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Discover why non-local functions may not behave as expected when arguments are missing, and learn how to correctly implement them in your R code.
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Understanding Function Scope: Why my_fun Works Differently with Arguments

In programming, especially in languages like R, the concept of function scope can greatly affect how your code runs. A common issue that many new programmers encounter is why a non-local function only works when arguments are provided. This guide dives into a specific scenario to clarify this important concept and improve your coding skills.

The Problem

Let's explore a case where a function, my_fun, defined outside the scope of another function my_big_fun, unexpectedly behaves differently when called without parameters. Here's the original function definition:

[[See Video to Reveal this Text or Code Snippet]]

When running my_big_fun, you would expect output like the sequence 1, 2, 3,..., N. Instead, you receive [1] 1 10 10 10 10 10 10 10 10 10. So, why is this happening?

Analysis of the Issue

Understanding Variable Scope

Global vs. Local Scope:

In R, variables can either be defined locally (within a function) or globally (outside of functions).

Your function my_fun() does not take an argument and relies on x, which is defined within my_big_fun. However, my_fun() will not be able to access x correctly from the following scope.

The Role of x:

Since x is not globally defined (in this context, outside of the functions), the call to my_fun() will throw an error or, as seen, not work as expected.

If x has been defined globally elsewhere (for instance, set to 9), my_fun() will always return 10 because it adds one to the global x.

Setting the First Value

In the for loop inside my_big_fun, the initial value is set manually with res[1] <- start. This explains why the first value is correctly 1 — it is being assigned explicitly.

The Solution

Modify my_fun to Accept Parameters

To ensure the function works as intended, the solution is to modify my_fun to accept an argument, x, as follows:

[[See Video to Reveal this Text or Code Snippet]]

Now, redefine my_big_fun to use my_fun2:

[[See Video to Reveal this Text or Code Snippet]]

Key Takeaways

Always pass relevant variables as arguments to functions when they are not globally defined.

Understand variable scope: local variables exist only within the function they are defined in, while global variables are accessible throughout the entire R session.

Conclusion

Understanding function scope is crucial for effective programming. When defining and using functions like my_fun, always consider how variables are accessed and modified. By ensuring that variables are passed as arguments, you can avoid unexpected behavior and achieve the intended output in your code.

By applying these principles, you will gain confidence in your coding abilities, make fewer mistakes, and write more efficient programs.