How to get row from R data.frame

x[r,]

where r is the row you're interested in. Try this, for example:

#Add your data
x <- structure(list(A = c(5,    3.5, 3.25, 4.25,  1.5 ), 
                    B = c(4.25, 4,   4,    4.5,   4.5 ),
                    C = c(4.5,  2.5, 4,    2.25,  3   )
               ),
               .Names    = c("A", "B", "C"),
               class     = "data.frame",
               row.names = c(NA, -5L)
     )

#The vector your result should match
y<-c(A=5, B=4.25, C=4.5)

#Test that the items in the row match the vector you wanted
x[1,]==y

This page (from this useful site) has good information on indexing like this.

Logical indexing is very R-ish. Try:

 x[ x$A ==5 & x$B==4.25 & x$C==4.5 , ] 

Or:

subset( x, A ==5 & B==4.25 & C==4.5 )