RFP - Part 6: R «- Assignment Operator

by Guangming Lang
4 min read

Categories

  • r

If you have been following this R Functional Programming (RFP) series1, you know by now we have discussed:

In summary, we have the following facts about the R language:

  • Variable assignment binds symols to objects.
  • Reassignment changes the binding. It doesn’t modify data.
  • Mutability is a property of objects. It has nothing to do with variables or variable assignment.
  • Copy-n-modify is everywhere.

To better understand these facts, let’s walk through two examples. The first example consists of the following code block:

change = function(x) {
        x = x - 5
        print(paste("do work and get:", x, "at addr", pryr::address(x)))
}
main = function() {
        x = 5
        print(paste("start          :", x, "at addr", pryr::address(x)))
        
        change(x) # x is not changed. It still points to the same value 5
        print(paste("end            :", x, "at addr", pryr::address(x)))
}

What will happen if we call main()?

  1. x will be bound to 5.
  2. 5 and its memory address will be printed.
  3. change(x) will not change the value of x.
  4. 5 and the same address will be printed.

Let’s call it now:

main()
## [1] "start          : 5 at addr 0x7f875baea088"
## [1] "do work and get: 0 at addr 0x7f875ebda4e8"
## [1] "end            : 5 at addr 0x7f875baea088"

The output confirms our reasoning. When the line change(x) is executed, a new environment is created, then the value 5 is copied into this new environment and bound to a new variable x2. When the line x = x - 5 is run, R looks up the value 5 of this new x from this new environment, carries out the subtraction and rebinds x to the result 0. When the program steps out of the function change(), everything (x, 0, 5) inside of this new environment will be garbage collected. I want to point out that the original x in main() is never modified. See the following diagram:

<<- Reassigns Variables in Outer Scope

Now let’s look at the second example:

main = function() {
        change = function(x) {
                x <<- x - 5
                print(paste("do work and get:", x, "at addr", pryr::address(x)))
        }
        
        x = 5
        print(paste("start          :", x, "at addr", pryr::address(x)))
        
        change(x) # x is changed. It now points to a new value 0 at a different location 
        print(paste("end            :", x, "at addr", pryr::address(x)))
}

This example differs from the first one at two places:

  • change() is defined inside of main().
  • <<- is used instead of <- or = (x <<- x - 5 instead of x = x - 5).

The <<- operator is usually only used in functions. While operators <- and = perform variable assignment inside the environment (or scope) where they are evaluated, <<- causes a search through parent environments for the variable being assigned. If such a variable is found (and its binding is not locked) then its value is redefined, otherwise assignment takes place in the global environment. I’d like to point out that like <- and =, <<- also only changes the binding and doesn’t modify data.

In this example:

  • change() defines an inner scope and main() defines an outer scope.
  • because of <<-, change(x) changes the value of x defined in the outer scope.

When we call main(), the following will happen:

  1. change will be bound to function(x) { x <<- x - 5 ...} in the outer scope.
  2. x will be bound to 5 in the outer scope.
  3. 5 and its memory address will be printed.
  4. change(x) will change the value of outer x to 0 and print 5 and a different location.
  5. 5 and yet another different location will be printed.

Let’s call main() and look at the output:

main()
## [1] "start          : 5 at addr 0x7f875f47f810"
## [1] "do work and get: 5 at addr 0x7f875e8daa80"
## [1] "end            : 0 at addr 0x7f875fcd4340"

The 2nd line of output is particularly interesting. You’re probably expecting it to print 0, but that wouldn’t be correct. When the line change(x) is executed, a new environment is created, then the value 5 is copied into this new environment and bound to a new variable x2. When the line x <<- x - 5 is run, R looks up the value 5 of this new x from this new environment, carries out the subtraction and rebinds the x in the outer scope to the result 0. Notice that the inner variable x is still bound to 5. When R executes the line print(paste("do work and get:", x, "at addr", pryr::address(x))), it looks up the value and address of the inner x, and hence prints 5. Of course, because this 5 is a copy of the outer 5, it is a different 5 and hence has a different memory address. See the following diagram:

To summarize, here’re the facts about <<-:

  • it’s usually only used in functions.
  • it reassigns outer variables by changing its binding.
  • it never modifies data directly.

Here’s an example of how <<- can be used in real world:

Mortgage Payment Schedule Calculator

  1. If you haven’t, I recommend you to read the linked posts first. ↩

  2. different from the x in main(). ↩ ↩2

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