[R] Taking the sum of only some columns of a data frame

[Bruce Ratner PhD]

Hi R'ers:
Given a data.frame of five columns and ten rows. 
I would like to take the sum of, say, the first and third columns only.
For the remaining columns, I do not want any calculations, thus rending their "values" on the "total" row blank. The sum/total row is to be combined to the original data.frame, yielding a data.frame with five columns and eleven rows. 

Thanks, in advance. 
Bruce 


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Bruce Ratner PhD
The Significant Statistician™




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