seq_len
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