R: Calculate means for subset of a group
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27-10-2019 - |
Question
I want to calculate the mean for each "Day" but for a portion of the day (Time=12-14). This code works for me but I have to enter each day as a new line of code, which will amount to hundreds of lines.
This seems like it should be simple to do. I've done this easily when the grouping variables are the same but dont know how to do it when I dont want to include all values for the day. Is there a better way to do this?
sapply(sap[sap$Day==165 & sap$Time %in% c(12,12.1,12.2,12.3,12.4,12.5,13,13.1,13.2,13.3,13.4,13.5, 14), ],mean)
sapply(sap[sap$Day==166 & sap$Time %in% c(12,12.1,12.2,12.3,12.4,12.5,13,13.1,13.2,13.3,13.4,13.5, 14), ],mean)
Here's what the data looks like:
Day Time StomCond_Trunc
165 12 33.57189926
165 12.1 50.29437636
165 12.2 35.59876214
165 12.3 24.39879768
Solution
Try this:
aggregate(StomCond_Trunc~Day,data=subset(sap,Time>=12 & Time<=14),mean)
OTHER TIPS
If you have a large dataset, you may also want to look into the data.table
package. Converting a data.frame
to a data.table
is quite easy.
Example:
Large(ish) dataset
df <- data.frame(Day=1:1000000,Time=sample(1:14,1000000,replace=T),StomCond_Trunc=rnorm(100000)*20)
Using aggregate on the data.frame
>system.time(aggregate(StomCond_Trunc~Day,data=subset(df,Time>=12 & Time<=14),mean))
user system elapsed
16.255 0.377 24.263
Converting it to a data.table
dt <- data.table(df,key="Time")
>system.time(dt[Time>=12 & Time<=14,mean(StomCond_Trunc),by=Day])
user system elapsed
9.534 0.178 15.270
Update from Matthew. This timing has improved dramatically since originally answered due to a new optimization feature in data.table 1.8.2.
Retesting the difference between the two approaches, using data.table 1.8.2 in R 2.15.1 :
df <- data.frame(Day=1:1000000,
Time=sample(1:14,1000000,replace=T),
StomCond_Trunc=rnorm(100000)*20)
system.time(aggregate(StomCond_Trunc~Day,data=subset(df,Time>=12 & Time<=14),mean))
# user system elapsed
# 10.19 0.27 10.47
dt <- data.table(df,key="Time")
system.time(dt[Time>=12 & Time<=14,mean(StomCond_Trunc),by=Day])
# user system elapsed
# 0.31 0.00 0.31
Using your original method, but with less typing:
sapply(sap[sap$Day==165 & sap$Time %in% seq(12, 14, 0.1), ],mean)
However this is only a slightly better method than your original one. It's not as flexible as the other answers since it depends on 0.1 increments in your time values. The other methods don't care about the increment size, which makes them more versatile. I'd recommend @Maiasaura's answer with data.table