How to get the mean for each group in a data frame

Question

Here's my data:

  v        x
  0:0       96
  0:0      119
  0:0      108
  1:0       73
  1:0       65
  2:0       83
  2:0       73
  2:0       23

How can I get the mean by groups of v i.e. the mean of x for all 0:0s, 1:0 etc.

My failed attempt:

df = read.csv(input.file.path, header=TRUE)
df$v <- as.factor(df$v)
ave(df$x, df$v)

Answer 1

ave isn't the best option here. It will return a vector the same length as your inputs, while I'm guessing that you want to lump the results together into a more compact table.

If that's the case, try aggregate or tapply instead:

> aggregate(x ~ v, df, mean)
    v         x
1 0:0 107.66667
2 1:0  69.00000
3 2:0  59.66667

> tapply(df$x, df$v, mean)
      0:0       1:0       2:0 
107.66667  69.00000  59.66667