[R] Can't compute row means of two columns of a dataframe.

[Sorkin, John]

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 = c(111, 115, 119, 123, 127, 131, 
                   135, 140, 144, 148, 153, 157, 162, 167, 171, 176, 181, 186, 191
                   )), class = "data.frame", row.names = c(NA, -19L))
xxxz
str(xxxz)
xxxz$Average20 <- by(xxxz[,c("Low20","High20")],xxxz[,"TotalInches"],mean)
warnings()

When I run the code above, I don't get the means by row. I get the following warning messages, one for each row of the dataframe.

Warning messages:
1: In mean.default(data[x, , drop = FALSE], ...) :
  argument is not numeric or logical: returning NA
2: In mean.default(data[x, , drop = FALSE], ...) :
  argument is not numeric or logical: returning NA

 Can someone tell my what I am doing wrong, and how I can compute the row means?

Thank you,
John

John David Sorkin M.D., Ph.D.
Professor of Medicine, University of Maryland School of Medicine;
Associate Director for Biostatistics and Informatics, Baltimore VA Medical Center Geriatrics Research, Education, and Clinical Center; 
PI Biostatistics and Informatics Core, University of Maryland School of Medicine Claude D. Pepper Older Americans Independence Center;
Senior Statistician University of Maryland Center for Vascular Research;

Division of Gerontology and Paliative Care,
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524
Cell phone 443-418-5382