loop to paste columns together if condition is met - r

Question

I would like to create a loop that will create a new column, then paste together two columns if a condition is met in a separate column. If the condition is not met, then the column would equal whatever value is in the existing column. Finally, I would like to delete the old columns and rename the new columns to match the old columns. In my example below, I create columns called a1_t, a2_t, a3_t. Then, if a1 == A, paste a1 and a1_c together and place the value in a1_t, otherwise copy the value from a1 into a1_t. Repeat this procedure for a2_t and a3_t.
Here is the data:
set.seed(1)
dat <- data.frame(a1 = sample(LETTERS[1:9],15,replace=T),
a1_c = sample (1:100,15),
a2 = sample(LETTERS[1:9],15,replace=T),
a2_c = sample (1:100,15),
a3 = sample(LETTERS[1:9],15,replace=T),
a3_c = sample (1:100,15))
Here is the long hand way of creating my end goal:
dat$a1_t <- 'none'
dat$a1_t[dat$a1=="A"] <- paste((dat$a1[dat$a1=="A"]),(dat$a1_c[dat$a1=="A"]),sep="_")
dat$a1_t[dat$a1=="B"] <- 'B'
dat$a1_t[dat$a1=="C"] <- 'C'
dat$a1_t[dat$a1=="D"] <- 'D'
dat$a1_t[dat$a1=="E"] <- 'E'
dat$a1_t[dat$a1=="F"] <- 'F'
dat$a1_t[dat$a1=="G"] <- 'G'
dat$a1_t[dat$a1=="H"] <- 'H'
dat$a1_t[dat$a1=="I"] <- 'I'
dat$a2_t <- 'none'
dat$a2_t[dat$a2=="A"] <- paste((dat$a2[dat$a2=="A"]),(dat$a2_c[dat$a2=="A"]),sep="_")
dat$a2_t[dat$a2=="B"] <- 'B'
dat$a2_t[dat$a2=="C"] <- 'C'
dat$a2_t[dat$a2=="D"] <- 'D'
dat$a2_t[dat$a2=="E"] <- 'E'
dat$a2_t[dat$a2=="F"] <- 'F'
dat$a2_t[dat$a2=="G"] <- 'G'
dat$a2_t[dat$a2=="H"] <- 'H'
dat$a2_t[dat$a2=="I"] <- 'I'
dat$a3_t <- 'none'
dat$a3_t[dat$a3=="A"] <- paste((dat$a3[dat$a3=="A"]),(dat$a3_c[dat$a3=="A"]),sep="_")
dat$a3_t[dat$a3=="B"] <- 'B'
dat$a3_t[dat$a3=="C"] <- 'C'
dat$a3_t[dat$a3=="D"] <- 'D'
dat$a3_t[dat$a3=="E"] <- 'E'
dat$a3_t[dat$a3=="F"] <- 'F'
dat$a3_t[dat$a3=="G"] <- 'G'
dat$a3_t[dat$a3=="H"] <- 'H'
dat$a3_t[dat$a3=="I"] <- 'I'
-al

Answer 1

If you are dealing with a small number of columns, you might just want to use within and ifelse, like this:
within(dat, {
a1_t <- ifelse(a1 == "A", paste(a1, a1_c, sep = "_"),
as.character(a1))
a2_t <- ifelse(a2 == "A", paste(a2, a2_c, sep = "_"),
as.character(a2))
a3_t <- ifelse(a3 == "A", paste(a3, a3_c, sep = "_"),
as.character(a3))
})
You can, however, extend the idea programatically, if necessary.
Ive added comments throughout the code below so you can see what it's doing.
## What variables are we checking?
checkMe <- c("a1", "a2", "a3")
## Let's convert those to character first
dat[checkMe] <- lapply(dat[checkMe], as.character)
cbind(dat, ## We'll combine the original data using cbind
setNames( ## setNames is for the resulting column names
lapply(checkMe, function(x) { ## lapply is an optimized loop
Get <- c(x, paste0(x, "_c")) ## We need this for the "if" part
ifelse(dat[, x] == "A", ## logical comparison
## if matched, paste together the value from
## the relevant column
paste(dat[, Get[1]], dat[, Get[2]], sep = "_"),
dat[, x]) ## else return the original value
}),
paste0(checkMe, "_t"))) ## the column names we want
# a1 a1_c a2 a2_c a3 a3_c a1_t a2_t a3_t
# 1 C 50 E 79 I 90 C E I
# 2 D 72 F 3 C 86 D F C
# 3 F 98 E 47 E 39 F E E
# 4 I 37 B 72 C 76 I B C
# 5 B 75 H 67 F 93 B H F
# 6 I 89 G 46 C 42 I G C
# 7 I 20 H 81 E 67 I H E
# 8 F 61 A 41 G 38 F A_41 G
# 9 F 12 G 23 A 30 F G A_30
# 10 A 25 D 7 H 69 A_25 D H
# 11 B 35 H 9 D 19 B H D
# 12 B 2 F 29 H 64 B F H
# 13 G 34 H 95 D 11 G H D
# 14 D 76 E 58 D 22 D E D
# 15 G 30 E 35 E 13 G E E