如何在python中随机通过double for循环

Question

Consider the following code:

for i in range(size-1):
    for j in range(i+1,size):
        print((i,j))

I need to go through this for-loop in a random fashion. I attempt to write a generator to do such a thing

def Neighborhood(size):
    for i in shuffle(range(size-1)):
        for j in shuffle(range(i+1), size):
            yield i, j
for i,j in Neighborhood(size):
    print((i,j))

However, shuffle cannot be applied to whatever object range is. I do not know how to remedy the situation, and any help is much appreciated. I would prefer a solution avoid converting range to a list, since I need speed. For example, size could be on the order of 30,000 and i will do perform this for loop around 30,000 times.

I also plan to escape the for loop early, so I want to avoid solutions that incorporate shuffle(list(range(size)))

Answer 1

You can use random.sample .

The advantage of using random.sample over random.shuffle , is , it can work on iterators , so in :

• Python 3.X you don't need to convert range() to list
• In Python 2,X, you can use xrange
• Same Code can work in Python 2.X and 3.X

Sample code :

n=10
l1=range(n)
for i in sample(l1,len(l1)):
    l2=range(i,n)
    for j in sample(l2,len(l2)):
        print(i,j)

Edit :

As to why I put in this edit, go through the comments.

def Neighborhood(size):
    range1 = range(size-1)
    for i in sample(range1, len(range1)):
        range2 = range(i+1)
        for j in sample(range2, len(range2)):
            yield i, j

Answer 2

A simple way to go really random, not row-by-row:

def Neighborhood(size):
    yielded = set()
    while True:
        i = random.randrange(size)
        j = random.randrange(size)
        if i < j and (i, j) not in yielded:
            yield i, j
            yielded.add((i, j))

Demo:

for i, j in Neighborhood(30000):
    print(i, j)

Prints something like:

2045 5990
224 5588
1577 16076
11498 15640
15219 28006
8066 10142
7856 8248
17830 26616
...

Note: I assume you're indeed going to "escape the for loop early" . Then this won't have problems with slowing down due to pairs being produced repeatedly.

Answer 3

I don't think you can randomly traverse an Iterator. You can predefine the shuffled lists, though

random iteration in Python

L1 = list(range(size-1))
random.shuffle(L1) 
for i in L1:
    L2 = list(range(i+1, size))
    random.shuffle(L2) 
    for j in L2:
        print((i,j))

Of course, not optimal for large lists