在 python 中的循环中更改循环变量

Question

I am building a small script to perform a task. I am using a for loop in a section of my code to give me a list, however, I need the list to not be in perfect order as its being generated. To do this I attempted to reverse the string variable “chat” within the loop in the hopes of affecting the value of the loop variable.

However, my code doesn't seem to be doing that since I get a clean aaa , aab , etc. output when it runs, regardless if I comment out the line char = char[::-1] or not.

Is what I am trying to do possible with for loop? Or is there any other way to this? And how can I get a more random order in my result in another way?

Also, I am using the list to make sure I don't get duplicates and I am printing char at the end to make sure it's changed except it doesn't seem to be going through the loop.

Here is the code.

import time 
import itertools

char = "abc"
counter = 0
x = []
for i in itertools.product(char, repeat=3):
    if ''.join(i)in x:
        break

    print(''.join(i))
    counter += 1
    char = char[::-1]
    x.append(''.join(i))

print('[+] Printed ' + str(counter)+ ' charecters.')

print(char)

Answer 1

Not sure what you are asking about, but let me try to solve the problem you have :)

import itertools
from random import shuffle

char = "abc"

items = [''.join(x) for x in itertools.product(char, repeat=len(char))]
shuffle(items)

print('\n'.join(items))
print('[+] Printed {} charecters.'.format(len(items)))

python test123.py
bbb
aca
bcb
abb
aab
bcc
acb
cca
abc
ccc
cac
caa
ccb
bca
cbb
bba
aba
aac
baa
bab
bac
cbc
bbc
acc
aaa
cab
cba
[+] Printed 27 charecters.

every run it generates the same "products list" but in a different order

Superfast "product" list generator with randomized output under the hood!

I've spent some time and got the working example

import itertools
from contextlib import contextmanager
from random import shuffle, sample
from datetime import datetime

char = "abcdefgf"


def generate_product(char):
    items = []
    for i, item in enumerate(itertools.product(char, repeat=len(char))):
        items.append("".join(item))
    shuffle(items)

    return items


def generate(char):
    pools = (char,) * len(char)

    result = ['']
    for i, pool in enumerate(pools):
        # let's shuffle the pool, it will randomize output without extra performance hit
        pool_rnd = sample(pool, k=len(pool))

        result = [x + y for x in result for y in pool_rnd]

    return result


@contextmanager
def timeit(name):
    start = datetime.now()
    print('Started "{}" at {}. Please wait ...'.format(name, start))

    yield

    end = datetime.now()
    print('Finished "{}" at {}! Execution took {}'.format(name, start, end - start))


with timeit('generate_product'):
    items1 = generate_product(char)
    # # print('\n'.join(items1))
    print(' > Contains {} charecters.'.format(len(items1)))

with timeit('generate'):
    items2 = generate(char)
    # print('\n'.join(items2))
    print(' > Contains {} charecters.'.format(len(items2)))

print('Are equal? Sorted comparision result={}'.format(sorted(items1) == sorted(items2)))

Compare the speed!

#> python test123.py

Started "generate_product" at 2019-12-13 05:35:39.219348. Please wait ...
 > Contains 16777216 charecters.
Finished "generate_product" at 2019-12-13 05:35:39.219348! Execution took 0:00:30.075320
Started "generate" at 2019-12-13 05:36:09.294871. Please wait ...
 > Contains 16777216 charecters.
Finished "generate" at 2019-12-13 05:36:09.294871! Execution took 0:00:02.998755
Are equal? Sorted comparision result=True

Just 0:00:02.998755 against 0:00:30.075320 ! That's a victory I believe!

One more "stress-test" for the algorithm!

Started "generate" at 2019-12-13 05:39:01.434788. Please wait ... Contains 387420489 charecters. Finished "generate" at 2019-12-13 05:39:01.434788! Execution took 0:01:20.306413

80 seconds to generate list of 387_420_489 combinations! I consumed 25 GB or RAM :D

Answer 2

You are changing char inside the loop, which is iterating over product(char, repeat=3) .

Although char is changed every time (you can check this by printing it), it does not affect the data you're looping over. That makes sense if you think about it: what would you expect would happen exactly?

The loop simply completes iterating over the original product and every value will show up in order, reversing char does nothing.

If you were trying to show all three character combinations of characters from the original string, but in a random order, this would work:

import random
import itertools

chars = 'abc'

result = list(itertools.product(chars, repeat=3))
random.shuffle(result)

for x in result:
    print(''.join(x))