> -----Original Message----- > From: [email protected] > [mailto:[email protected]] On Behalf Of Darcy Webber > Sent: Tuesday, February 15, 2011 4:10 PM > To: [email protected] > Subject: [R] distance between consecutive points > > Dear R users, > > I have two coloumns of data, say x and y, referring to a list of > points in 2D space. I am trying to develop a code that will give me > the distances (using Pythagoras) between consecutive points (xi,yi) > and (xi+1,yi+1). So far I have come up with the following: > > for (i in 1:length(x)) d<-sqrt((x[i+1]-x[i])^2+(y[i+1]-y[i])^2)
You want to assign to d[i] (where d is a preallocated numeric vector of length length(x)-1), not d, so you save all the distances and you don't want to do this for i==length(n), as there is no x[i+1] then. Here is more idiomatic R code that does it with a loop: > hypot <- function(x,y)sqrt(x^2 + y^2) # must be a built-in for this > d <- hypot( diff(x), diff(y) ) > d [1] 0.01 Bill Dunlap Spotfire, TIBCO Software wdunlap tibco.com > > For example, if I use the two points (note, I have hundreds > of points for x,y) > x<-c(64.59,64.60) > y<-c(-179.28,-179.28) > > d should be 0.01. > > But it just doesn't give me the correct answer and I can't figure out > why. Any help would be much appreciated. > > Cheers, > D'Arcy > > ______________________________________________ > [email protected] mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide > http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
