Switching values of a list with multiple conditions in R [duplicate]

I'm new to R and I have a little problem with switching values of list elements by using conditions.

I've got the following list:

listA <- list(",", ",", "_", "_", "A", "B")

First I tried to switch them with the following code:

listA[listA==","] <- as.character(".")

In this case everything works fine. Elements which are equal to "," are switched into a ".". Now I want to switch values by using multiple conditions. Something like this:

listA[listA!=","&& listA!="_"] <- as.character(".")

When I try that one nothing happens. Also no error message comes up. I also tested if the condition gives the right results but that's the next (maybe the main?) problem:

which(listA!=","&& listA!="_")

That snippet gives the result integer(0). Seems like R does not accept multiple conditions for indexing lists in the way I tried. Hope you can help me with this one.