#5963: Fixed format floating point conversion does not round to even
------------------------------+---------------------------------------------
Reporter: augustss | Owner:
Type: bug | Status: new
Priority: normal | Component: libraries/base
Version: 7.4.1 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Conversion of floating point numbers to a fixed numbers of decimals should
use round-to-even rather that rounding up when the number is right between
two possible results.
E.g., printf "%.1f" 0.45 should produce "0.4" rather than "0.5".
The heart of the problem is in libraries/base/GHC/Float.lhs.
The roundTo function should be replaced by this
{{{
roundTo :: Int -> Int -> [Int] -> (Int,[Int])
roundTo base d is =
case f d True is of
x@(0,_) -> x
(1,xs) -> (1, 1:xs)
_ -> error "roundTo: bad Value"
where
b2 = base `quot` 2
f n _ [] = (0, replicate n 0)
f 0 e (x:xs) | x == b2 && e && all (== 0) xs = (0, []) -- Round to
even when at exactly half the base
| otherwise = (if x >= b2 then 1 else 0, [])
f n _ (i:xs)
| i' == base = (1,0:ds)
| otherwise = (0,i':ds)
where
(c,ds) = f (n-1) (even i) xs
i' = c + i
}}}
I also (as the original author of the code) note that the large parts of
that module was taken verbatim from the hbc libraries without attribution.
:)
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5963>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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