How to write program in python to generate all n digit numbers with only 4 and 7 as their digits?

Question

我尝试使用itertools.permutations生成排列，但是我对如何使用n位数字感到困惑。

Answer 1

I would use itertools.product instead:

In [26]: for i in itertools.product(['4', '7'], repeat=2):
   ....:     print int(''.join(i))
   ....:
44
47
74
77

The repeat argument is your n .

Answer 2

I would use binary, if you need all 2-digits numbers with only 7 , 4 as digits:

max 2 digits number in base-2 is 11b ie 3 , so:

0 => 00b
1 => 01b
2 => 10b
3 => 11b

then replace 0 by 4 and 1 by 7 (arbitrary), giving: 44, 47, 74, 77