r - Using filter_ in dplyr where both field and value are in variables

Question

I want to filter a dataframe using a field which is defined in a variable, to select a value that is also in a variable. Say I have

df <- data.frame(V=c(6, 1, 5, 3, 2), Unhappy=c("N", "Y", "Y", "Y", "N"))
fld <- "Unhappy"
sval <- "Y"

The value I want would be df[df$Unhappy == "Y", ].

I've read the nse vignette to try use filter_ but can't quite understand it. I tried

df %>% filter_(.dots = ~ fld == sval)

which returned nothing. I got what I wanted with

df %>% filter_(.dots = ~ Unhappy == sval)

but obviously that defeats the purpose of having a variable to store the field name. Any clues please? Eventually I want to use this where fld is a vector of field names and sval is a vector of filter values for each field in fld.

Answer 1

You can try with interp from lazyeval

 library(lazyeval)
 library(dplyr)
 df %>%
     filter_(interp(~v==sval, v=as.name(fld)))
 #   V Unhappy
 #1 1       Y
 #2 5       Y
 #3 3       Y

For multiple key/value pairs, I found this to be working but I think a better way should be there.

  df1 %>% 
    filter_(interp(~v==sval1[1] & y ==sval1[2], 
           .values=list(v=as.name(fld1[1]), y= as.name(fld1[2]))))
 #  V Unhappy Col2
 #1 1       Y    B
 #2 5       Y    B

For these cases, I find the base R option to be easier. For example, if we are trying to filter the rows based on the 'key' variables in 'fld1' with corresponding values in 'sval1', one option is using Map. We subset the dataset (df1[fld1]) and apply the FUN (==) to each column of df1[f1d1] with corresponding value in 'sval1' and use the & with Reduce to get a logical vector that can be used to filter the rows of 'df1'.

 df1[Reduce(`&`, Map(`==`, df1[fld1],sval1)),]
 #   V Unhappy Col2
 # 2 1       Y    B
  #3 5       Y    B

data

df1 <- cbind(df, Col2= c("A", "B", "B", "C", "A"))
fld1 <- c(fld, 'Col2')
sval1 <- c(sval, 'B')