If I have
(float)value = 10.50
and do
int new_value = (int)value
what rules will round number?
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8
When a finite value of floating type is converted to an integer type, the fractional part is discarded (i.e., the value is truncated toward zero).
So in the case of -10.5, it's converted to -10.
C++11 4.9 Floating-integral conversions [conv.fpint]
An rvalue of a floating point type can be converted to an rvalue of an integer type. The conversion truncates; that is, the fractional part is discarded. The behavior is undefined if the truncated value cannot be represented in the destination type. [ Note: If the destination type is bool, see 4.12. —end note ]
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Although it is in the quote you mention, I would explicitly [mention that this can overflow](http://stackoverflow.com/a/24348037/1708801) and recommend `round` or something similar instead. – Shafik Yaghmour Jul 01 '14 at 19:08
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The rule is quite simple: the number simply gets truncated to its integral part, in this case, to 10. The fractional part gets dropped entirely. The same applies to negative numbers: -10.5 would be converted to -10.
juanchopanza
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I tried something, and found, that float -20.50 will be -21 in integer - I guess, it's not so simple as truncating... Or I, as usual, doing wrong :) – therainycat Sep 29 '13 at 14:49
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@therainycat Interesting. What platform are you on? That behaviour looks strange to me. I would expect to get `-20`. – juanchopanza Sep 29 '13 at 14:55
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I write on PAWN, and sometimes work with files, made by C++. There's no construction as (int) in PAWN, so someone made include, in which (inc) just replaced to floatround(value, floatround_floor). So, I had bad values for numbers below 0 – therainycat Sep 29 '13 at 15:07
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@therainycat sorry, I have no idea what you are talking about or how that applies to your question. – juanchopanza Sep 29 '13 at 15:08
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When converted to integers, the fractional part of the float is dropped, meaning the float 10.5 will be converted to the integer 10, and the float -10.5 will be converted to the integer -10.
guyzyl
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1It is rounded towards zero, whether the number is positive or negative, so -10.5 becomes -10. – Mats Petersson Sep 29 '13 at 14:52
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You mean “fractional” (but you wrote “decimal”) about the part of the floating-point number that is dropped by truncation. – Pascal Cuoq Sep 29 '13 at 15:17