-1 #increment the innermost loop
print result
[1, 0, 3]
[1, 0, 4]
[1, 0, 5]
[1, 1, 3]
[1, 1, 4]
[1, 1, 5]
[1, 2, 3]
[1, 2, 4]
[1, 2, 5]
[2, 0, 3]
[2, 0, 4]
[2, 0, 5]
[2, 1, 3]
[2, 1, 4]
[2, 1, 5]
[2, 2, 3]
[2, 2, 4]
[2, 2, 5]
[3, 0, 3]
[3, 0, 4]
[3, 0, 5]
[3, 1, 3]
[3, 1, 4]
[3, 1, 5]
[3, 2, 3]
[3, 2, 4]
[3, 2, 5]
[1, 0, 4]
Testing with the following would give the same result:
for x in range(*ranges[0]):
for y in range(*ranges[1]):
for z in range(*ranges[2]):
print [x,y,z]
回答6:
Have you considered xrange ?
for x in xrange(y ** n):
whatever()
And if you overshoot even xrange limit, you can use itertool
import itertools
for x in itertools.product(xrange(y), repeat=n):
whatever()
(previous itertool answer incorrectly used n for the range instead of y)
回答7:
My reply is late, but supposing that you want to do multiple loops, e.g. print some range multiple times. Then the correct version of this recursion is:
def loop_rec(y, number):
if (number > 1):
loop_rec( y, number - 1 )
for i in range(y):
print(i, end=' ')
else:
for i in range(y):
print(i, end=' ')
loop_rec(4,3)
This will create three for loops with the range(4)
If you want to play around with dynamic range, here are some variants:
def loop_rec(y, number):
if (number > 1):
loop_rec( y+1, number - 1 )
for i in range(y):
print(i, end=' ')
print(' ;')
else:
for i in range(y):
print(i, end=' ')
print(';')
loop_rec(6,4)
which will print out:
0 1 2 3 4 5 6 7 8 ;
0 1 2 3 4 5 6 7 ;
0 1 2 3 4 5 6 ;
0 1 2 3 4 5 ;
or
def loop_rec(y, number):
if (number > 1):
loop_rec( y-1, number - 1 )
for i in range(y):
print(i, end=' ')
print(' ;')
else:
for i in range(y):
print(i, end=' ')
print(';')
loop_rec(6,4)
which will output:
0 1 2 ;
0 1 2 3 ;
0 1 2 3 4 ;
0 1 2 3 4 5 ;
A better variant which is using only one for loop (less typing) is the following:
def loop_rec(y, number):
if (number >= 1):
loop_rec( y+1, number - 1 )
for i in range(y):
print(i, end=' ')
print('')
else:
return
loop_rec(1,5)
will output:
0 1 2 3 4
0 1 2 3
0 1 2
0 1
0
回答8:
A nice implementation of the varying number of For Loops problem:
def for_recursive(number_of_loops, range_list, execute_function, current_index=0, iter_list = []):
if iter_list == []:
iter_list = [0]*number_of_loops
if current_index == number_of_loops-1:
for iter_list[current_index] in range_list[current_index]:
execute_function(iter_list)
else:
for iter_list[current_index] in range_list[current_index]:
for_recursive(number_of_loops, iter_list = iter_list, range_list = range_list, current_index = current_index+1
An example of how to use it:
def do_whatever(index_list):
return print(index_list)
for_recursive(range_list = [range(0,3), range(0,3) , range(1,3)], execute_function = do_whatever)
The code returns the same that this code:
for i in range(0,3):
for j in range(0,3):
for k in range(1,3):
print([i,j,k])
来源：https://stackoverflow.com/questions/7186518/function-with-varying-number-of-for-loops-python