Identify open issues / tasks

[HFactors]

Please identify tasks that others can do. Examples:

• Write code for problem X
• Check code for X
• Check comments for X
  etc.

[jinlong25]

• Write a sorting function for a list of files extracted from a zip file. It should sort files in a alphabetically ascending order;
• Add warning messages for all functions using stop(). This will help us identify which function(s) went wrong when execute;