[R] making operators act on rows of a data frame

[Simon Fear]

The generic solution is apply(myArray, 1, myfun)
- see ?apply (while you're there see ?lapply and ?sapply
as well!).

For you particular case, you can use rowSums().

There is a distinction between `array`, `matrix` and
`data.frame` - these are not interchangeable terms.

As it happens, both apply() and rowSums() will coerce
the first argument to an array if possible - thus they
might work on a data.frame. But be very sure
your data.frame does not contain any factors or
character values.

> -----Original Message-----
> From: Jonathan Williams
> [mailto:jonathan.williams at pharmacology.oxford.ac.uk]
> Sent: 11 March 2004 12:14
> To: Ethz. Ch
> Subject: [R] making operators act on rows of a data frame
> 
> 
> Security Warning: 
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> ________________________________________________________________ 
>  
> 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
> 
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Simon Fear 
Senior Statistician 
Syne qua non Ltd 
Tel: +44 (0) 1379 644449 
Fax: +44 (0) 1379 644445 
email: Simon.Fear at synequanon.com 
web: http://www.synequanon.com 
  
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