Function which uses different dataframes with same column names - r

Question

I have two dataframes which look like follows:
df1 <- data.frame(V1 = 1:4, V2 = rep(2, 4), V3 = 7:4)
df2 <- data.frame(V2 = rep(NA, 4), V1 = rep(NA, 4), V3 = rep(NA, 4))
I need to write a function which assigns the values of df1 to df2, if the columnnames of both dataframes are the same. The structure of the function should look like this:
fun <- function(x){
if(# If the name of x is the same like the name of a column in df1)
out <- df1$? # Here I need to assign df1$"x" somehow
out
}
fun(df2$V1)
The output should look like this:
[1] 1 2 3 4
Unfortunately I couldnt find a solution by myself. Is there a way how I could do this? Thank you very much in advance!

Answer 1

I need to write a function which assigns the values of df1 to df2, if
the columnnames of both dataframes are the same.
Are you sure you need a function?
names_in_common <- intersect(names(df1),names(df2))
df2[,names_in_common] <- df1[,names_in_common]

Answer 2

Using Joachim Schork's code:
names_in_common <- intersect(names(df1),names(df2))
df2[,names_in_common] <- df1[,names_in_common]
and if you want to change a single column of df2:
names_in_common <- intersect(names(df1), names(df2[, "V1", drop=FALSE]))
df2[,names_in_common] <- df1[,names_in_common]

Answer 3

This is impossible, because when you access a column of a data.frame using the dollar syntax you lose the column name. There's no way for fun() to determine the column name of the vector that was passed in as an argument.
Instead, you can simply call fun() using the column name itself as the argument, rather than the vector of NAs, which are not useful and not used at all inside the function. In other words, the call becomes
fun('V1');
Then you can write the function as follows:
fun <- function(name) df1[[name]];
Demo:
fun('V1');
## [1] 1 2 3 4
Although now that I think about it, you might as well just index df1 directly, since that's all the function does now:
df1$V1;
## [1] 1 2 3 4
Rereading your question, you said you want to assign the column from df1 to df2, although your example code doesn't do that. Assuming you did want to carry out this assignment inside the function, you could do this:
fun <- function(name) df2[[name]] <<- df1[[name]];
Demo:
fun('V1');
df2;
## V2 V1 V3
## 1 NA 1 NA
## 2 NA 2 NA
## 3 NA 3 NA
## 4 NA 4 NA
This makes use of the superassignment operator <<-.