joining the first n factors (with different n) in R - r

Question

A data frame contains ID, group, n (numeric), and several factor variables
ID <- c(1,2,3,4,5,6,7,8,9,10)
group <- c("m", "m", "m", "f", "f", "m", "m", "f", "f", "m")
n <- c(1,2,6,3,6,8,4,1,4,2)
b1 <- c("a", "b", "", "a", "d", "d", "a", "c", "c", "b")
b2 <- c("a", "", "e", "a", "d", "d", "a", "c", "c", "b")
b3 <- c("a", "b", "", "a", "", "d", "a", "c", "c", "b")
b4 <- c("a", "b", "e", "a", "", "d", "a", "c", "c", "b")
b5 <- c("a", "b", "e", "a", "d", "", "", "", "c", "b")
b6 <- c("a", "", "", "", "d", "d", "", "c", "c", "b")
df <- data.frame(ID, group, n, b1, b2, b3, b4, b5, b6)
I need to create a new character column (call it y).
They way to compute y is by joining the first n variables (b1,b2,b3,b4,b5,b6) and use comma to seperate them.
Note, in case a column is a blank, then remove it from the join.
For example, for ID=1, y = "a"; for ID = 2, y = "b" (not "b, "); for ID = 3, y = "e,e,e", etc.
And, the faster the code, the better.

Answer 1

A possible sollution, the speed might still be an issue:
df$y <- sapply(seq_len(nrow(df)), function(i){
cvec <- head(unlist(df[i, 4:9]), df$n[i])
cvec <- cvec[!cvec == '']
paste(cvec, collapse = ',')
})
# ID group n b1 b2 b3 b4 b5 b6 y
# 1 1 m 1 a a a a a a a
# 2 2 m 2 b b b b b
# 3 3 m 6 e e e e,e,e
# 4 4 f 3 a a a a a a,a,a
# 5 5 f 6 d d d d d,d,d,d
# 6 6 m 8 d d d d d d,d,d,d,d
# 7 7 m 4 a a a a a,a,a,a
# 8 8 f 1 c c c c c c
# 9 9 f 4 c c c c c c c,c,c,c
# 10 10 m 2 b b b b b b b,b

Answer 2

Here is an option using gsub and paste. We paste the 'b' columns of 'df' (do.call(paste0, df[-(1:3)]), then use substring to keep only the characters that suggested by 'n' column, use gsub to create the , in between each character.
df$y <- gsub("(?<=\\S)(?=\\S)", ",",
substring(do.call(paste0, df[-(1:3)]), 1, df$n), perl = TRUE)
df
# ID group n b1 b2 b3 b4 b5 b6 y
#1 1 m 1 a a a a a a a
#2 2 m 2 b b b b b,b
#3 3 m 6 e e e e,e,e
#4 4 f 3 a a a a a a,a,a
#5 5 f 6 d d d d d,d,d,d
#6 6 m 8 d d d d d d,d,d,d,d
#7 7 m 4 a a a a a,a,a,a
#8 8 f 1 c c c c c c
#9 9 f 4 c c c c c c c,c,c,c
#10 10 m 2 b b b b b b b,b

Answer 3

df$y <- apply(df, 1, function(r) {
gsub("\\s+", "\\,", trimws(paste(head(r[4:9], r["n"]), sep= " ", collapse = " ")))})
df
# ID group n b1 b2 b3 b4 b5 b6 y
# 1 1 m 1 a a a a a a a
# 2 2 m 2 b b b b b
# 3 3 m 6 e e e e,e,e
# 4 4 f 3 a a a a a a,a,a
# 5 5 f 6 d d d d d,d,d,d
# 6 6 m 8 d d d d d d,d,d,d,d
# 7 7 m 4 a a a a a,a,a,a
# 8 8 f 1 c c c c c c
# 9 9 f 4 c c c c c c c,c,c,c
# 10 10 m 2 b b b b b b b,b