[Solved] in R, How to sum by flowing row in a data frame

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We could use shift from data.table

library(data.table)
m1 <- na.omit(do.call(cbind, shift(df1$col1, 0:4, type="lead")))
rowSums(m1*(1:5)[col(m1)]/5) 
#[1] 13.60 12.20 31.24 25.58 30.48 32.58 44.88

Or another option 

 m1 <- embed(df1$col1,5)
 rowSums(m1*(5:1)[col(m1)]/5)
 #[1] 13.60 12.20 31.24 25.58 30.48 32.58 44.88

solved in R, How to sum by flowing row in a data frame