# 1. We should use functions at all because:
# Decrease the volume of source code
# Easy to manage because the program is divided into several sections
# Source code is bright
# Reuse the source code
# Hide information
# 2. define/declare functions:
# To define/declare a function, simply record the function name together with the parentheses (),
# function behind "def"
# 3. call/ use:
# A function call has been marked as a comment because it is called before the definition of a function,
# a function must always be defined before it is called, ie it must be written before it is called.
# 4. What is return, why and how do we use it?
# Keyword return
# A function is created to perform certain tasks.
# Declare a variable to store the total value and then browse through a given list to sum it and save it to the sum variable,
# but when the function ends, the total variable is deleted, you can not get the total variable outside the function. 
# that's it. At this point you need to use the return keyword, 
# the return keyword has the effect of "returning" a value of a function.
# 5. Do we have to use return in every function?
# - yep
# 6. What are parameter / function parameters, why and how do we use it?
# The sequence of numbers that you give to the function so that it does the calculation job called parameter.
# 7. How to use function from a different file other than our currently working file?
# - declare a function in a file, have "def" -> give parameter -> call function in file other 

