Most efficient way to look for the last digit of a number?

Question

I recently had a question in a Python exam where we were asked to check for numbers to be ending in 8 and I came across two ways and I was wondering which one was the most efficient or if there was an even better way to do so.

(If n is the desired number) So, method 1:

if n % 10 == 8:
    //do stuff here

Method 2:

if str(n)[-1] == 8:
    //do stuff here

Answer 1

You could easily test this yourself with Python's built-in timeit.timeit function:

>>> from timeit import timeit
>>> n = 12345678
>>> timeit("n % 10 == 8", "from __main__ import n")
0.45508130223254284
>>> timeit("str(n)[-1] == '8'", "from __main__ import n")
1.689859186013905
>>>

As you can see from the results above, the first solution is very efficient and outperforms the second by almost four times.

Answer 2

For more fun and excitement in addition to iCodez answer, you can use the dis module to see what the difference in instructions is:

wayne@wango ~ ⚘ python3.4                                                                                                                                                                                                                                               14:24:49
Python 3.4.0 (default, Mar 25 2014, 15:24:33) 
[GCC 4.6.3] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import dis
>>> dis.dis('n % 10 == 8')
  1           0 LOAD_NAME                0 (n)
              3 LOAD_CONST               0 (10)
              6 BINARY_MODULO
              7 LOAD_CONST               1 (8)
             10 COMPARE_OP               2 (==)
             13 RETURN_VALUE
>>> dis.dis('str(n)[-1] == 8')
  1           0 LOAD_NAME                0 (str)
              3 LOAD_NAME                1 (n)
              6 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
              9 LOAD_CONST               2 (-1)
             12 BINARY_SUBSCR
             13 LOAD_CONST               1 (8)
             16 COMPARE_OP               2 (==)
             19 RETURN_VALUE

If I were to make a guess, I'd say the major time difference has to do with the string conversion. And a quick test:

>>> from timeit import timeit
>>> n = 12345678
>>> m = "12345678"
>>> timeit("n % 10 == 8", "from __main__ import n")
0.09333206800511107
>>> timeit("m[-1] == '8'", "from __main__ import m")
0.05890634500246961
>>> 

Shows this to be the case.

Additionally in your code, the second example would always equate to False , since a string will never == a number.