user3206440 user3206440 - 1 month ago 10
R Question

R - aggregate by group with some function

I want to aggregate a data frame by a certain group and operation

data

> df <- data.frame(replicate(9, 1:4))
X1 X2 X3 X4 X5 X6 X7 X8 X9
1 1 1 1 1 1 1 1 1 1
2 2 2 2 2 2 2 2 2 2
3 3 3 3 3 3 3 3 3 3
4 4 4 4 4 4 4 4 4 4


aggregation

> aggregate(df[,2], list(df[,1]), mean)
Group.1 x
1 1 1
2 2 2
3 3 3
4 4 4


The above aggregation works, which is great. However instead of
mean
, in place of that I need to use combination of functions like
mean*sd/length^2
. Should we be using something other than aggregate here ?

Answer

I modified your sample data frame in order to get a length and standard deviation for each group (you can't do this with only one data point per group).

> df
   X1 X2 X3 X4 X5 X6 X7 X8 X9
1   1  1  1  1  1  1  1  1  1
2   2  2  2  2  2  2  2  2  2
3   3  3  3  3  3  3  3  3  3
4   4  4  4  4  4  4  4  4  4
5   1  1  1  1  1  1  1  1  1
6   2  2  2  2  2  2  2  2  2
7   3  3  3  3  3  3  3  3  3
8   4  4  4  4  4  4  4  4  4
9   1  4  4  4  4  4  4  4  4
10  2  5  5  5  5  5  5  5  5
11  3  6  6  6  6  6  6  6  6
12  4  7  7  7  7  7  7  7  7
13  1  4  4  4  4  4  4  4  4
14  2  5  5  5  5  5  5  5  5
15  3  6  6  6  6  6  6  6  6
16  4  7  7  7  7  7  7  7  7

To aggregate by a more elaborated formula do:

aggregate(df[,2], list(df[,1]), function(x){mean(x)*sd(x)/length(x)^2})
  Group.1         x
1       1 0.2706329
2       2 0.3788861
3       3 0.4871393
4       4 0.5953925

If you want to have the same column labels you could do:

aggregate(list(X2 = df[,2]), list(X1 = df[,1]), function(x){mean(x)*sd(x)/length(x)^2})
  X1        X2
1  1 0.2706329
2  2 0.3788861
3  3 0.4871393
4  4 0.5953925

(or rename them afterwards with colnames)

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