convert only some factors into a different factor - r

Question

I'm trying to build a factor column that relates to two other factor columns with completely different factor levels. Here's example data.
set.seed(1234)
a<-sample(LETTERS[1:10],50,replace=TRUE)
b<-sample(letters[11:20],50,replace=TRUE)
df<-data.frame(a,b)
df$a<-as.factor(df$a)
df$b<-as.factor(df$b)
The rule I want to make creates a new column, c, that bases it's factor level value based on the value of column a.
if any row in column a ="F", that row in column c will equal whatever the entry is for column b. The code I'm trying:
dfn<-dim(df)[1]
for (i in 1:dfn){
df$c[i]<-ifelse(df$a[i]=="F",df$b[i],df$a[i])
}
df
only spits out the numbered index of the factor level for column b and not the actual entry. What have I done wrong?

Answer 1

I think you'll need to do a little finagling of character values. This seems to do it.
w <- df$a == "F"
df$c <- factor(replace(as.character(df$a), w, as.character(df$b)[w]))
Here is a quick look at the new column,
factor(replace(as.character(df$a), w, as.character(df$b)[w]))
# [1] B G G G I G A C G s G k C J C I C C B C D D B A C I n J I A
# [31] E C D p B H C C J I l G D G D p G E C H
# Levels: A B C D E G H I J k l n p s

Answer 2

As my previous comment, a solution with dplyr:
df %>% mutate(c = ifelse(a == "F", as.character(b), as.character(a)))

Answer 3

If you plan on doing anything involving combinations of the columns as factors, for example, comparisons, you should refactor to the same set of levels.
u<-union(levels(df$a),levels(df$b))
df$a<-factor(df$a,u)
df$b<-factor(df$b,u)
df$c<-df$a
ind<-df$a=="F"
df$c[ind]<-df$b[ind]
By taking this precaution, you can sensibly do
> sum(df$c==df$b)
[1] 6
> sum(df$a=="F")
[1] 6
otherwise the first line will fail.