Calculate group mean, sum, or other summary stats. and assign column to original data

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:

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

Have a look at the ave function. Something like

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)