Jin Shusong wrote: > Dear R Users, > > Are there any documents on the usage of apply, tapply, > sapply so that I avoid explicit loops. I found that these > three functions were quite hard to be understood. Thank you > in advance.
If you have read the help pages for each and possibly even consulted the reference on those help pages, you may need to elaborate on what parts of these functions you don't understand. You might also describe a loop you are contemplating and ask how it might be replaced by one of these functions. Here is a very simple example of a loop that could be avoided with one of these functions: > for(i in 1:4){print(mean(iris[,i]))} [1] 5.843333 [1] 3.057333 [1] 3.758 [1] 1.199333 Here is how you would do that with apply(): > apply(iris[,1:4], 2, mean) Sepal.Length Sepal.Width Petal.Length Petal.Width 5.843333 3.057333 3.758000 1.199333 Even better in this particular case would be: > colMeans(iris[,1:4]) Sepal.Length Sepal.Width Petal.Length Petal.Width 5.843333 3.057333 3.758000 1.199333 but you don't always want mean() or sum() as the function, so the functions you mention above are more general than colMeans() and similar functions. > > ------------------------------------------------------------------------ > > ______________________________________________ > R-help@stat.math.ethz.ch 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. -- Chuck Cleland, Ph.D. NDRI, Inc. 71 West 23rd Street, 8th floor New York, NY 10010 tel: (212) 845-4495 (Tu, Th) tel: (732) 512-0171 (M, W, F) fax: (917) 438-0894 ______________________________________________ R-help@stat.math.ethz.ch 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.