[Numpy-discussion] A proposal for a round(x,n) enhancement

Christopher Smith csmith at blakeschool.org
Wed Sep 12 14:08:02 EDT 2001


I would like to see an addition to the round(x,n) function syntax that
would allow 
one to specify that x should be rounded to the nth significant digit as
opposed to 
the nth digit to the left or right of the 1's digit.  I am testing the
waters right 
now on python-list and have not yet submitted a PEP.  

Some there have suggested that it be called a different function.  Someone
else has 
suggested that perhaps it could be added into SciPy or Numeric as a
function there.

I prefer the name round because it describes what you are doing.  Someone
that MATLAB has a function called chop that does what I am proposing and at


the document says that the "MATLAB function chop(x,n) rounds (not chops!)
the number 
x to n significant digits."  If this new function was called "chop" then
any previous 
MATLAB users would know what to expect.  (But why call it chop if you are

What I think would be nice is to modify round so it can round to a given 
number of sig. figs.  Here is a def called Round that simulates what I am

from math import floor,log10
def Round(x,n=0):
	if n%1:
		return round(x,d-1-int(floor(log10(abs(x)))))
		return round(x,n)
print Round(1.23456,2)   #returns 1.23
print Round(1.23456,0.2) #returns 1.2

The way that I have enhanced round's syntax is to have it check to see if
there is a 
non-zero decimal part to n.  If there is, n is multiplied by 10 and the
value is used to compute which digit to round x to.  n=0.1 will round to
the first 
significant digit while n=0.4 and n=1.1 will round to the 4th and 11th,

I don't believe you will run into any numerical issues since even though
like .3 may be represented internally as 0.29999999999999999, multiplying
it by 10 
gets you back to 3 and this value is what is used in the call to round()

I am open to comments about implementation and function name.


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