[Numpy-discussion] A proposal for a round(x,n) enhancement
Christopher Smith
csmith at blakeschool.org
Wed Sep 12 14:08:02 EDT 2001
Hello,
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
suggested
that MATLAB has a function called chop that does what I am proposing and at
http://www.math.colostate.edu/~tavener/M561/lab5/lab5.pdf
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
actually
rounding?)
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
proposing:
from math import floor,log10
def Round(x,n=0):
if n%1:
d=int(10*n)
return round(x,d-1-int(floor(log10(abs(x)))))
else:
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
resulting
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,
respectively.
I don't believe you will run into any numerical issues since even though
something
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.
/c
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