[問題] 請教 null 的用法

[netsm (小黑)]

請教各位高手，下面的寫法是甚麼意思?

謝謝!



 if (Math.Abs(t - 0.3) < dt / 2)
    {
      write("..\\(2D)u" + filename + ".plt", u);
      write("..\\(2D)uexact_" + filename + ".plt", null);
    }

//////

public static void write(string fname, fun ux)
{
 FileStream fs = new FileStream(fname, FileMode.Create);
 StreamWriter sw = new StreamWriter(fs, Encoding.Default);
 sw.WriteLine("VARIABLES=x, y, u1");
 sw.WriteLine("ZONE T=\"dx={0}, dy={1}\", I={2}, J={3}, F=POINT",
                            dx,     dy,    Ni/2,   Nj);
 for (double y = 0; y <= MaxY; y += dy)
 for (double x = 0; x <= MaxX; x += dx)
   {

    double uk=(ux==null)? (exact(t,x,y)): ux.f1(0, x, y, 0, 0, 0);

    sw.WriteLine("{0},{1},{2}", x, y, uk);
   }
   sw.Close();
}

請教: double uk=(ux==null)? (exact(t,x,y)): ux.f1(0, x, y, 0, 0, 0);

是甚麼樣的邏輯?
我讀出來的檔案 write("..\\(2D)uexact_" + filename + ".plt", null);

只有時間t有值，而x,y皆為零，是否與 null 有關?





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