[R] select from data frame

[David Winsemius]

> On Jul 15, 2017, at 4:01 AM, Andras Farkas via R-help <r-help at r-project.org> wrote:
> 
> Dear All,
> 
> wonder if you could please assist with the following 
> 
> df<-data.frame(ID=c(1,1,1,2,2,3,3,4,4,5,5),samples=c("A","B","C","A","C","A","D","C","B","A","C"))
> 
> from this data frame the goal is to extract the value of 3 from the ID column based on the logic that the ID=3 in the data frame has NO row that would pair 3 with either "B", AND/OR "C" in the samples column...
> 

This returns a vector that determines if either of those characters are in the character values of that factor column you created. Coercing to character is needed because leaving samples as a factor generated an invalid factor level warning and gave useless results.

 with( df, ave( as.character(samples), ID, FUN=function(x) {!any(x %in% c("B","C"))}))
 [1] "FALSE" "FALSE" "FALSE" "FALSE" "FALSE" "TRUE"  "TRUE"  "FALSE" "FALSE"
[10] "FALSE" "FALSE"

You can then use it to extract and consolidate to a single value (although wrapping with as.logical was needed because `ave` returned character class values):

 unique( df$ID[ as.logical(   # fails without this since "FALSE" != FALSE
                    with( df, 
                       ave( as.character(samples), ID, FUN=function(x) {!any(x %in% c("B","C"))})))
              ] )
#[1] 3

The same sort of logic could also be constructed with a for-loop:

> for (x in unique(df$ID) ) { if ( !any( df$samples[df$ID==x] %in% c("b","C")) ) print(x) }
[1] 3

Although you are warned that for-loops do not return values and you might need to make an assignment rather than just printing.

-- 

David Winsemius
Alameda, CA, USA