如何将函数应用于data.frame中的行对，类似于diff？

Question

I've got a data frame that represents points in a plane.

     x     y
1  0.0   0.0
2  0.2   0.0
3  0.3  -0.1
...

I want to write a diff -like function for that data. I've written the distance function myDist (not Euclidean), but how can I apply it to the data frame?

That is: By calling someFunction( myData, myDist ) , I want to obtain a vector with the values myDist(myData[1,],myData[2,]) , myDist(myData[2,],myData[3,]) , myDist(myData[3,],myData[4,]) , ...

Is there a function that does this, or do I have to write a loop?

Answer 1

Solved it with lapply :

lapply(seq_len(nrow(myData) - 1),
    function(i){
        myDist( myData[i,], myData[i+1,] )
    })

Answer 2

You could use head(myData, -1) and tail(myData, -1) to get 2 datasets with the rows lined up the way you want, then if myDist is vectorized you would just pass the results to myDist , if it is not vectorized you could use mapply (or the Vectorize function), or sapply , lapply , apply , loops or other functionality.

myData <- data.frame( x=1:10, y=sample(1:10), z=rnorm(10) )

myDist <- function( one, two ) {
    sqrt( rowSums( (one-two)^2 ) )
}

myDist( head(myData,-1), tail(myData,-1) )