# Complete the function sd$\_$or$\_$na below. The function takes as an input a vector

# called vec.

# If vec is numeric, the function returns the standard deviation of vec.

# If vec is not numeric, the function returns the special value NA

# This is commented out to start with, so that you don't get errors.

# Uncomment all the lines of this function by deleting the $\text{'}$#$\text{'}$ symbols.

# Then you can add the missing parts to complete the function

#sd$\_$or$\_$na $=$ function$($vec$)\{$

# if $()\{$

# return$()$

#