To change the color of bars in a bar chart in R, you would use the col parameter. By providing a vector of colors corresponding to each bar, you can assign different colors to different bars in the chart.

One situation where you might need to use a nested function in R for a complex task is when handling complex data manipulation or transformations, implementing intricate statistical calculations, or developing custom modeling or simulation procedures. Challenges in such scenarios may include managing variable scopes, dealing with large datasets, optimizing performance, and ensuring code readability. These challenges can be mitigated by carefully planning the nested functions, testing and debugging extensively, and breaking down complex tasks into smaller, more manageable components.

In R, exponentiation is performed using the ^ operator. For example, to calculate 2 to the power of 3, we would use 2^3.

When dealing with large data objects, global variables in R can lead to memory inefficiency if not managed properly. Global variables persist throughout the program's execution and occupy memory even when they are no longer needed. This can result in excessive memory usage, especially if multiple large data objects are stored as global variables. Proper management, such as removing or resetting global variables when no longer needed, is crucial to avoid memory-related issues.

The lapply() function in R can be used as an alternative to a for loop to apply a function to each element of a list. It returns a list containing the results of applying the function to each element of the list.

If a variable with the same name exists in both the local and global environment in R, the local variable will be used. R follows the scoping rules where variables defined in the local environment take precedence over variables with the same name in the global environment.

To square a number in R, you use the ^ operator with 2 as the exponent. For example, to calculate the square of 4, you would use 4^2, which would return 16.

The substr() function in R is used to extract a specific substring from a string. For example, substr("Hello", 2, 3) would return "el".

If you have two character vectors in R, you can concatenate corresponding elements from each vector with a hyphen in between using the 'paste()' function with 'sep = "-"'. For example, 'paste(c("Hello", "Goodbye"), c("world!", "friends!"), sep = "-")' would return a vector containing "Hello-world!" and "Goodbye-friends!".

To create a pie chart in R that color-codes segments based on a specific criteria, you would use the pie() function. Provide a vector of colors corresponding to each segment, ensuring that the colors align with the specific criteria you want to represent.