How to calculate group mean and assign it to new data in R

Question

I want to calculate mean (or any other summary statistics of length one, e.g. min, max, length, sum) of a numeric variable ("value") within each level of a grouping variable ("group").

The summary statistic should be assigned to a new variable which has the same length as the original data. That is, each row of the original data should have a value corresponding to the current group value - the data set should not be collapsed to one row per group. For example, consider group mean

down votefaI want to calculate mean (or any other summary statistics of length one, e.g. min, max, length, sum) of a numeric variable ("value") within each level of a grouping variable ("group")The summary statistic should be assigned to a new variable which has the same length as the original data. That is, each row of the original data should have a value corresponding to the current group value - the data set should not be collapsed to one row per group. For example, consider group 

Before

id  group  value
1   a      10
2   a      20
3   b      100
4   b      200

After

id  group  value  grp.mean.values
1   a      10     15
2   a      20     15
3   b      100    150
4   b      200    150

Answer 1

You can use something like this:

df$grp.mean.values <- ave(df$value, df$group)

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If you want to use ave to calculate something else per group, you need to specify FUN = your-desired-function, e.g. FUN = min:

df$grp.min <- ave(df$value, df$group, FUN = min)