Rounding to a power of 10

Question

I have a variable, tauMax, that I want to round up to the nearest power of ten(1, 10, 100, 1000...). I am using the below expression to find the closest integer to the max value in the tau array. I am finding the max value because I am trying to calculate the power of ten that should be the x axis cutoff. In this cause, tauMax is equal to 756, so I want to have an expression that outputs either 1000, or 3(for 10^3).

tauMax = round(max(tau));

I'd really appreciate any help!

You could also do this I believe based off of the number of digits. — , Jul 20 '11 at 22:43
That's a great idea. Do you know what command i would use to find the number of digits in a given number? — Alex Nichols, Jul 20 '11 at 22:47
@Alex Nichols - Since you're talking base 10 just use `log10` (see my answer below). — b3., Jul 20 '11 at 22:49
Seems to be some different ways to do that: http://www.mathworks.co.jp/matlabcentral/answers/10795-counting-the-number-of-digits, but the other method may be easier. — , Jul 20 '11 at 22:50

score 42 · Accepted Answer · edited Jan 31 '12 at 22:12

42

Since you're talking base 10, you could just use log10 to get the number of digits.

How about:

>> ceil(log10(756))

ans =

     3

edited Jan 31 '12 at 22:12

Hannele

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answered Jul 20 '11 at 22:47

b3.

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Very nice, and works just the same using python `math` as @steveha answers also – vpipkt Oct 03 '18 at 13:19

steveha · Answer 2 · 2018-08-17T05:39:33.660

7

I don't really do Matlab, but the usual way to do this in any language I do know is: take the logarithm base 10, then round up that number to the nearest integer, then compute 10 to the power of that number. In Python:

from math import ceil, log

def ceil_power_of_10(n):
    exp = log(n, 10)
    exp = ceil(exp)
    return 10**exp

>>> print(ceil_power_of_10(1024))  # prints 10000

edited Aug 17 '18 at 05:39

answered Jul 20 '11 at 22:47

steveha

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Doesn't it print 10000? – Nils_M Aug 16 '18 at 13:32
@Nils_M Yes, it prints 10000. Thanks for the correction! I'll edit the answer and correct the comment. – steveha Aug 17 '18 at 05:37

score -2 · Answer 3 · answered Jul 23 '14 at 13:50

You could also look at the source of the built-in Matlab function nextpow2(N) (simply open nextpow2.m) to see how Mathworks engineers implemented this for a power of 2 and create a new function adapting this source to a power of 10.

http://www.mathworks.it/it/help/matlab/ref/nextpow2.html

Rounding to a power of 10

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