[R] Trying to understand factors

[David Winsemius]

On Mar 30, 2012, at 12:50 PM, Julio Sergio wrote:

>
> I'm trying to figure out about factors, however the on-line  
> documentation is
> rather sparse. I guess, factors are intended for grouping arrays  
> members into
> categories, which R names "Levels". And so we have:
>
> * state <- c("tas", "sa",  "qld", "nsw", "nsw", "nt",  "wa",  "wa",
>                  "qld", "vic", "nsw", "vic", "qld", "qld", "sa",   
> "tas",
>                  "sa",  "nt",  "wa",  "vic", "qld", "nsw", "nsw",  
> "wa",
>                  "sa",  "act", "nsw", "vic", "vic", "act")
> * statef <- factor(state)
> * statef
> [1] tas sa  qld nsw nsw nt  wa  wa  qld vic nsw vic qld qld sa  tas  
> sa  nt  wa
> [20] vic qld nsw nsw wa  sa  act nsw vic vic act
> Levels: act nsw nt qld sa tas vic wa
>
> With this, just visually, I know what the cateogries or Levels are.  
> Nonetheless,
> two questions arise here: How can I have, computationally as opposed  
> to
> visually, access to the names of these categories, and how do I get  
> the indexes
> of the original array elements that belong to a particular category,  
> say, "act"?
> This is, for instance, to select from another "parallel" array, the
> corresponding elements, say
>
>
> * incomes <- c(60, 49, 40, 61, 64, 60, 59, 54, 62, 69, 70, 42, 56,
>                    61, 61, 61, 58, 51, 48, 65, 49, 49, 41, 48, 52, 46,
>                    59, 46, 58, 43)
>
> So to select, the corresponding elements to "act":
>
>  46 43

I think you need to understand indexing more than you need to  
understand factors.

incomes [ which(statef == "act") ]

>
>
> Do you have any comments on this?

If you want to understand how to  programmatically access levels, then  
you only need to follow the "See also" links on the ?factor page.

-- 

David Winsemius, MD
West Hartford, CT