dataframe is collapsed to a vector when given to function - r

Question

I am trying to make use of the content of a dataframe in a function, here is a simplified example of my problem.
df <- data.frame(v1=1:10,v2=23:32)
df2 <- data.frame(v1=1:3,v2=3:5)
fxm <- function(x,y,q)
{
return(cbind(q[q[,2]==x,],y))
}
mapply(fxm,df[,1],df[,2],q=df2)
Error in q[, 2] : incorrect number of dimensions
if I add a print statement:
df <- data.frame(v1=1:10,v2=23:32)
df2 <- data.frame(v1=1:3,v2=3:5)
fxm <- function(x,y,q)
{
print(q)
return(cbind(q[q[,2]==x,],y))
}
mapply(fxm,df[,1],df[,2],q=df2)
I get:
[1] 1 2 3
Error in q[, 2] : incorrect number of dimensions
The data frame is converted to a vector of its first column for some reason. How can I stop this from happening, and have the whole dataframe accessible to my function?
I am trying to select a subset of the dataframe and returning it based on the other two parameters of the function, which is why I need the whole dataframe to be passed to the function.

Answer 1

If I understand you correctly, you want the whole thing q = df2 passed to the fxm function you define, am I right?
The problem is that in your code mapply will extract elements from q = df2 as some additional parameters just same as extracting elements from df[,1] and df[,2]. You need to set MoreArgs parameter for mapply to pass the whole thing to the function like this:
df <- data.frame(v1=1:10,v2=23:32)
df2 <- data.frame(v1=1:3,v2=3:5)
fxm <- function(x,y,q)
{
print(q)
return(cbind(q[q[,2]==x,],y))
}
mapply(fxm,df[,1],df[,2], MoreArgs = list(q=df2))
This still doesn't work for me and there is some error elsewhere. From the printing result you can see the whole data.frame prints out, which solves your original problem.