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I want to reshape my dataframe from long to wide format and I loose some data that I'd like to keep. For the following example: df <- data.frame(Par1 = unlist(strsplit("AABBCCC","")), Par2 = unlist(strsplit("DDEEFFF","")), ParD = unlist(strsplit("foo,bar,baz,qux,bla,xyz,meh",",")), Type = unlist(strsplit("pre,post,pre,post,pre,post,post",",")), Val = c(10,20,30,40,50,60,70)) # Par1 Par2 ParD Type Val # 1 A D foo pre 10 # 2 A D bar post 20 # 3 B E baz pre 30 # 4 B E qux post 40 # 5 C F bla pre 50 # 6 C F xyz post 60 # 7 C F meh post 70 dfw <- dcast(df, formula = Par1 + Par2 ~ Type, value.var = "Val", fun.aggregate = mean) # Par1 Par2 post pre # 1 A D 20 10 # 2 B E 40 30 # 3 C F 65 50 this is almost what I need but I would like to have some field keeping data from ParD field (for example, as single merged string), number of observations used for aggregation. i.e. I would like the resulting data.frame to be as follows: # Par1 Par2 post pre Num.pre Num.post ParD # 1 A D 20 10 1 1 foo_bar # 2 B E 40 30 1 1 baz_qux # 3 C F 65 50 1 2 bla_xyz_meh I would be grateful for any ideas. For example, I tried to solve the second task by writing in dcast: fun.aggregate=function(x) c(Val=mean(x),Num=length(x)) - but this causes an error.

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