Replace a value in a data frame based on a conditional (`if`) statement

In the R data frame coded for below, I would like to replace all of the times that B appears with b.

junk <- data.frame(x <- rep(LETTERS[1:4], 3), y <- letters[1:12])
colnames(junk) <- c("nm", "val")

this provides:

   nm val
1   A   a
2   B   b
3   C   c
4   D   d
5   A   e
6   B   f
7   C   g
8   D   h
9   A   i
10  B   j
11  C   k
12  D   l

My initial attempt was to use a for and if statements like so:

for(i in junk$nm) if(i %in% "B") junk$nm <- "b"

but as I am sure you can see, this replaces ALL of the values of junk$nm with b. I can see why this is doing this but I can't seem to get it to replace only those cases of junk$nm where the original value was B.

NOTE: I managed to solve the problem with gsub but in the interest of learning R I still would like to know how to get my original approach to work (if it is possible)

Solution 1:

Easier to convert nm to characters and then make the change:

junk$nm <- as.character(junk$nm)
junk$nm[junk$nm == "B"] <- "b"

EDIT: And if indeed you need to maintain nm as factors, add this in the end:

junk$nm <- as.factor(junk$nm)

Solution 2:

another useful way to replace values

library(plyr)
junk$nm <- revalue(junk$nm, c("B"="b"))