Describe a situation where you had to use nested loops in R for a complex data processing task. How did you optimize your code?

One situation where you might need to use nested loops in R for a complex data processing task is when working with hierarchical data structures, such as nested lists or data frames. To optimize the code, you can use techniques like preallocating output objects, vectorizing operations within the loops, and utilizing R's apply family of functions to avoid explicit use of nested loops.