get the mean of a variable subset of data in R [duplicate] - r

Question

This question already has answers here:
Adding a column of means by group to original data [duplicate]
(4 answers)
Closed 6 years ago.
Imagine I have the following data:
Year Month State ppo
2011 Jan CA 220
2011 Feb CA 250
2012 Jan CA 230
2011 Jan WA 200
2011 Feb WA 210
I need to calculate the mean for each state for the year, so the output would look something like this:
Year Month State ppo annualAvg
2011 Jan CA 220 230
2011 Feb CA 240 230
2012 Jan CA 260 260
2011 Jan WA 200 205
2011 Feb WA 210 205
where the annual average is the mean of any entries for that state in the same year. If the year and state were constant I would know how to do this, but somehow the fact that they are variable is throwing me off.
Looking around, it seems like maybe ddply is what I want to be using for this (https://stats.stackexchange.com/questions/8225/how-to-summarize-data-by-group-in-r), but when I tried to use it I was doing something wrong and kept getting errors (I have tried so many variations of it that I won't bother to post them all here). Any idea how I am actually supposed to be doing this?
Thanks for the help!

Answer 1

Try this:
library(data.table)
setDT(df)
df[ , annualAvg := mean(ppo) , by =.(Year, State) ]

Answer 2

Base R: df$ppoAvg <- ave(df$ppo, df$State, df$Year, FUN = mean)

Answer 3

Using dplyr with group_by %>% mutate to add a column:
library(dplyr)
df %>% group_by(Year, State) %>% mutate(annualAvg = mean(ppo))
#Source: local data frame [5 x 5]
#Groups: Year, State [3]
# Year Month State ppo annualAvg
# (int) (fctr) (fctr) (int) (dbl)
#1 2011 Jan CA 220 235
#2 2011 Feb CA 250 235
#3 2012 Jan CA 230 230
#4 2011 Jan WA 200 205
#5 2011 Feb WA 210 205
Using data.table:
library(data.table)
setDT(df)[, annualAvg := mean(ppo), .(Year, State)]
df
# Year Month State ppo annualAvg
#1: 2011 Jan CA 220 235
#2: 2011 Feb CA 250 235
#3: 2012 Jan CA 230 230
#4: 2011 Jan WA 200 205
#5: 2011 Feb WA 210 205
Data:
structure(list(Year = c(2011L, 2011L, 2012L, 2011L, 2011L), Month = structure(c(2L,
1L, 2L, 2L, 1L), .Label = c("Feb", "Jan"), class = "factor"),
State = structure(c(1L, 1L, 1L, 2L, 2L), .Label = c("CA",
"WA"), class = "factor"), ppo = c(220L, 250L, 230L, 200L,
210L), annualAvg = c(235, 235, 230, 205, 205)), .Names = c("Year",
"Month", "State", "ppo", "annualAvg"), class = c("data.table",
"data.frame"), row.names = c(NA, -5L), .internal.selfref = <pointer: 0x105000778>)