Concatenate dichotome columns to semicolon-separated column - r

Question

I have data frame containing the results of a multiple choice question. Each item has either 0 (not mentioned) or 1 (mentioned). The columns are named like this:
F1.2_1, F1.2_2, F1.2_3, F1.2_4, F1.2_5, F1.2_99
etc.
I would like to concatenate these values like this: The new column should be a semicolon-separated string of the selected items. So if a row has a 1 in F1.2_1, F1.2_4 and F1.2_5 it should be: 1;4;5
The last digit(s) of the dichotome columns are the item codes to be used in the string.
Any idea how this could be achieved with R (and data.table)? Thanks for any help!
edit:
Here is a example DF with the desired result:
structure(list(F1.2_1 = c(0L, 1L, 0L, 1L), F1.2_2 = c(1L, 0L,
0L, 1L), F1.2_3 = c(0L, 1L, 0L, 1L), F1.2_4 = c(0L, 1L, 0L, 0L
), F1.2_5 = c(0L, 0L, 0L, 0L), F1.2_99 = c(0L, 0L, 1L, 0L), desired_result = structure(c(3L,
2L, 4L, 1L), .Label = c("1;2;3", "1;3;4", "2", "99"), class = "factor")), .Names = c("F1.2_1",
"F1.2_2", "F1.2_3", "F1.2_4", "F1.2_5", "F1.2_99", "desired_result"
), class = "data.frame", row.names = c(NA, -4L))
F1.2_1 F1.2_2 F1.2_3 F1.2_4 F1.2_5 F1.2_99 desired_result
1 0 1 0 0 0 0 2
2 1 0 1 1 0 0 1;3;4
3 0 0 0 0 0 1 99
4 1 1 1 0 0 0 1;2;3

Answer 1

In his comment, the OP asked how to deal with more multiple choice questions.
The approach below will be able to handle an arbitrary number of questions and choices for each question. It uses melt() and dcast() from the data.table package.
Sample input data
Let's assume the input data.frame DT for the extended case contains two questions, one with 6 choices and the other with 4 choices:
DT
# F1.2_1 F1.2_2 F1.2_3 F1.2_4 F1.2_5 F1.2_99 F2.7_1 F2.7_2 F2.7_3 F2.7_11
#1: 0 1 0 0 0 0 0 1 1 0
#2: 1 0 1 1 0 0 1 1 1 1
#3: 0 0 0 0 0 1 1 0 1 0
#4: 1 1 1 0 0 0 1 0 1 1
Code
library(data.table)
# coerce to data.table and add row number for later join
setDT(DT)[, rn := .I]
# reshape from wide to long format
molten <- melt(DT, id.vars = "rn")
# alternatively, the measure cols can be specified (in case of other id vars)
# molten <- melt(DT, measure.vars = patterns("^F"))
# split question id and choice id
molten[, c("question_id", "choice_id") := tstrsplit(variable, "_")]
# reshape only selected choices from long to wide format,
# thereby pasting together the ids of the selected choices for each question
result <- dcast(molten[value == 1], rn ~ question_id, paste, collapse = ";",
fill = NA, value.var = "choice_id")
# final join for demonstration only, remove row number as no longer needed
DT[result, on = "rn"][, rn := NULL][]
# F1.2_1 F1.2_2 F1.2_3 F1.2_4 F1.2_5 F1.2_99 F2.7_1 F2.7_2 F2.7_3 F2.7_11 F1.2 F2.7
#1: 0 1 0 0 0 0 0 1 1 0 2 2;3
#2: 1 0 1 1 0 0 1 1 1 1 1;3;4 1;2;3;11
#3: 0 0 0 0 0 1 1 0 1 0 99 1;3
#4: 1 1 1 0 0 0 1 0 1 1 1;2;3 1;3;11
For each question, the final result shows which choices were selected in each row.
Reproducible data
The sample data can be created with
DT <- structure(list(F1.2_1 = c(0L, 1L, 0L, 1L), F1.2_2 = c(1L, 0L,
0L, 1L), F1.2_3 = c(0L, 1L, 0L, 1L), F1.2_4 = c(0L, 1L, 0L, 0L
), F1.2_5 = c(0L, 0L, 0L, 0L), F1.2_99 = c(0L, 0L, 1L, 0L), F2.7_1 = c(0L,
1L, 1L, 1L), F2.7_2 = c(1L, 1L, 0L, 0L), F2.7_3 = c(1L, 1L, 1L,
1L), F2.7_11 = c(0L, 1L, 0L, 1L)), .Names = c("F1.2_1", "F1.2_2",
"F1.2_3", "F1.2_4", "F1.2_5", "F1.2_99", "F2.7_1", "F2.7_2",
"F2.7_3", "F2.7_11"), row.names = c(NA, -4L), class = "data.frame")

Answer 2

We can try
j1 <- do.call(paste, c(as.integer(sub(".*_", "",
names(DF)[-7]))[col(DF[-7])]*DF[-7], sep=";"))
DF$newCol <- gsub("^;+|;+$", "", gsub(";*0;|0$|^0", ";", j1))
DF$newCol
#[1] "2" "1;3;4" "99" "1;2;3"