How to round a number to significant figures in Python


Question

I need to round a float to be displayed in a UI. E.g, to one significant figure:

1234 -> 1000

0.12 -> 0.1

0.012 -> 0.01

0.062 -> 0.06

6253 -> 6000

1999 -> 2000

Is there a nice way to do this using the Python library, or do I have to write it myself?

1
122
8/5/2010 2:37:21 AM

Accepted Answer

You can use negative numbers to round integers:

>>> round(1234, -3)
1000.0

Thus if you need only most significant digit:

>>> from math import log10, floor
>>> def round_to_1(x):
...   return round(x, -int(floor(log10(abs(x)))))
... 
>>> round_to_1(0.0232)
0.02
>>> round_to_1(1234243)
1000000.0
>>> round_to_1(13)
10.0
>>> round_to_1(4)
4.0
>>> round_to_1(19)
20.0

You'll probably have to take care of turning float to integer if it's bigger than 1.

130
1/7/2016 6:43:05 AM

%g in string formatting will format a float rounded to some number of significant figures. It will sometimes use 'e' scientific notation, so convert the rounded string back to a float then through %s string formatting.

>>> '%s' % float('%.1g' % 1234)
'1000'
>>> '%s' % float('%.1g' % 0.12)
'0.1'
>>> '%s' % float('%.1g' % 0.012)
'0.01'
>>> '%s' % float('%.1g' % 0.062)
'0.06'
>>> '%s' % float('%.1g' % 6253)
'6000.0'
>>> '%s' % float('%.1g' % 1999)
'2000.0'

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