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Comments on Compile-time rounding with the pre-processor
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Compile-time rounding with the pre-processor
When searching the Internet, I came across this SO post Rounding in C Preprocessor but immediately noted that none of the answers are showing a way to perform actual rounding - rather, they are doing flooring to the nearest integer.
The post is actually misleading and got nothing to do with rounding, but with getting the appropriate integer type back from the macro. Yet it is the #1 search engine hit when searching for rounding the the preprocessor, which is kind of sad.
Is there a way to round a floating point expression to an integer at compile-time, using the pre-processor?
I suppose such code could also either be written in a type-generic way accepting any of the common arithmetic types as input, or alternatively in a type safe manner only accepting one specific type as input. Is there an advantage in using one form or the other?
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If you can truncate (get the floor() of a number), then you can round the number by adding 0.5 first. Or, I suppose that, for the sake of completeness, you add "the equivalent of 0.5," since a given program might not represent the values in decimal or you might want to round to a non-ones position in the number.
When you round numbers (and you know this, otherwise you wouldn't make the distinction, but to explain beyond "add a half"), a (decimal) digit of five or higher bumps the next-most-significant place up by one, otherwise you round. Assuming rounding to the nearest integer, then, from 0.0 to 0.49̅, adding 0.5 increases to 0.5 to 0.99̅, so truncation goes to the lower value. Anything higher crosses the threshold to the next integer, so truncation gives you that higher value.
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