csv_database.md

CSV as a Database (DB)

A database is a system of storing and organizing information on a computer. In this case a CSV will act as our database. In most cases a database can be thought of as a table or group of tables:

ID	Name
1	Kari
2	Jeremy
3	Crystal
4	Cynthia

We're going to quickly create a CSV to read later, don't worry about this code for now, just copy and paste it. Notice that it created a file named ada_people.csv on your computer.

people = [[1,"Kari"], [2, "Jeremy"], [3, "Crystal"], [4, "Cynthia"]]
CSV.open("ada_people.csv", "w") do |file|
  people.each do |person|
    file << person
  end
end

Ruby includes a CSV library used to read and write CSV files. This class is very similar to the File class, it's just specifically suited to read and write CSV files.

require 'csv'
# Reads the contents of the file into an array of arrays.
CSV.read("ada_people.csv")
# => [["1", "Kari"],["2", "Jeremy"],["3", "Crystal"],["4", "Cynthia"]]

# Returns a CSV Object, to be read or manipulated.
csv = CSV.open("ada_people.csv")
# <#CSV io_type:File io_path:"file.csv" encoding:UTF-8 ...>

The open method requires at least one argument and up to three arguments as well as an optional block.

CSV.open(filename, mode='r', options) {|file| block}

The required argument is the CSV filename and path. The second argument (mode) sets what permissions Ruby has to the file when it is opened. This defaults to r (read onle). Possible values are:

Mode	Meaning
"r"	Read-only, starts at beginning of file (default mode)
"r+"	Read-write, starts at beginning of file.
"w"	Write-only, truncates existing file to zero length
"w+"	Read-write, truncates existing file to zero length.
"a"	Write-only, starts at end of file if file exists.
"a+"	Read-write, starts at end of file if file exists.
"b"	Binary file mode
"t"	Text file mode

Most modes will create a new file if one doesn't already exist with the given name.

If you pass a block to open, Ruby will open the file, execute the code within the block (the file contents is given to the block as a local variable), and then close the file.

require 'csv'
CSV.open("moar_ada_peeps.csv", 'w') do |csv|
  csv << "1,Elise\n2,Karen"
end

We can iterate over each line of the file using the each method:

require 'csv'
CSV.open("markets.csv", 'r').each do |line|
  puts line
end

The .read method requires one arguement, the filename, and allows for additional options. .read first opens the CSV file, then reads the contents of it into a new Array. Each row of CSV data is translated into an array as well.

# ada_peeps.csv
1,Kari
2,Jeremy
3,Cynthia
4,Crystal

require 'csv'
require 'awesome_print'
array_of_ada_peeps = CSV.read("ada_peeps.csv")
ap array_of_ada_peeps
# [
#     [0] [
#         [0] 1,
#         [1] "Kari"
#     ],
#     [1] [
#         [0] 2,
#         [1] "Jeremy"
#     ],
#     [2] [
#         [0] 3,
#         [1] "Cynthia"
#     ],
#     [3] [
#         [0] 4,
#         [1] "Crystal"
#     ]
# ]