Hi,
On 6/21/2016 16:33, Andreas Prescher wrote:
void f(double d) {
int i = vector.size() * d;
}
A quick note to add to the other answers: You don't want to be using
`int` here.
Especially since you mention a 64-bit platform -- but IMHO it's a bad
idea in general.
If you'd like a simple rule of thumb, here goes: Avoid using `int` for
anything by default -- especially anything involving
sizes/indexes/counting. Yes, there are exceptional lucky cases when you
may be able to get away with an `int`, but taking these into account is
brittle and anything but "simple."
Your `vector.size()`, assuming you're dealing with `vector` of type
`std::vector<int>` (as in your later example), has type
`std::vector<int>::size_type` -- which (on your 64-bit platform) will be
64-bits.
See: http://en.cppreference.com/w/cpp/container/vector/size
In contrast, type `int` is at most 32-bits:
http://en.cppreference.com/w/cpp/language/types
If the value of the expression involving the vector size value is larger
than what you can store in an `int`, then the size_type->double->int
conversion you're doing when storing to `i` will cause undefined
behavior due to overflow:
http://blog.frama-c.com/index.php?post/2013/10/09/Overflow-float-integer
While you may (or may not) be in a particularly lucky special case when
the size value just happens to fit in the 32-bits on one occasion, you
may not be on another.
Instead of spending time on deciding "is `int` luckily going to work
today" (or performing an interval analysis involving making guesses
about potential input sample sizes / the ranges of `vector.size()` and
`d`) it's simpler to just use the proper type at all times.
Now, regarding the rounding and the "adding 0.5" trick, there are some
subtleties here worth being aware of:
http://blog.frama-c.com/index.php?post/2013/05/02/nearbyintf1
If you can use C++11 the right choice would be to use these:
http://en.cppreference.com/w/cpp/numeric/math/round
Refs:
https://randomascii.wordpress.com/2012/04/05/floating-point-complexities/
https://randomascii.wordpress.com/2012/02/25/comparing-floating-point-numbers-2012-edition/
Best,
Matt
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