[R] logical condition in vector operation

[Christoph Buser]

Dear Frederico

>From your example it is not clear to me what you like to obtain: 
Please have a look on the slightly changed example here (I
changed two values to show a potentially undesired side effect of
your coding.


test <- data.frame(rbind(c(3,3,10,21,0), c(2,3,11,12,0), c(3,4,12,23,0),
                         c(3,5,13,24,0)))
names(test) <- c("x","y","p","d","su")
test

>>   x y  p  d su
>> 1 3 3 10 21  0
>> 2 2 3 11 12  0
>> 3 3 4 12 23  0
>> 4 3 5 13 24  0

j <- 3
test[test[,1] == j, 5] <- test[test[,1] == j,2] + test[test[,2] == j,1]

>> > > Warning message:
>> longer object length
>> 	is not a multiple of shorter object length in: 
>>         test[test[, 1] == j, 2] + test[test[, 2] == j, 1] 

Your code example produces now a warning for the adapted
data frame "test", since one tries to add two vectors of length 2
and 3, respectively. The result is based on recycling of the
smaller vector. In your example there was no warning since the
second column had only one entry.
The result with the adapted data frame is:

test
>>   x y  p  d su
>> 1 3 3 10 21  6
>> 2 2 3 11 12  0
>> 3 3 4 12 23  6
>> 4 3 5 13 24  8

Is that kind of recycling desired in your application. Otherwise
you should be careful with the coding example above.

Regards,

Christoph Buser

--------------------------------------------------------------
Christoph Buser <buser at stat.math.ethz.ch>
Seminar fuer Statistik, LEO C13
ETH (Federal Inst. Technology)	8092 Zurich	 SWITZERLAND
phone: x-41-44-632-4673		fax: 632-1228
http://stat.ethz.ch/~buser/
--------------------------------------------------------------






Federico Calboli writes:
 > HI All,
 > 
 > I have a data frame such as:
 > 
 > > test
 >      x y  p  d
 > [1,] 1 0 10 21 0
 > [2,] 2 3 11 12 0
 > [3,] 3 4 12 23 0
 > [4,] 3 5 13 24 0
 > 
 > 
 > and I want to perfor some operations on the first two coulums,
 > conditional on the uneqaulity values on the 3rd and 4th columns.
 > 
 > For instance:
 > 
 > j = 3
 > test[test[,1] == j, 5] = test[test[,1] == j,2] + test[test[,2] == j,1]
 > 
 > gives me the result:
 > 
 > test:
 > 
 >      x y  p  d
 > [1,] 1 0 10 21 0
 > [2,] 2 3 11 12 0
 > [3,] 3 4 12 23 6
 > [4,] 3 5 13 24 7
 > 
 > 
 > My probblem is the following: I want to perform the operation
 > test[test[,1] == j,2] + test[test[,2] == j,1] only if the value of
 > column p and column d are different at the positions where x or y = j.
 > In practice, I don't want to perform the first operation because
 > test[2,4 is 12 and test[1,3] is 12 as well.
 > 
 > I tried an if statement with little success:
 > 
 > if(test[test[,1] == j,3] != test[test[,2] == j,4]){
 > test[test[,1] == j, 5] = test[test[,1] == j,2] + test[test[,2] == j,1]
 > }
 > Warning message:
 > the condition has length > 1 and only the first element will be used in:
 > if (test[test[, 1] == j, 3] != test[test[, 2] == j, 4]) {
 > 
 > Could anyone lend some advice?
 > 
 > Cheers,
 > 
 > Federico
 > -- 
 > Federico C. F. Calboli
 > Department of Epidemiology and Public Health
 > Imperial College, St Mary's Campus
 > Norfolk Place, London W2 1PG
 > 
 > Tel  +44 (0)20 7594 1602     Fax (+44) 020 7594 3193
 > 
 > f.calboli [.a.t] imperial.ac.uk
 > f.calboli [.a.t] gmail.com
 > 
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