Summary table by group in R

Question

Consider the following dataframe:

 df <- data.frame(group = c("group1", "group1", "group2", "group2", "group2", "group3"), factor = paste("factor", 1:6, sep=""), vol = seq(from = 0.02, length.out = 6, by = 0.02))

The first column defines a top-level group for each factor in the second column. The third column is the value of standard deviation for each factor.

I would like to generate a summary table with the groups only, and the standard deviation for each group defined as:

• If group1 contrains factors f1 and f2, and vol(f1) and vol(f2) are the standard deviations for factors f1 and f2 respectively, then the standard deviation of group1 is:

  std(group1) = sqrt[vol(f1)^2 + vol(f2)^2]

Is there any easy way of creating the summary table, where vol of each group is computed using this custom function?

Any help would be appreciated! Thank you.

Answer 1

A base solution for good measure.

by(df,df$group,function(x) sqrt(sum(x$vol^2)))

If you need it to look prettier:

as.table(df,df$group,function(x) sqrt(sum(x$vol^2))))

df$group
    group1     group2     group3 
0.04472136 0.14142136 0.12000000