for loop on R function

Question

I'm new to R (and programming generally), and confused about why the following bits of code yield different results:

x <- 100

for(i in 1:5){
  x <- x + 1
  print(x)
}

This incrementally prints the sequence 101:105 as I would expect.

x <- 100

f <- function(){
  x <- x + 1
  print(x)
}

for(i in 1:5){
  f()
}

But this just prints 101 five times.

Why does packaging the logic into a function cause it to revert to the original value on each iteration rather than incrementing? And what can I do to make this work as a repeatedly-called function?

joneshf · Accepted Answer · 2013-05-18 14:52:02Z

The problem

It is because in your function you are dealing with a local variable x on the left side, and a global variable x on the right side. You are not updating the global x in the function, but assigning the value of 101 to the local x. Each time you call the function, the same thing happens, so you assign local x to be 101 5 times, and print it out 5 times.

To help visualize:

# this is the "global" scope
x <- 100

f <- function(){
  # Get the "global" x which has value 100,
  # add 1 to it, and store it in a new variable x.
  x <- x + 1
  # The new x has a value of 101
  print(x)
}

This would be similar to the following code:

y <- 100

f <- function(){
  x <- y + 1
  print(x)
}

One possible fix

As for what to do to fix it. Take the variable as the argument, and pass it back as the update. Something like this:

f <- function(old.x) {
    new.x <- old.x + 1
    print(new.x)
    return(new.x)
}

You would want to store the return value, so your updated code would look like:

x <- 100

f <- function(old.x) {
    new.x <- old.x + 1
    print(new.x)
    return(new.x)
}

for (i in 1:5) {
  x <- f(x)
}

Matthew Lundberg · Accepted Answer · 2013-05-19 00:20:27Z

2

This does what you want:

f <- function(){
  x <<- x + 1
  print(x)
}

But you shouldn't do this. Globals are not a good construct. Functions with side-effects cause code to be hard to understand and hard to debug.

A safer way to use a global is to encapsulate it into another environment. Here is an example:

create.f <- function(x) {
  return(function() {
    x <<- x + 1
    print(x)
  })
}

f <- create.f(100)
for (i in 1:5) f()
## [1] 101
## [1] 102
## [1] 103
## [1] 104
## [1] 105

Here, the "global" x is in the environment of the body of create.f, where f is defined, and not the global environment. The environment of a function is the environment in which it is defined (and not that in which it is called).

edited May 19, 2013 at 0:20

answered May 18, 2013 at 14:42

Matthew Lundberg

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1 Comment

Paul Hiemstra Over a year ago

It might solve your problem now, but please avoid using global variables and do not get too comfortable with them. For small scripts, using a global is probably not that bad, but small scripts get big and then these things can really bite you.

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