[R] Calculating a daily average

[ecvetano at uwaterloo.ca]

I have a set of data with 12 readings for temperature per day, with  
180 days. I want to find the average temperature of each day. I am  
able to do this one by one, but with that many days to calculate the  
average for, it will get very long. I'm sure there is a faster way to  
do this, I just don't know how. What i have so far is:


av1 <-  subset(ER9r, Day == 98, select = c (Depth1j:Depth0.75j))
av1 <- mean(av1 [av1>0])
av2 <-  subset(ER9r, Day == 99, select = c (Depth1j:Depth0.75j))
av2 <- mean(av2 [av2>0])
av3 <-  subset(ER9r, Day == 100, select = c (Depth1j:Depth0.75j))
av3 <- mean(av3 [av3>0])
av4 <-  subset(ER9r, Day == 101, select = c (Depth1j:Depth0.75j))
av4 <- mean(av4 [av4>0])
av5 <-  subset(ER9r, Day == 102, select = c (Depth1j:Depth0.75j))
av5 <- mean(av5 [av5>0])

Depth<- c("SML")
Day<- c(98, 99, 100, 101, 102)
Average<- c(av1, av2, av3, av4, av5)
chlsummaryER9 <- data.frame (Depth, Day, Average)
chlsummaryER9

Any ideas on how to do this in less steps?

Thanks,
Emilija