Add columns with predefined values to a dataframe

Question

I would like to add new columns to a dataframe, populated with predefined values stored in another dataframe. What is the best way to proceed?

Nice to have: is it possible to make the code dynamic in case new variables appear in d2?

data for reproducible code

d1 <- head(iris)                      # dataframe 1 ----------

var <- c("var1", "var2", "var3")
val <- c(5, 7, 1)
d2 <- as.data.frame(cbind(var, val))  # dataframe 2 ----------

Expected output

First dataframe

Second dataframe

Desired output

Best regards

Answer 1

One way would be (this will work for any number of vars)

d1[as.character(d2$var)] <- rep(d2$val, each = nrow(d1))
#   Sepal.Length Sepal.Width Petal.Length Petal.Width Species var1 var2 var3
# 1          5.1         3.5          1.4         0.2  setosa    5    7    1
# 2          4.9         3.0          1.4         0.2  setosa    5    7    1
# 3          4.7         3.2          1.3         0.2  setosa    5    7    1
# 4          4.6         3.1          1.5         0.2  setosa    5    7    1
# 5          5.0         3.6          1.4         0.2  setosa    5    7    1
# 6          5.4         3.9          1.7         0.4  setosa    5    7    1

Answer 2

Using data.table:

require(data.table)
setDT(d1)[, as.character(d2$var) := as.list(d2$val)]

as.character() is required because of the way you've created d2, which resulted in var being factor.