Is hlookup possible in R? (or, how do I conditionally populate a column?)

Question

I have two dataframes.

X1 is

size    count
37      1.181
38      0.421
39      0.054

and X2 is

size    v1    v1
37      1     5
...     ...   ...
37      1     5
38      3     3
...     ...   ...
38      3     3
39      5     6
...     ...   ...
39      5     6

Dataframe X2 is a large panel basically. I want to create a 3rd column, v3, in X2, such that it takes the value of count from X1 if size matches.

How is this possible?

Answer 1

You can join the two datasets together like this:

library(dplyr)
left_join(X2, X1, by="size")

Example with the Iris dataset:

I2<-data.frame(Species=c("setosa", "versicolor",  "virginica"),
               myData=c(1,2,3))
left_join(iris, I2, by="Species")

Answer 2

This can be accomplished via a left_join() from the dplyr package.

library(dplyr)

x1 <- tribble(~size, ~count,
              37, 1.181,
              38, 0.421,
              39, 0.054
)

x2 <- tribble(~size, ~v1, ~v2,
              37, 1, 5,
              37, 1, 5,
              38, 3, 3,
              38, 3, 3,
              39, 5, 6,
              39, 5, 6
)

left_join(x2, x1, by = "size")

Produces:

#    size    v1    v2 count
#   <dbl> <dbl> <dbl> <dbl>
# 1    37     1     5 1.18 
# 2    37     1     5 1.18 
# 3    38     3     3 0.421
# 4    38     3     3 0.421
# 5    39     5     6 0.054
# 6    39     5     6 0.054

Answer 3

The base R function merge would get you there as well.

merge(x2, x1)