[R] Calculating distances between points in a data frame?

[Sander Oom]

Dear list,

I would like to calculate the distance between consecutive points in a data 
frame. Of course the first point in the data frame does not have a point of 
origin, and should get a value NA. I have tried two different loops, which 
both result in error:

 > num <- seq(0,10,1)
 > X <- seq(0,30,3)
 > Y <- seq(0,40,4)
 > XY <- data.frame(num, X, Y)
 > attach(XY)
 > summary(XY)
       num             X              Y
  Min.   : 0.0   Min.   : 0.0   Min.   : 0
  1st Qu.: 2.5   1st Qu.: 7.5   1st Qu.:10
  Median : 5.0   Median :15.0   Median :20
  Mean   : 5.0   Mean   :15.0   Mean   :20
  3rd Qu.: 7.5   3rd Qu.:22.5   3rd Qu.:30
  Max.   :10.0   Max.   :30.0   Max.   :40
 > plot(X,Y)
 > rngNum <- range(num)
 > for (i in rngNum){
+     XY$DistXY[i] <- sqrt( ((X[i]-X[i-1])^2) + ((Y[i]-Y[i-1])^2) )
+ }
Error in "$<-.data.frame"(`*tmp*`, "DistXY", value = sqrt(((X[i] - X[i -  :
         replacement has 10 rows, data has 11
 > for (i in rngNum){
+     XY$DistXY2[i] <- ifelse(i=min(rngNum), NA, sqrt(((X[i]-X[i-1])^2) + 
((Y[i]-Y[i-1])^2)) )
+ }
Error in ifelse(i = min(rngNum), NA, sqrt(((X[i] - X[i - 1])^2) + ((Y[i] -  :
         unused argument(s) (i ...)
 > detach(XY)
 >

Any suggestions much appreciated,

Sander Oom.


--------------------------------------------------------------
Dr. Sander P. Oom
Animal, Plant and Environmental Sciences
University of the Witwatersrand
Private Bag 3
Wits 2050
South Africa

Tel (work)      +27 (0)11 717 64 04
Tel (home)      +27 (0)18 297 44 51
Fax             +27 (0)18 299 24 64

Email   sander at oomvanlieshout.net
Web     www.oomvanlieshout.net/sander