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I have a data frame with three columns, TotalInches, Low20, High20. For each row of the dataset, I am trying to compute the mean of Low20 and High20. xxxz <- structure(list(TotalInches = c(58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76), Low20 = c(84, 87, 90, 93, 96, 99, 102, 106, 109, 112, 116, 119, 122, 126, 129, 133, 137, 141, 144), High20...

..., Maybe you can clarify what you want the output to look like. It took me a while to realize what you may want as it is NOT properly described as wanting rowsums. There is a standard function called rowMeans() that probably does what you want if you want the mean of all rows as in: > rowMeans(xxxz) [1] 84.33333 87.00000 89.66667 92.33333 95.00000 97.66667 100.33333 103.66667 106.33333 109.00000 112.33333 115.00000 [13] 118.00000 121.33333 124.00000 127.33333 130.66667 134.00000 137.00000 It does not add the means to the original data.frame if you wanted it there but that is easy enou...

Use apply(), not by(). xxxz$av20 <- apply(xxxz[,c("Low20","High20")],1, mean) -- Bert On Sat, Jun 8, 2024 at 10:38?AM Sorkin, John <jsorkin at som.umaryland.edu> wrote: > I have a data frame with three columns, TotalInches, Low20, High20. For > each row of the dataset, I am trying to comp...