R: loop through list

Question

I've come over this problem a few times by now, and this time I am eager to get an efficient solution. The underlying problem is that I want to loop through a list of variables with the paste function.

dat <- read.csv("some file", header=TRUE)  

list.R <- c("IC","IG","DM","IM","IN","EN","RM")  

for (RO in list.R){
            paste("dat$",RO,"_I", sep="")[
            paste("dat$",RO,"_I", sep="") == 
            "Strongly disagree"] <- 1
            }

I pasted the name of the variable together, but this gives me a string in block quotes. I've tried the following but nothing worked so far:

eval(parse(text=paste("dat$",RO,"_I", sep="")))

or

get(paste("dat$",RO,"_I", sep=""))

do you know how to solve this so that the loop works? I would very much appreciate your help :)

(I know in this case I could also use as.numeric(levels(dat$IC_I))[dat$IC_I] but the order of the levels is wrong)

Solution

You can do this with simple assignment operators -- there's no need for a loop (as is usually the case in R). First, I'll construct a sample data frame, with your variables stored as the character type instead of as factors:

dat <- data.frame(id=1:2, ID_I=c("Agree", "Strongly Disagree"), IG_I=c("Strongly Disagree", "Agree"), stringsAsFactors=FALSE)
dat
#   id              ID_I              IG_I
# 1  1             Agree Strongly Disagree
# 2  2 Strongly Disagree             Agree

Now you can use column indexing to replace "Strongly Disagree" with 1:

cols <- c("ID_I", "IG_I")
dat[,cols][dat[,cols] == "Strongly Disagree"] <- 1
dat
#   id  ID_I  IG_I
# 1  1 Agree     1
# 2  2     1 Agree