[Tutor] Occurrence of number 2 in a range from 1 to 100

[eryksun]

On Sun, Dec 1, 2013 at 6:14 AM, Steven D'Aprano <steve at pearwood.info> wrote:
> On Sun, Dec 01, 2013 at 02:57:33PM +0530, Reuben wrote:
>> I mean occurrence of 2 from numbers 1 to 100.  The number could be the
>> first digit or second digit in a two digit number..for e.g. In number 21 it
>> appears as first digit. For number 92 it appears as second digit
>
> The most efficient way is to use a bit of reasoning:
>
> 2, 12, 20 through 29, then 32, 42, 52 etc.

    >>> [n for n in range(100) if n % 10 == 2 or n // 10 == 2]
    [2, 12, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29,
     32, 42, 52, 62, 72, 82, 92]

Floor division (//) and modulo (%) are explained here:

http://docs.python.org/2/reference/expressions.html#binary-arithmetic-operations