How to calculate mean in different columns in R

Question

I have a painfully simple question. Due to small sample sizes I want to group several species for analysis. I am trying to make a new variable which has calculated the means of columns 13, 15, 16 and 17.

Answer 1

to compute the mean of a column, just call the colMeans function, eg to get the mean of the first column in the following dataframe example:

df <- data.frame(rnorm(4),rnorm(4),rnorm(4))
meanFirstCol <- colMeans(df[1])