Examples of the perils of globals in R and Stata

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I also have the pleasure of teaching R to undergraduate students who have no experience with programming. The problem I found was that most examples of when globals are bad, are rather simplistic and don’t really get the point across.

Instead, I try to illustrate the principle of least astonishment. I use examples where it is tricky to figure out what was going on. Here are some examples:

  1. I ask the class to write down what they think the final value of i will be:

    i = 10
for(i in 1:5)
    i = i + 1
i

    Some of the class guess correctly. Then I ask should you ever write code like this?

    In some sense i is a global variable that is being changed.

  2. What does the following piece of code return:

    x = 5:10
x[x=1]

    The problem is what exactly do we mean by x

  3. Does the following function return a global or local variable:

     z = 0
 f = function() {
     if(runif(1) < 0.5)
          z = 1
     return(z)
  }

    Answer: both. Again discuss why this is bad.

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