Get most significant digit in python

Question

Say I have list [34523, 55, 65, 2]

What is the most efficient way to get [3,5,6,2] which are the most significant digits. If possible without changing changing each to str()?

Answer 1

Assuming you're only dealing with positive numbers, you can divide each number by the largest power of 10 smaller than the number, and then take the floor of the result.

>>> from math import log10, floor
>>> lst = [34523, 55, 65, 2]
>>> [floor(x / (10**floor(log10(x)))) for x in lst]
[3, 5, 6, 2]

If you're using Python 3, instead of flooring the result, you can use the integer division operator //:

>>> [x // (10**floor(log10(x))) for x in lst]
[3, 5, 6, 2]

However, I have no idea whether this is more efficient than just converting to a string and slicing the first character. (Note that you'll need to be a bit more sophisticated if you have to deal with numbers between 0 and 1.)

>>> [int(str(x)[0]) for x in lst]
[3, 5, 6, 2]

If this is in a performance-critical piece of code, you should measure the two options and see which is faster. If it's not in a performance-critical piece of code, use whichever one is most readable to you.