C How to round a double to an int

Question

I have a double  call it x   meant to be 55 but in actuality stored as 54 999999999999943157 which I just realised   So when I do   double x   54 999999999999943157  int y    int  x    y   54 instead of 55   This puzzled me for a long time  How do I get it to correctly round

User · Answer

It is worth noting that what you re doing isn t rounding  it s casting  Casting using  int  x truncates the decimal value of x  As in your example  if x   3 9995  the  9995 gets truncated and x   3   As proposed by many others  one solution is to add 0 5 to x  and then cast

User · Answer

add 0 5 before casting  if x   0  or subtract 0 5  if x  lt  0   because the compiler will always truncate   float x   55     stored as 54 999999    x   x   0 5 -  x lt 0      x is now 55 499999    int y    int x     truncated to 55   C  11 also introduces std  round  which likely uses a similar logic of adding 0 5 to  x  under the hood  see the link if interested  but is obviously more robust   A follow up question might be why the float isn t stored as exactly 55  For an explanation  see this stackoverflow answer

User · Answer

Casting to an int truncates the value  Adding 0 5 causes it to do proper rounding   int y    int  x   0 5

User · Answer

include  lt iostream gt   include  lt cmath gt  using namespace std   int main         double x 54 999999999999943157      int y ceil x    The ceil   function returns the smallest integer no less than x     return 0

User · Answer

Casting is not a mathematical operation and doesn t behave as such   Try  int y    int round x

[c++] C++: How to round a double to an int?

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