Études for Elixir

Chapter 7. Hashes

Étude 7-1: Creating a HashDict from a File

Your local college has given you a text file that contains data about which courses are taught in which rooms. Here is part of the file. The first column is the course ID number. The second column is the course name, and the third column is the room number.

64850,ENGL 033,RF141
64851,ENGL 080,SC103
64853,ENGL 102,C101B

Your job in this étude is to read the file and create a HashDict whose key is the room number and whose value is a list of all the courses taught in that room.

Opening Files

To open file test.csv, which you will find in the example download area at URL goes here, use File.open/2, which takes the path to a file as its first argument and a list of options as its second argument. Here is a shell session that opens the file, reads one line, and then closes the file.

iex(1)> {result, device} = File.open("courses.csv", [:read, :utf8])
{:ok,#PID<0.39.0>}
iex(2)> data = IO.readline(device)
"64850,ENGL 033,RF141\n"
iex(3)> File.close(device)
:ok

If you successfully open the file, result will be :ok, and the device will be the variable you when reading or closing the file. If there is some error, result will be :error, and the device variable will contain the reason that the file open failed.

IO.readline/1 reads a line from the file (including the ending \n character) unless there is no more data, in which case you will get the atom :eof.

Note

If you do not use the :utf8 option, the file will open in binary mode, and you will ...