在R中应用外部函数

Question

Given this data.frame

  x y z
1 1 3 5
2 2 4 6

I'd like to add the value of columns x and z plus a coefficient 10, for every rows in dat . The intended result is this

  x y z result
1 1 3 5 16      #(1+5+10)
2 2 4 6 18      #(2+6+10)

But why this code doesn't produce the desired result?

 dat <- data.frame(x=c(1,2), y=c(3,4), z=c(5,6))
 Coeff <- 10

 # Function
 process.xz <- function(v1,v2,cf) {
    return(v1+v2+cf)
 }

# It breaks here
sm <- apply(dat[,c('x','z')], 1, process.xz(dat$x,dat$y,Coeff ))

# Later I'd do this:
# cbind(dat,sm);

Answer 1

I wouldn't use an apply here. Since the addition + operator is vectorized, you can get the sum using

> process.xz(dat$x, dat$z, Coeff)
[1] 16 18

To write this in your data.frame , don't use cbind , just assign it directly:

dat$result <- process.xz(dat$x, dat$z, Coeff)

Answer 2

The reason it fails is because apply doesn't work like that - you must pass the name of a function and any additional parameters. The rows of the data frame are then passed (as a single vector) as the first argument to the function named.

 dat <- data.frame(x=c(1,2), y=c(3,4), z=c(5,6))
 Coeff <- 10

 # Function
 process.xz <- function(x,cf) {
    return(x[1]+x[2]+cf)
 }

sm <- apply(dat[,c('x','z')], 1, process.xz,cf=Coeff)

I completely agree that there's no point in using apply here though - but it's good to understand anyway.