FillIn: A function for filling in missing values of a variable from...
In DataCombine: Tools for Easily Combining and Cleaning Data Sets

FillIn uses values of a variable from one data set to fill in missing values in another.

1 2	FillIn(D1, D2, Var1, Var2, KeyVar = c("iso2c", "year"), allow.cartesian = FALSE, KeepD2Vars = FALSE)

`D1`	the data frame with the variable you would like to fill in.
`D2`	the data frame with the variable you would like to use to fill in `D1`.
`Var1`	a character string of the name of the variable in `D1` you want to fill in.
`Var2`	an optional character string of variable name in `D2` that you would like to use to fill in. Note: must be of the same class as `Var1`.
`KeyVar`	a character vector of variable names that are shared by `D1` and `D2` that can be used to join the data frames.
`allow.cartesian`	logical. See the `data.table` documentation for more details.
`KeepD2Vars`	logical, indicating whether or not to keep the variables from D2 in the output data frame. The default is `KeepD2Vars = FALSE`. Hint: avoid having variables in your `D2` data frame that share names with variables in `D1` other than the `KeyVar`s

# Create data set with missing values
naDF <- data.frame(a = sample(c(1,2), 100, rep = TRUE),
                   b = sample(c(3,4), 100, rep = TRUE),
                   fNA = sample(c(100, 200, 300, 400, NA), 100, rep = TRUE))

# Created full data set
fillDF <- data.frame(a = c(1, 2, 1, 2),
                     b = c(3, 3, 4, 4),
                     j = c(5, 5, 5, 5),
                     fFull = c(100, 200, 300, 400))

# Fill in missing f's from naDF with values from fillDF
FilledInData <- FillIn(naDF, fillDF, Var1 = "fNA",
                       Var2 = "fFull", KeyVar = c("a", "b"))