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digitalmars.D.learn - Issue with small floating point numbers

reply Tim <tim.oliver tutanota.com> writes:
Hello all,

I have this piece of code
```D
/**
Rotate a 2D array (Vector) by phi radians

Params:
     vec = 2D Vector to rotate
     phi = Degree with which to rotate the Vector in radians

Returns:
     Rotated 2D array (Vector)

Example:

*/
pragma(inline, true)
Point2 rotate2D(in Point2 vec, in float phi) pure nothrow {
     double x = (vec[0]*cos(phi)) - (vec[1]*sin(phi));
     double y = (vec[0]*sin(phi)) + (vec[1]*cos(phi));
     return [x, y];
}

unittest{
     auto p = rotate2D([0.0, 10.0], PI_2);
     assert(p == [-10.0, 0.0]);
}
```

When I run the unittest, I get ```[-10, -4.37114e-07]``` back, 
which is obviously wrong. Any idea as to why it's not making the 
y-axis zero? Is it a rounding issue with the types I'm using?

Thanks in advance
May 12
next sibling parent reply Alain De Vos <devosalain ymail.com> writes:
Not is is not wrong it is wright.
Because you use not pi but an approximation of pi the result is 
not zero but an approximation of zero.
May 12
parent reply Tim <tim.oliver tutanota.com> writes:
On Thursday, 13 May 2021 at 03:46:28 UTC, Alain De Vos wrote:
 Not is is not wrong it is wright.
 Because you use not pi but an approximation of pi the result is 
 not zero but an approximation of zero.
Oh, of course. Jesus that sucks big time. Any idea on how to use assert with an approximate number like this?
May 12
next sibling parent JG <someone somewhere.com> writes:
On Thursday, 13 May 2021 at 03:48:49 UTC, Tim wrote:
 On Thursday, 13 May 2021 at 03:46:28 UTC, Alain De Vos wrote:
 Not is is not wrong it is wright.
 Because you use not pi but an approximation of pi the result 
 is not zero but an approximation of zero.
Oh, of course. Jesus that sucks big time. Any idea on how to use assert with an approximate number like this?
You could try and use this [this](https://dlang.org/library/std/math/is_close.html)
May 12
prev sibling parent Alain De Vos <devosalain ymail.com> writes:
I would calculate the squared distance to the point (-10,0) and 
check it is small enough for practical use.
```
double squared_distance=(p.x+10) * (p.x+10)+p.y * p.y
assert (squared_distance < 1e-10);

```
May 12
prev sibling next sibling parent Zardoz <luis.panadero gmail.com> writes:
On Thursday, 13 May 2021 at 03:03:37 UTC, Tim wrote:
 Hello all,

 I have this piece of code
 ```D
 /**
 Rotate a 2D array (Vector) by phi radians

 Params:
     vec = 2D Vector to rotate
     phi = Degree with which to rotate the Vector in radians

 Returns:
     Rotated 2D array (Vector)

 Example:

 */
 pragma(inline, true)
 Point2 rotate2D(in Point2 vec, in float phi) pure nothrow {
     double x = (vec[0]*cos(phi)) - (vec[1]*sin(phi));
     double y = (vec[0]*sin(phi)) + (vec[1]*cos(phi));
     return [x, y];
 }

 unittest{
     auto p = rotate2D([0.0, 10.0], PI_2);
     assert(p == [-10.0, 0.0]);
 }
 ```

 When I run the unittest, I get ```[-10, -4.37114e-07]``` back, 
 which is obviously wrong. Any idea as to why it's not making 
 the y-axis zero? Is it a rounding issue with the types I'm 
 using?

 Thanks in advance
You should try to use isClose to compare for floats equality : https://dlang.org/phobos/std_math.html#.isClose Float arithmetic isn't exact, and could give unexpected results like 0.1f + 0.2f != 0.3f
May 12
prev sibling parent Berni44 <someone somemail.com> writes:
On Thursday, 13 May 2021 at 03:03:37 UTC, Tim wrote:
 ```
 unittest{
     auto p = rotate2D([0.0, 10.0], PI_2);
     assert(p == [-10.0, 0.0]);
 }
 ```
I suggest ``` unittest { auto p = rotate2D([0.0, 10.0], PI_2); assert(isClose(p[0], -10.0)); assert(isClose(p[1], 0.0, 0.0, 1e-6)); } ``` In the second test, the value is compared against zero, which is somewhat special - you need to specify an acceptable distance from zero to get it right. You could also improve your result by making the `phi` a `double` value. In this case you can replace the `1e-6` above by `1e-15` which is much closer to zero.
May 13