It is quite strange, but the following code sometimes produces a "core.exception.AssertError@C:\d\dmd2\src\phobos\std\array.d(253): Attempting to fetch the front of an empty array" while trying to sort the array and sometimes runs just fine:

import std.stdio,
       std.random,
       std.math,
       std.algorithm;

struct Point {
    double x, y;
    static Point opCall( double x, double y ) {
        Point nPoint;
        nPoint.x = x;
        nPoint.y = y;
        return nPoint;
    }
    double angle() {
        return (atan2( y, x ) / PI * 180);
    }
}

void printAngles( Point[] contour ) {
    foreach( Point p; contour )
        writef( "%0.1f ", p.angle() );
    writeln();
}

int main()
{
    Point[] path;
    path.length = 5;
    foreach( inout p; path )
        p = Point( uniform( 0, 100 ), uniform( 0, 100 ) );

    printAngles( path );
    sort!("a.angle() < b.angle()")(path);
    printAngles( path );

    return 0;
}

Using "a.x < b.x" as a sorting criterium produces normal behavior, so I guess the problem is caused by using a function there. I'm new to D, and so I have no idea, where exactly the source of this problem lies. Can somebody point me in the right direction, please?

I'm using the version 2.037 of the compiler with phobos.

nnk wrote:
> It is quite strange, but the following code sometimes produces a "core.exception.AssertError@C:\d\dmd2\src\phobos\std\array.d(253): Attempting to fetch the front of an empty array" while trying to sort the array and sometimes runs just fine:
> 
> import std.stdio,
>        std.random,
>        std.math,
>        std.algorithm;
> 
> struct Point {
>     double x, y;
>     static Point opCall( double x, double y ) {
>         Point nPoint;
>         nPoint.x = x;
>         nPoint.y = y;
>         return nPoint;
>     }
>     double angle() {
>         return (atan2( y, x ) / PI * 180);
>     }
> }
> 
> void printAngles( Point[] contour ) {
>     foreach( Point p; contour )
>         writef( "%0.1f ", p.angle() );
>     writeln();
> }
> 
> int main()
> {
>     Point[] path;
>     path.length = 5;
>     foreach( inout p; path )
>         p = Point( uniform( 0, 100 ), uniform( 0, 100 ) );
> 
>     printAngles( path );
>     sort!("a.angle() < b.angle()")(path);
>     printAngles( path );
> 
>     return 0;
> }
> 
> Using "a.x < b.x" as a sorting criterium produces normal behavior, so I guess the problem is caused by using a function there. I'm new to D, and so I have no idea, where exactly the source of this problem lies. Can somebody point me in the right direction, please?
> 
> I'm using the version 2.037 of the compiler with phobos.

It's a compiler bug.

change Point.angle to

    double angle() {
	double rv = atan2( y, x ) / PI * 180;
        return rv;
    }

and the problem goes away.

However use a real or the direct return and it happens.

    double angle() {
	real rv = atan2( y, x ) / PI * 180;
        return rv;
    }
Some how it must be returning NaN, which will then bugger the sort.

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