On 2/16/12, Ian Kelly <ian.g.ke...@gmail.com> wrote: > On Thu, Feb 16, 2012 at 1:36 AM, Daniel Fetchinson > <fetchin...@googlemail.com> wrote: >>>> Hi folks, often times in science one expresses a value (say >>>> 1.03789291) and its error (say 0.00089) in a short way by parentheses >>>> like so: 1.0379(9) >>>> >>>> One can vary things a bit, but let's take the simplest case when we >>>> only keep 1 digit of the error (and round it of course) and round the >>>> value correspondingly. I've been searching around for a simple >>>> function that would take 2 float arguments and would return a string >>>> but didn't find anything although something tells me it's been done a >>>> gazillion times. >>>> >>>> What would be the simplest such function? >>> >>> Well, this basically works: >>> >>>>>> def format_error(value, error): >>> ... precision = int(math.floor(math.log(error, 10))) >>> ... format = "%%.%df(%%d)" % max(-precision, 0) >>> ... return format % (round(value, -precision), >>> ... int(round(error / 10 ** precision))) >>> ... >>>>>> format_error(1.03789291, 0.00089) >>> '1.0379(9)' >>> >>> Note that "math.floor(math.log(error, 10))" may return the wrong >>> decimal precision due to binary floating point rounding error, which >>> could produce some strange results: >>> >>>>>> format_error(10378929, 1000) >>> '10378900(10)' >>> >>> So you'll probably want to use decimals instead: >>> >>> def format_error(value, error): >>> value = decimal.Decimal(value) >>> error = decimal.Decimal(error) >>> value_scale = value.log10().to_integral(decimal.ROUND_FLOOR) >>> error_scale = error.log10().to_integral(decimal.ROUND_FLOOR) >>> precision = value_scale - error_scale >>> if error_scale > 0: >>> format = "%%.%dE" % max(precision, 0) >>> else: >>> format = "%%.%dG" % (max(precision, 0) + 1) >>> value_str = format % value.quantize(decimal.Decimal("10") ** >>> error_scale) >>> error_str = '(%d)' % error.scaleb(-error_scale).to_integral() >>> if 'E' in value_str: >>> index = value_str.index('E') >>> return value_str[:index] + error_str + value_str[index:] >>> else: >>> return value_str + error_str >>> >>>>>> format_error(1.03789291, 0.00089) >>> '1.0379(9)' >>>>>> format_error(103789291, 1000) >>> '1.03789(1)E+08' >>> >>> I haven't tested this thoroughly, so use at your own risk. :-) >> >> Thanks a lot, this indeed mostly works, except for cases when the >> error needs to be rounded up and becomes two digits: >> >>>>> format_error( '1.34883', '0.0098' ) >> '1.349(10)' >> >> But in this case I'd like to see 1.35(1) > > A small adjustment to the scale fixes that. Also tidied up the string > formatting part: > > import decimal > > def format_error(value, error): > value = decimal.Decimal(value) > error = decimal.Decimal(error) > error_scale = error.adjusted() > error_scale += error.scaleb(-error_scale).to_integral().adjusted() > value_str = str(value.quantize(decimal.Decimal("1E%d" % error_scale))) > error_str = '(%d)' % error.scaleb(-error_scale).to_integral() > if 'E' in value_str: > index = value_str.index('E') > return value_str[:index] + error_str + value_str[index:] > else: > return value_str + error_str > > Cheers, > Ian
Thanks, it's simpler indeed, but gives me an error for value=1.267, error=0.08: Traceback (most recent call last): File "/home/fetchinson/bin/format_error", line 26, in <module> print format_error( sys.argv[1], sys.argv[2] ) File "/home/fetchinson/bin/format_error", line 9, in format_error error_scale += error.scaleb( -error_scale ).to_integral( ).adjusted( ) File "/usr/lib64/python2.6/decimal.py", line 3398, in scaleb ans = self._check_nans(other, context) File "/usr/lib64/python2.6/decimal.py", line 699, in _check_nans other_is_nan = other._isnan() AttributeError: 'int' object has no attribute '_isnan' Which version of python are you using? Cheers, Daniel -- Psss, psss, put it down! - http://www.cafepress.com/putitdown -- http://mail.python.org/mailman/listinfo/python-list