Extended Example: A Function to Display the Contents of a Call Frame with R

Extended Example: A Function to Display the Contents of a Call Frame

In R, a call frame contains the local variables and arguments of a function at the time of its invocation. It can be useful to visualize this information for debugging or understanding the state of variables during function execution.

Here’s how to create a function that displays the contents of a call frame in R.

Using sys.frame() and ls()

The sys.frame() function allows you to access a specific call frame, while ls() lists the objects in that environment. We will combine these two functions to create a function that displays the local variables and arguments of the call frame.

Syntax of sys.frame(): 

sys.frame(which = sys.nframe())

• which: Indicates which call frame to access (0 for the current call frame, 1 for the calling frame, etc.).

Creating the Function

Let’s create a function called display_call_frame_contents() that displays the contents of the current call frame. 

# Function to display the contents of the call frame
display_call_frame_contents <- function() {
  # Get the current call frame
  frame <- sys.frame(which = sys.parent())
  # Check if the frame exists
  if (!is.null(frame)) {
    # Display object names and their values in the frame
    objects <- ls(envir = frame)
    cat("Contents of the call frame:\n")
    for (obj in objects) {
      cat(sprintf("%s:\n", obj))
      print(get(obj, envir = frame))
      cat("\n")
    }
  } else {
    cat("The call frame is empty or does not exist.\n")
  }
}
# Example usage
my_function <- function(a, b) {
  x <- a + b
  y <- a * b
  display_call_frame_contents()
  return(x + y)
}
# Call the function
result <- my_function(3, 4)
print(result)

Explanation of the Code

• sys.frame(which = sys.parent()): Accesses the call frame of the parent (the frame where the current function was called).
• ls(envir = frame): Lists the names of objects in the call frame.
• get(obj, envir = frame): Retrieves the value of each object in the call frame.
• cat() and print(): Display the objects and their values.

Output

When you call my_function(3, 4), the display_call_frame_contents() function will print the variables a, b, x, and y, along with their values at the time of the call.

Expected Output:

Contents of the call frame: 

a:
[1] 3
b:
[1] 4
x:
[1] 7
y:
[1] 12
[1] 19

Summary

• Function display_call_frame_contents(): Displays the local variables and arguments in the current call frame.
• Using sys.frame() and ls(): Accesses and lists the objects in the call frame.
• Display: Shows the names and values of variables in the call frame for debugging and analysis.