[R] Creating a factor from a combination of vectors

[Yves Brostaux]

Thank you Richard, but I dismissed the 'ifelse' solution because it 
needs explicit manual definition of the factor levels and corresponding 
vectors' combinations and does not define it automaticaly  from the 
'cas' data-frame (from which values, number of levels and rownames can 
vary).

Eric Lecoutre's code does exactly what I want, many thanks to both of you.

Richard A. O'Keefe a écrit :

>Yves Brostaux <brostaux.y at fsagx.ac.be> wrote:
>	I want to produce a factor from a subset of the combination of two 
>	vectors. I have the vectors a et b in a data-frame :
>	
>	 > df <- expand.grid(a=c(0, 5, 10, 25, 50), b=c(0, 25, 50, 100, 200))
>	...
>	and want to create a factor which levels correspond to particular 
>	combinations of a and b (let's say Low for a=0 & b=0, Medium for a=10 & 
>	b=50, High for a=50 & b=200, others levels set to NA), reading them from 
>	a data-frame which describes the desired subset and corresponding levels.
>	
>	Here's my own solution (inputs are data-frames df and cas, output is the 
>
>Why not do it the obvious way?
>
>	ifelse(a == 0 & b == 0, "Low",
>	  ifelse(a == 10 & b == 50, "Medium",
>	    ifelse(a == 50 & b == 200, "High", 
>	      "Other")))
>
>gives you the mapping from vectors a and b to strings you want.
>To get at the vectors locally, you need
>
>	with(df, ...)
>
>To convert the vector of strings you get to an ordered factor,
>with "Other" mapped to NA, just do
>
>	ordered(..., levels = c("Low","Medium","High"))
>
>because any string not listed in levels= will be mapped to NA.
>Put these pieces together, and you get
>
>    output <- ordered(with(df,
>		ifelse(a == 0 & b == 0, "Low",
>		  ifelse(a == 10 & b == 50, "Medium",
>		    ifelse(a == 50 & b == 200, "High",
>		      "Other")))),
>		levels = c("Low","Medium","High"))
>
>
>
>  
>

-- 
Ir. Yves BROSTAUX
Unité de Statistique et Informatique
Faculté universitaire des Sciences agronomiques de Gembloux (FUSAGx)
8, avenue de la Faculté
B-5030 Gembloux
Belgique
Tél: +32 81 62 24 69
Email: brostaux.y at fsagx.ac.be