Averaging variables by ID

gistfile1.r

# create dummy data
df = data.frame(id=c(1,1,1,2,2,3,3,3,4,4,4), var1=c(-2.1, 1.1, 3.14, 2.17828, 9, 10, 8,2,1,0,-1), var2=c(1,2,3,4,5,6,7,8,9,10,11))

# This is what you'd see if you called this data set from the interactive prompt (or REPL, as it's called (Read-Evaluate-Print-Loop)):
#> df
#   id     var1 var2
#1   1 -2.10000    1
#2   1  1.10000    2
#3   1  3.14000    3
#4   2  2.17828    4
#5   2  9.00000    5
#6   3 10.00000    6
#7   3  8.00000    7
#8   3  2.00000    8
#9   4  1.00000    9
#10  4  0.00000   10
#11  4 -1.00000   11

# What you're looking for is the aggregate function:
means = aggregate(df, by=list(df$id), FUN=mean)

# This forms groups based on the id column(the "by=list(df$id)" part), 
# and takes means (the "FUN=mean" part) of the rest of your columns.
#> means
#  Group.1 id      var1 var2
#1       1  1 0.7133333  2.0
#2       2  2 5.5891400  4.5
#3       3  3 6.6666667  7.0
#4       4  4 0.0000000 10.0