[R] iterating over groups of columns

[Greg Snow]

You could try something like this (untested):


> tmpfun <- function(i) {
+  tmp <- paste( "^k.", i, sep="")
+  apply( subset(mydf, select=grep(tmp, names(mydf) ) ), 1, min ) 
+ }
> out1 <- lapply(1:10, tmpfun)
> names(out1) <- paste( "min.k.", 1:10, sep="" )
> mydf2 <- cbind(mydf, out1)



-- 
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at imail.org
801.408.8111


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of 09wkj
> Sent: Tuesday, June 08, 2010 4:32 PM
> To: r-help at r-project.org
> Subject: Re: [R] iterating over groups of columns
> 
> In the code fragment, I used 'by' to actually compute the min value
> (part of the statement with the eval) - and I agree that an apply would
> work there wonderfully.
> 
> However, my hope was to use an apply for the subsetting of the
> data.frame's columns, so that I could then use an apply to compute the
> min across each row of the subsets.
> 
> Something that would give me the results of the following, but
> programmatically:
> apply(the.data[,1], 1, min)        #min of the first column
> apply(the.data[,2:3], 1, min)    #min of the next 2 columns
> apply(the.data[,4:6], 1, min)    #min of the next 3 columns
> apply(the.data[,7:10], 1, min)  #min of the next 4 columns
> ...
> apply(the.data[,46:55], 1, min)#min of the next 10 columns
> 
> 
> 
> Like, can I make a vector of levels with 'rep(1:10,1:10)', and then
> apply the function across all columns in each level? And then how could
> I cbind them together?
> 
> 
> Thanks for any help,
> Bill
> 
> 
> 
> 
> On Jun 8, 2010, at 5:08 PM, Jannis wrote:
> 
> > you should have found a solution for that in the help page of apply.
> >
> > just run
> >
> > min.values = apply(the.data,1,min)
> >
> > the '1' marks the direction (e.g. whether apply is applied to rows or
> columns), it could be a 2 as well. Check that yourself in the apply
> documentation.
> >
> > Then run rbind(the.data,min.values) (could be cbind as well, I am not
> sure again ;-) ) and you get what you want.
> >
> > 09wkj schrieb:
> >> I am mainly a Java/C++ programmer, so my mind is used to iterating
> over data with for loops. After a long break, I am trying to get back
> into the "R mindset", but I could not find a solution in the
> documentation for the applys, aggregate, or by.
> >>
> >> I have a data.frame where each row is an entry with 10 groups of
> measurements. The first measurement spans 1 column, the second spans 2
> columns, third 3, and so on (55 total columns). What I want to do is
> add to my data.frame 10 new columns containing the minimum value of
> each measurement.
> >>
> >> dim(the.data)
> >> [1] 1679  55
> >>
> >>
> >>> colnames(the.data)
> >>>
> >>  [1] "k.1.1"   "k.2.1"   "k.2.2"   "k.3.1"   "k.3.2"   "k.3.3"
> "k.4.1"    [8] "k.4.2"   "k.4.3"   "k.4.4"   "k.5.1"   "k.5.2"
> "k.5.3"   "k.5.4"   [15] "k.5.5"   "k.6.1"   "k.6.2"   "k.6.3"
> "k.6.4"   "k.6.5"   "k.6.6"   [22] "k.7.1"   "k.7.2"   "k.7.3"
> "k.7.4"   "k.7.5"   "k.7.6"   "k.7.7"   [29] "k.8.1"   "k.8.2"
> "k.8.3"   "k.8.4"   "k.8.5"   "k.8.6"   "k.8.7"   [36] "k.8.8"
> "k.9.1"   "k.9.2"   "k.9.3"   "k.9.4"   "k.9.5"   "k.9.6"   [43]
> "k.9.7"   "k.9.8"   "k.9.9"   "k.10.1"  "k.10.2"  "k.10.3"  "k.10.4"
> [50] "k.10.5"  "k.10.6"  "k.10.7"  "k.10.8"  "k.10.9"  "k.10.10"
> >>
> >> I want to add to the.data new columns: min.k.1, min.k.2, ...,
> min.k.10
> >>
> >> This is the section of code I would like to improve, hopefully
> getting rid of the eval and the for loop:
> >>
> >> for(k in 1:10){
> >>    s <- subset(the.data, select=paste("k", k, 1:k, sep="."))
> >>    eval(parse(text = paste("the.data$min.k.", k, "<-as.vector(by(s,
> 1:nrow(s), min))", sep="")))
> >> }
> >>
> >> Thanks for any help,
> >> Bill
> >> ______________________________________________
> >> R-help at 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.
> >>
> >>
> >
> 
> ______________________________________________
> R-help at 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.