[Python-Dev] Rounding float to int directly (Re: struct module and coercing floats to integers) (original) (raw)
Nick Coghlan ncoghlan at gmail.com
Thu Aug 3 13:43:27 CEST 2006
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Ron Adam wrote:
Consider an example where you are combining data that had different number of significant digits. Keeping all the digits of your answer gives a false since of accuracy. The extra digits are meaningless because the margin of error is greater than any of the digits that exceed the minimum number of significant digits in your data. So you need to remove the extraneous digits by rounding. Then this answer can be further used as data, and sense the number of significant digits is maintained, the margin of error can be calculated accurately.
This is a fallacy though - add two numbers together each with an error of +/- 0.05, and the error in the total is +/- 0.1. The approach you propose gives the misleading impression that the error in the total is still only +/- 0.05 (as the answer will contain 1 decimal place).
If you really care about error, you need to do it properly (e.g. stddev or adding the error margins).
Anyway, it's not proposed to remove the "round to x decimal places" capability entirely - merely remove it from the builtin round() so that the return type can be changed.
It makes since for round have an argument to specify the number of digits of precision to round to and also for it to return floating point numbers because rounding results to a float of n digits of precision is a necessary operation.
If you have the default round() case of 0 (and negative) digits of precision return integers, you then have a function that may sometimes returns integers, and sometimes returns floats.
That's not the proposal, as Greg has already said (the fround() mentioned would probably be 'math.fround' rather than a builtin):
[Greg Ewing]
No, round() wouldn't have that option at all. If you wanted it, you would use fround() instead, which would have the option and return a float always.
This would be a Py3k thing, obviously. If done before then, the new function would have to be given a different name.
[Ron Adam]
This can be problematic if the values are further used in division operations. Which will result in many cases of.. float(round(x)) in order to convert integer results back to floats.
Not in Py3k where int/int will return a float as needed.
And in regard to using round() combined with division operators in floating point math, it is an issue of least surprises.
And in Py3k, with a round that always returned an integer there wouldn't be any surprises at all:
- division would do the right thing, because true division becomes the only behaviour for '/'
- the result of the builtin rounding operation would be usable directly as an index without requiring subsequent coercion to an integer
Cheers, Nick.
-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia
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