C++

Forum

 
How numbers are rounded

CuddleBunniezzz12 (9)
How are floats rounded to integers in C++? Does it only round up, or does it work like this:

1
2
3
 
1.1 = 1
1.4 = 1
1.9 = 2
tummychow (1210)
I'm pretty sure truncation is compiler independent but will almost always be down (floor).
CuddleBunniezzz12 (9)
Yea, your right, just tested it, it rounds down.
firedraco (6243)
AFAIK it just removes the non-integer part.

1
2
3
4
 
1.1 = 1
2.6 = 2
-1.2 = -1
//etc
chrisname (7395)
If you need integer numbers that are correctly rounded, just add 0.5 before converting to integer.
closed account (z05DSL3A)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
 
double round(double num) 
{
    return (num > 0.0) ? floor(num + 0.5) : ceil(num - 0.5);
}
int round2int(double num)
{
    return (int) round( num);
}

int main()
{
    cout << "  x    (int)x   (int)x +0.5   round(x)  round2int(x)" << endl;
    cout << "====   ======   ===========   ========  ============" << endl;
    for(int i(0); i < 21; ++i)
    {
        double x = 1.0 - (0.1 * i);
        cout.width(6);
        cout << x;
        cout.width(7);
        cout << (int)x;
        cout.width(14);
        cout << (int) (x + 0.5);
        cout.width(11);
        cout << round(x);
        cout.width(14);
        cout << round2int(x) << endl;
    }
}

  x    (int)x   (int)x +0.5   round(x)  round2int(x)
====   ======   ===========   ========  ============
     1      1             1          1             1
   0.9      0             1          1             1
   0.8      0             1          1             1
   0.7      0             1          1             1
   0.6      0             1          1             1
   0.5      0             1          1             1
   0.4      0             0          0             0
   0.3      0             0          0             0
   0.2      0             0          0             0
   0.1      0             0          0             0
     0      0             0         -0             0
  -0.1      0             0         -0             0
  -0.2      0             0         -0             0
  -0.3      0             0         -0             0
  -0.4      0             0         -0             0
  -0.5      0             0         -1            -1
  -0.6      0             0         -1            -1
  -0.7      0             0         -1            -1
  -0.8      0             0         -1            -1
  -0.9      0             0         -1            -1
    -1     -1             0         -1            -1
Press any key to continue . . .


Edit:
The round() function should probably be the following to stop it rounding 0.0 to -0.0
1
2
3
4
 
double round(double num) 
{
    return (num < 0.0) ? ceil(num - 0.5) : floor(num + 0.5);
}

Last edited on
Topic archived. No new replies allowed.