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comp.lang.c FAQ list · Question 14.6

Q: How do I round numbers?

A: The simplest and most straightforward way is with code like

(int)(x + 0.5)
C's floating to integer conversion truncates (discards) the fractional part, so adding 0.5 before truncating arranges that fractions >= 0.5 will be rounded up. (This technique won't work properly for negative numbers, though, for which you could use something like (int)(x < 0 ? x - 0.5 : x + 0.5), or play around with the floor and ceil functions.)

You can round to a certain precision by scaling:

	(int)(x / precision + 0.5) * precision
Handling negative numbers, or implementing even/odd rounding, is slightly trickier.

Note that because truncation is otherwise the default, it's usually a good idea to use an explicit rounding step when converting floating-point numbers to integers. Unless you're careful, it's quite possible for a number which you thought was 8.0 to be represented internally as 7.999999 and to be truncated to 7.

Additional links: further reading

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