R help - Sep 2004 - Indexing dataframe

I am sorry to ask such question, but I can't find a solution...

I have a dataframe 'd2004' and I want to remove two columns:
'd2004$concentration' and 'd2004$stade".

I could do it just as follows:
> names(d2004)
 [1] "Localite"       "Date"           "parcelle" 
"maille"
"presence.plant" "concentration"  "stade.culture"
 [8] "stade"          "Trou"           "Horizon"  
"Profondeur"
> d2004 <- d2004[, -c(6, 8)]
but I'd like to use column names (to avoid finding column numbers each
time).

I cannot find an easy way to operate...

I wonder why that works:
> d2004[, "concentration"]
and this don't:
> d2004 <- d2004[, -c("concentration", "stade")]
Thanks...

Jacques VESLOT / CIRAD

Jacques VESLOT wrote:
> I am sorry to ask such question, but I can't find a solution...
> 
> I have a dataframe 'd2004' and I want to remove two columns:
> 'd2004$concentration' and 'd2004$stade".
d2004$concentration <- NULL
d2004$stade         <- NULL

Hope it helps!

Thomas P.