I have the following data:
a <- c(1,1,1,1,2,2,2,2)
b <- c(2,4,6,8,2,3,4,1)
c <- factor(c("A","B","A","B","A","B","A","B"))
df <- data.frame(
sp=a,
length=b,
method=c)
I can use the following to get a count of the number of samples of each species by method:
n <- with(df,tapply(sp,method,function(x) count(x)))
How do I also get the mean length by method for each species?
Answer
Personally I would use aggregate:
aggregate(length ~ sp, data = df, FUN= "mean" )
# by species only
# sp length
#1 1 5.0
#2 2 2.5
aggregate(length ~ sp + method, data = df, FUN= "mean" )
# by species and method
# sp method length
#1 1 A 4
#2 2 A 3
#3 1 B 6
#4 2 B 2
for everything together you may want:
aggregate(length ~ method, data = df, function(x) c(m = mean(x), counts = length(x)) )
# counts and mean for each method
# method length.m length.counts
#1 A 3.5 4.0
#2 B 4.0 4.0