Jonathan - Petr Pikal suggests RowSums() for your specific task.
A more general solution is given by apply() and sapply(). These functions will apply any R function (even one that you would write yourself) to individual members of any dimension of an array, matrix or data frame, or any combination of dimensions (in the context of a three or four-dimensional [or more] array). See help("apply"), help("sapply"). - tom blackwell - u michigan medical school - ann arbor - On Thu, 11 Mar 2004, Jonathan Williams wrote: > Dear R helpers, > I wish to use the "sum" operator for each row of a data frame. > However, it appears that the operator acts on the entire data > frame, over all columns. What is the best way to obtain row- > wise operation? > > The following code shows my attempts so far, and their problems:- > > test1=array(rbinom(120,1,0.5),c(20,3)) > test1[,3]=NA > sum(test1[,1:2]) > test1[,3][sum(test1[,1:2])>=2]=1 > test1[,3][sum(test1[,1:2])] > test1 > > test2=array(rbinom(120,1,0.5),c(20,3)) > test2[,3]=NA > sum(test2[,1:2]) > test2[,3][(test2[,1]+test2[,2])>=2]=1 > test2[,3][sum(test2[,1:2])] > test2 > > In the 1st section, I try to use "sum" to add the first two columns > of a data frame. Here, sum(test1[,1:2]) evaluates to a single integer > but this modifies *all* the rows of test1. > > In the 2nd section, specifying addition of the first two columns with > a '+' acts row-by row, as I want. This is OK for this demonstration, > but would be impractical in the program I am trying to write (where > the columns I wish to sum are numerous and change from time to time). > > I would be very grateful to know if it is possible to get operators > to act on rows of a data frame, and if so, how. > > I am running R1.8.1 on Windows NT. > > Jonathan Williams > OPTIMA > Radcliffe Infirmary > Woodstock Road > OXFORD OX2 6HE > Tel +1865 (2)24356 > > ______________________________________________ > [EMAIL PROTECTED] mailing list > https://www.stat.math.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html > ______________________________________________ [EMAIL PROTECTED] mailing list https://www.stat.math.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html