[R] iterating over groups of columns

From: 09wkj <Bill.K.Jannen_at_williams.edu>
Date: Tue, 08 Jun 2010 16:15:06 -0400


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



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