"Round towards even" in sprintf

7 次查看（过去 30 天）
K E
K E 2011-12-23
It seems that sprintf doesn't always round up if a value is halfway between two integers:
sprintf('%4.1f', 0.95) ; % Returns 0.9
But it also seems to disobey the IEEE standard of "round toward the even number", though I could be misunderstanding. Left column is the input to the sprintf statement above and right column is the output.
-0.95 -1.0
-0.85 -0.8
-0.75 -0.8
-0.65 -0.7
-0.55 -0.6
-0.45 -0.5
-0.35 -0.4
-0.25 -0.3
-0.15 -0.1
-0.05 -0.1
0.05 0.1
0.15 0.1
0.25 0.3
0.35 0.4
0.45 0.5
0.55 0.6
0.65 0.7
0.75 0.8
0.85 0.8
0.95 1.0
[Note that roundsd.m enforces the "rounding up" of trailing decimals which can then be fed into sprintf. http://www.mathworks.com/matlabcentral/fileexchange/26212-round-with-significant-digits]

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Walter Roberson
Walter Roberson 2011-12-24
I looked through the documentation earlier today, but I could not find any evidence that sprintf() is required to conform to IEEE rounding rules. The most detailed sprintf() documentation simply indicates that the value will be rounded, without indicating that any particular rounding scheme will be used.
It is not clear at the moment that MATLAB supports round-towards-even. There is an undocumented call
feature('setround',VALUE)
but none of the four known VALUE are for round towards even.
Historically MATLAB is known not to have used round to even; see http://www.mathworks.com/matlabcentral/newsreader/view_thread/16691
  2 个评论
K E
K E 2012-1-3
Holy inconsistency, Batman! Guess I will stick with roundsd.m where rounding follows a predictable pattern of rounding up.
Walter Roberson
Walter Roberson 2012-1-3
The discussion goes back at least a decade; see http://www.mathworks.com/matlabcentral/newsreader/view_thread/24439

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