Charilaos Skiadas wrote: > > On Dec 8, 2007, at 1:02 AM, Joe W. Byers wrote: > >> In a post on R-devel, Prof Ripley add the following comment >> | > BTW, 1:dim(names)[1] is dangerous: it could be 1:0. That was the >> | > motivation for seq_len. >> >> I use the dim(names)[1] and dim(x)[2] along with length(x) with varying >> levels of frustration depending on the object which I am trying to get >> the dimensions. I found the reference to seq_len interesting since it >> is a function that I have never seen (probably just missed it reading >> the docs). >> >> I was hoping someone could expand on the benefits of seq_len. > > I think that example says it all. But in simpler form, suppose x is a > vector, and you want to produce a regular sequence of integers of the > same length. What should happen i the vector x has length 0? Here's > the output of the two commands. > > x<-numeric(0) > > y<-length(x) > > y > [1] 0 > > 1:y > [1] 1 0 > > seq_len(y) > integer(0) > > Other than treating the edge case correctly, the only other advantage > of seq_len, that I am aware of, is that it is faster. Not sure how > often that ends up mattering though. > >> Happy Holidays >> Joe >> > > Haris Skiadas > Department of Mathematics and Computer Science > Hanover College > > > >
The essence of this feature is to get the correct index sequences when performing matrix lookups and loops without causing errors in the matrices or subscripts. If I build the correct x=seq_len or x=seq(along=) and use the 'in x' not 'in 1:dim(x)[1]' or 'in 1:length(x)', my code will execute correctly without the NA/NAN error or subscript out of bounds, etc. Is this correct? Examples > x=numeric() > x numeric(0) > for ( i in 1:x) print(i) Error in 1:x : NA/NaN argument > for ( i in x) print(i) > x=2 > for ( i in x) print(i) [1] 2 > seq_len(x) [1] 1 2 > x=seq_len(x) > for ( i in x) print(i) [1] 1 [1] 2 > for ( i in 1:x) print(i) [1] 1 Warning message: In 1:x : numerical expression has 2 elements: only the first used > for ( i in x) print(i) [1] 1 [1] 2 > x=2 > for ( i in 1:x) print(i) [1] 1 [1] 2 > for ( i in x) print(i) [1] 2 > for (i in 1:length(x)) print(i) [1] 1 > Thank you Joe ______________________________________________ R-help@r-project.org 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.
