Why is using `<<-` frowned upon and how can I avoid it?

Question

I followed the discussion over HERE and am curious why is using<<- frowned upon in R. What kind of confusion will it cause?

I also would like some

Answer 1

You can think of <<- as global assignment (approximately, because as kohske points out it assigns to the top environment unless the variable name exists in a more proximal environment). Examples of why this is bad are here:

Examples of the perils of globals in R and Stata

Answer 2

First point

<<- is NOT the operator to assign to global variable. It tries to assign the variable in the nearest parent environment. So, say, this will make confusion:

f <- function() {
    a <- 2
    g <- function() {
        a <<- 3
    }
}

then,

> a <- 1
> f()
> a # the global `a` is not affected
[1] 1

Second point

You can do that by using Reduce:

Reduce(function(a, b) {a[a==b] <- a[a==b]-1; a}, 2:6, df)

or apply

apply(df, c(1, 2), function(i) if(i >= 2) {i-1} else {i})

But

simply, this is sufficient:

ifelse(df >= 2, df-1, df)