Getting substrings and storing them in variables using a loop

Question

My issue consists of splitting a path, getting all the subpaths till the next "$" ocurrence (a kind of cumulative subpath) and generating a new variable for each of them.

Doing it step by step I get the desired output:

data<-data.frame(path=c("A/A/$/B/$/A/$","B/C/$","B/C/$/C/$/A/B/$"),stringsAsFactors=FALSE)
library(stringr)
data$tr<-str_count(data$path,"\\$")
data$tr_1<-substr(sapply(strsplit(data$path, "\\$"), `[[`, 1),1,nchar(sapply(strsplit(data$path, "\\$"), `[[`, 1))-1)
data$tr_2<-ifelse(is.na(sapply(strsplit(data$path, "\\$"), `[`, 2))==TRUE,
                  "",
                  paste0(data$tr_1,substr(sapply(strsplit(data$path, "\\$"), `[`, 2),1,nchar(sapply(strsplit(data$path, "\\$"), `[`, 2))-1)))
data$tr_3<-ifelse(is.na(sapply(strsplit(data$path, "\\$"), `[`, 3))==TRUE,
                  "",
                  paste0(data$tr_2,substr(sapply(strsplit(data$path, "\\$"), `[`, 3),1,nchar(sapply(strsplit(data$path, "\\$"), `[`, 3))-1)))

Trying to do the same in a loop according to Creating new named variable in dataframe using loop and naming convention, the output fails.

data<-data[,-c(4,5)]
for (i in 2:max(data$tr)) {
  data[[paste0("tr_",i)]]<-ifelse(is.na(sapply(strsplit(data$path, "\\$"), `[`, i))==TRUE,
                  "",
                  paste0(data$tr_i-1,substr(sapply(strsplit(data$path, "\\$"), `[`, i),1,nchar(sapply(strsplit(data$path, "\\$"), `[`, i))-1)))
}

Is there another way of doing it recursively? (each new variable uses the previous one).
Thanks in advance!

Roland · Accepted Answer · 2017-01-12 09:27:19Z

1

I'd do this:

data<-data.frame(path=c("A/A/$/B/$/A/$","B/C/$","B/C/$/C/$/A/B/$"),stringsAsFactors=FALSE)

#split strings
tmp <- strsplit(data$path, "/$", fixed = TRUE) #thanks to David
data$tr <- lengths(tmp)

#paste them together cumulatively
tmp <- lapply(tmp, Reduce, f = paste0, accumulate = TRUE)

#create data.frame
tmp <- lapply(tmp, `length<-`, max(lengths(tmp)))
tmp <- setNames(as.data.frame(do.call(rbind, tmp), stringsAsFactors = FALSE), 
                paste0("tr_", seq_len(max(data$tr))))

data <- cbind(data, tmp)
#             path tr tr_1  tr_2      tr_3
#1   A/A/$/B/$/A/$  3  A/A A/A/B   A/A/B/A
#2           B/C/$  1  B/C  <NA>      <NA>
#3 B/C/$/C/$/A/B/$  3  B/C B/C/C B/C/C/A/B

If you must, you can replace NA values with empty strings in another lapply loop.

edited Jan 12, 2017 at 9:27

answered Jan 12, 2017 at 9:09

Roland

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