[R] Applying function to parts of a matrix based on a factor

John Sorkin jsorkin at grecc.umaryland.edu
Wed Jan 13 18:10:45 CET 2010


R 2.9
Windows XP

I have a matrix, Data, which contains a factor Sex and a continuous variable Age.
I want to get mean age by sex. I know I can do this with two statements,
mean(Data["Age,Data[,"Sex"]=="Male") and  
mean(Data["Age,Data[,"Sex"]=="Female")

I know this can be done in a single command, but I can remember how. There is a function that allows another function work within factors, something like
magicfunction(Data,Factor=Sex). n.b. I know the function I am looking for is not in the lapply, sapply etc. family

Please put me out of my misery (and senior moment) and remind me what function I should be using. 




John David Sorkin M.D., Ph.D.
Chief, Biostatistics and Informatics
University of Maryland School of Medicine Division of Gerontology
Baltimore VA Medical Center
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524
(Phone) 410-605-7119
(Fax) 410-605-7913 (Please call phone number above prior to faxing)

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