how to access global/outer scope variable from R apply function?

Question

I can't seem to make apply function access/modify a variable that is declared outside... what gives?

    x = data.frame(age=c(11,12,13), weight=c(100,105,110))     x      testme <- function(df) {         i <- 0         apply(df, 1, function(x) {             age <- x[1]             weight <- x[2]             cat(sprintf("age=%d, weight=%d\n", age, weight))             i <- i+1   #this could not access the i variable in outer scope             z <- z+1   #this could not access the global variable         })         cat(sprintf("i=%d\n", i))         i     }      z <- 0     y <- testme(x)     cat(sprintf("y=%d, z=%d\n", y, z)) 

Results:

    age=11, weight=100     age=12, weight=105     age=13, weight=110     i=0     y=0, z=0 

Answer 1

Using the <<- operator you can write to variables in outer scopes:

x = data.frame(age=c(11,12,13), weight=c(100,105,110)) x  testme <- function(df) {     i <- 0     apply(df, 1, function(x) {         age <- x[1]         weight <- x[2]         cat(sprintf("age=%d, weight=%d\n", age, weight))         i <<- i+1   #this could not access the i variable in outer scope         z <<- z+1   #this could not access the global variable     })     cat(sprintf("i=%d\n", i))     i }  z <- 0 y <- testme(x) cat(sprintf("y=%d, z=%d\n", y, z)) 

The result here:

age=11, weight=100 age=12, weight=105 age=13, weight=110 i=3 y=3, z=3 

Note that the usage of <<- is dangerous, as you break up scoping. Do this only if really necessary and if you do, document that behavior clearly (at least in bigger scripts)