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ugly loop

8 messages · Dimitris Rizopoulos, Marc Schwartz, Bill Simpson

Bill Simpson
# 
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!

Bill
Dimitris Rizopoulos
# 
maybe you want something like this:

x <- c(t(pt$x))


Best,
--D

----
Dimitris Rizopoulos
Ph.D. Student
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/16/336899
Fax: +32/16/337015
Web: http://www.med.kuleuven.ac.be/biostat/
     http://www.student.kuleuven.ac.be/~m0390867/dimitris.htm


----- Original Message ----- 
From: "Bill Simpson" <William.Simpson at drdc-rddc.gc.ca>
To: "r-help" <r-help at stat.math.ethz.ch>
Sent: Friday, April 22, 2005 2:58 PM
Subject: [R] ugly loop
Bill Simpson
# 
To clarify: I want to get rid of the loop over i,j
Here is a simpler example. ver is a 2D matrix

count<-0
for(i in 1:nrow(ver))
  for(j in 1:ncol(ver))
    {
    count<-count+1
    x[count]<-ver[i,j]
    }

Bill
Marc Schwartz
#
On Fri, 2005-04-22 at 08:58 -0400, Bill Simpson wrote:
The following should work:
+                  y = sample(1:16, 16),
+                  z = sample(1:16, 16))
[,1] [,2] [,3] [,4]
[1,]    8    9    5   13
[2,]   14   16    1   10
[3,]   12    2   11    7
[4,]    6    3    4   15
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
[1] 10  3  7  1  4 12  9 11 13  6 16 14  2  8 15  5
[1]  5  9 13 14 12  3  2  1  7 15 11  4 16  8 10  6
[1] 12 16  9  7  2  6  3  5 14 15  1 10 11 13  4  8


Keep in mind that a matrix is a vector with dims, so you can fill a
vector from the matrix simply by doing the indexing with a single value,
which will do the fill indexed column by column.

HTH,

Marc Schwartz
Dimitris Rizopoulos
# 
then use

x <- c(t(ver))


Best,
--D

----
Dimitris Rizopoulos
Ph.D. Student
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/16/336899
Fax: +32/16/337015
Web: http://www.med.kuleuven.ac.be/biostat/
     http://www.student.kuleuven.ac.be/~m0390867/dimitris.htm


----- Original Message ----- 
From: "Bill Simpson" <William.Simpson at drdc-rddc.gc.ca>
To: "r-help" <r-help at stat.math.ethz.ch>
Sent: Friday, April 22, 2005 3:17 PM
Subject: Re: [R] ugly loop
Bill Simpson
# 
Thanks Marc for your help.
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
Marc Schwartz
#
On Fri, 2005-04-22 at 09:31 -0400, Bill Simpson wrote:
That's what I get for not comparing your results against my own.  I just
noted that you are going by row and not by column. So, using the same
data above:
+   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]]
+     }
[1] 10  4 13  2  3 12  6  8  7  9 16 15  1 11 14  5
[1]  5 12  7 16  9  3 15  8 13  2 11 10 14  1  4  6
[1] 12  2 14 11 16  6 15 13  9  3  1  4  7  5 10  8


Thus, I just need to use t(ver) instead of ver:
[1] 10  4 13  2  3 12  6  8  7  9 16 15  1 11 14  5
[1]  5 12  7 16  9  3 15  8 13  2 11 10 14  1  4  6
[1] 12  2 14 11 16  6 15 13  9  3  1  4  7  5 10  8

That should do it?

HTH,

Marc
<Off to make another pot of coffee....>
Bill Simpson
#
On Fri, 22 Apr 2005, Marc Schwartz wrote:
Yep!!

Thanks very much Marc and others who suggested this.

Cheers
Bill