Ruby and CSV - How to get a column value after filtering rows - ruby

Question

I have parsed a CSV file with headers of ID, Name, and Address. I need to find the row(s) in the data that have a particular ID and Name. Once I find that row, I need to access the Address value.
data = CSV.parse(my_csv, headers: true)
rows = data.select { |row| row['ID'] == someId } //ids are not unique
row = rows.select { |row| row['Name'] == name } //names are unique
How do I get the Address value in this row of data? row[2] doesn't work.

Answer 1

I need to find the row(s) in the data that have a particular ID and Name.
You can combine as many criteria as you wish in a single select():
rows = data.select {|row| row['ID'] == someId && row['Name'] == name}
In fact, you can make this boolean statement as simple or as complex as you wish. You will need to determine the exact boolean logic necessary to get the results you want.
I need to access the Address value.
To get the 'Address' column of each row selected, use map():
addresses = rows.map {|row| row['Address']}
To get the 'Address' column of the first row selected:
rows[0]['Address']
Be careful with this last one, though. It will cause an error if rows is empty.

Answer 2

Let's first create some test data.
my_csv = <<~_
ID,Name,Address
1,cat,Mars
2,dog,Jupyter
1,dog,Venus
_
#=> "ID,Name,Address\n1,cat,Mars\n2,dog,Jupyter\n1,dog,Venus\n"
puts my_csv
ID,Name,Address
1,cat,Mars
2,dog,Jupyter
1,dog,Venus
As it is guaranteed that the values of 'Names' are unique, we can use Enumerable#find, which terminates as soon as the condition in its block evaluates true.
require 'csv'
def find_addr(csv_str, id, name)
row = CSV.parse(csv_str, headers: true).find do |row|
row['ID'] == id && row['Name'] == name
end
row.nil? ? nil : row['Address']
end
find_addr(my_csv, "1", "cat") #=> "Mars"
find_addr(my_csv, "1", "dog") #=> "Venus"
find_addr(my_csv, "2", "dog") #=> "Jupyter"
find_addr(my_csv, "2", "pig") #=> nil
Note that row is assigned nil if no row satisfies the block condition.
We could shorten this by using the Safe Navigation Operator, &.
def find_addr(csv_str, id, name)
CSV.parse(csv_str, headers: true).find do |row|
row['ID'] == id && row['Name'] == name
end&.[]('Address')
end
find_addr(my_csv, "1", "cat") #=> "Mars"
find_addr(my_csv, "1", "dog") #=> "Venus"
find_addr(my_csv, "2", "dog") #=> "Jupyter"
find_addr(my_csv, "2", "pig") #=> nil
Note that when using the Safe Navigation Operator we cannot use syntactic sugar versions of methods, here
...end&['Address']
would raise an exception.
If find_addr returning nil signals that there is a problem with the data we could either handle that in code or remove the Safe Navigation Operator in the second method above, which would cause a NoMethodError exception to be raised if no row satisfies the required condition.