How to define a field inside a dataframe based on the columnname and the rowname ?

I have a dataframe called HMDAYTIME with the columnname "A" and the rownames "2" and "3", and based on a raw data csv called "DATA".

My question is (i didn´t found anything about that on the internet), if i want to do a for loop to change each value of the "A" column, how can I make the bolded code show the "A" column value based on the rowname.

Thanks very much in advance,


HMDAYTIME <- data.frame(1:(length(unique(DATA$Wochentag))))

colnames(HMDAYTIME) <- "A"
rownames(HMDAYTIME) <- unique(DATA$Wochentag)

for (i in rownames(HMDAYTIME))
HMDAYTIME$A[#the row of i] <- sum(DATA$Brutto.Preis[DATA$Wochentag == DATA$Wochentag])