Does casting double to int returns the round down number?

Question

I have the folowing example :

double x = 2.5;
int n = (int)x;
int k = (int) floor(x);

Does casting double to int returns the round down number? or I should use floor function?

it´ll return *truncated* value: `2.5 -> 2`; `2.99 -> 2` (fractional part removed) — Dmitry Bychenko, Feb 12 '16 at 15:55
Related: [Why does a float when converted to an int be rounded off below in C?](http://stackoverflow.com/q/35008621/3049655) — Spikatrix, Feb 12 '16 at 16:53
"should use floor function?" --> consider using the `(int) round(x)` to round to nearest. — chux - Reinstate Monica, Feb 12 '16 at 20:14

user3813674 · Answer 1 · 2016-02-12T16:00:50.880

16

Be careful with negative numbers. Casting will truncate towards 0. floor will truncate towards negative infinite.

If the value is positive, then both methods return the same truncated value.

edited Feb 12 '16 at 16:00

answered Feb 12 '16 at 15:57

user3813674

Right. I totally forgot about negative rounding rules and what rounding up or down means there. – SergeyA Feb 12 '16 at 15:59

score 7 · Answer 2 · answered Feb 12 '16 at 16:00

Well (int) truncates the initial double value (fractional part removed)

  2.1  -> 2
  2.5  -> 2
  2.99 -> 2

it looks like floor, but they're not the same in case of negative numbers:

  (int)-2.1   -> -2
  floor(-2.1) -> -3.0

since floor returns max integer value that less or equal to the given argument

score 1 · Answer 3 · answered Feb 12 '16 at 16:27

1

Beware of the limitation of int and double.

int main(void)
{
    double val1 = INT_MAX + 10.5;
    int    val2 = val1;

    printf("val1 = %f\nval2 = %d\n", val1, val2);

    return (0);
}

This produce the output below :

val1 = 2147483657.500000
val2 = -2147483648

(I don't know if INT_MAX is standard, sorry).

answered Feb 12 '16 at 16:27

Tom's

1

casting an out of range double to int is undefined behavior – bolov Feb 12 '16 at 16:35
I didn't know, that's one more reason to be careful when casting. – Tom's Feb 12 '16 at 16:40
INT_MAX is standard. Please add information about undefined behavior into the answer and also answer the actual question. – 2501 Feb 21 '16 at 16:31

3 Answers3