Mean by factor by level

Question

Maybe this is simple but I can't find answer on web. I have problem with mean calculation by factors by level. My data looks typicaly:

factor, value a,1 a,2 b,1 b,1 b,1 c,1 

I want to get vector A contains mean only for level "a" If I type A on consol I want to get 1.5 And this method for calculating mean, must use factors.

Thank you in advance for help.

Answer 1

take a look at tapply, which lets you break up a vector according to a factor(s) and apply a function to each subset

> dat<-data.frame(factor=sample(c("a","b","c"), 10, T), value=rnorm(10)) > r1<-with(dat, tapply(value, factor, mean)) > r1          a          b          c  0.3877001 -0.4079463 -1.0837449 > r1[["a"]] [1] 0.3877001 

You can access your results using r1[["a"]] etc.

Alternatively, one of the popular R packages (plyr) has very nice ways of doing this.

> library(plyr) > r2<-ddply(dat, .(factor), summarize, mean=mean(value)) > r2   factor       mean 1      a  0.3877001 2      b -0.4079463 3      c -1.0837449 > subset(r2,factor=="a",select="mean")        mean 1 0.3877001 

You can also use dlply instead (which takes a dataframe and returns a list instead)

> dlply(dat, .(factor), summarize, mean=mean(value))$a        mean 1 0.3877001 

Answer 2

The following code asks for the mean of value when factor = a:

mean(data$value[data$factor == "a"])