[R] ugly loop

[Bill Simpson]

Thanks Marc for your help.

> > The following code is slow and ugly:
> > 
> > count<-0
> > for(i in 1:nrow(ver))
> >   for(j in 1:ncol(ver))
> >     {
> >     count<-count+1
> >     x[count]<-pt$x[ver[i,j]]
> >     y[count]<-pt$y[ver[i,j]]
> >     z[count]<-pt$z[ver[i,j]]
> >     }
> > 
> > Please help me make it better.
> > 
> > Thanks!
> 
> The following should work:
> 
> > ver <- matrix(sample(1:16, 16), ncol = 4)
> > pt <- data.frame(x = sample(1:16, 16), 
> +                  y = sample(1:16, 16),
> +                  z = sample(1:16, 16))
>  
> > ver
>      [,1] [,2] [,3] [,4]
> [1,]    8    9    5   13
> [2,]   14   16    1   10
> [3,]   12    2   11    7
> [4,]    6    3    4   15
> > pt
>     x  y  z
> 1   6 15 15
> 2   9  2  3
> 3  11  1  5
> 4  14  4 10
> 5  13  7 14
> 6   1 14  7
> 7  15 10  4
> 8  10  5 12
> 9   4 12  2
> 10  8  8 13
> 11 16 11  1
> 12  7 13  9
> 13  2 16 11
> 14  3  9 16
> 15  5  6  8
> 16 12  3  6
> 
> > x <- pt$x[ver]
> > y <- pt$y[ver]
> > z <- pt$z[ver]
This doesn't give the same results as my original code -- it scrambles the 
order.

OK I will explain my example.

pts contains the x, y, z coordinates of some 3D points. These points are 
the vertices of 3D triangles.

ver contains the indexes into pts.

each line of ver contains 3 vertices -- they are the corners of a 
triangle. For example, if line 1 of ver is
10 9 7
That means I need to draw a triangle whose coordinates are
pt$x[10],pt$y[10],pt$z[10]
pt$x[9],pt$y[9],pt$z[9]
pt$x[7],pt$y[7],pt$z[7]

Now it should be clear why the ordering is critical.
I am using rgl.triangles() to plot. It requires the x,y,z coordinates in 
the order I gave in my original code.

Cheers
Bill