Check file header and then loop through - ruby

Question

Suppose I have a file in the following format.
date|time|account
2010-01-01|07:00:00|A1
2010-01-01|07:00:01|A2
....
Suppose I have the following function.
def ReadLongFile(longFile)
CSV.foreach(longFile, :headers => true, :col_sep => '|') do |row|
p row.to_hash
end
end
I like this function because it allows me to store each line as a hash where the header entries are the keys, and the line entries are the corresponding values. However, what is the most efficient way to modify it such that I can verify the header contains the correct entries? I was considering two options. First, I could open it another function and check the first line. Second, I could check within the function, but it would perform the check each iteration.

Answer 1

I would suggest using the CSV::header_row function to perform the check, and raising an error if it's not what you expect. Something like:
def ReadLongFile(longFile)
CSV.foreach(longFile, :headers => true, :return_headers => true, :col_sep => '|') do |row|
if row.header_row? then
raise ArgumentError, "Bad headers" unless header_sane?(row)
end
# Otherwise do the processing
end
end
Your implemenation of header_sane? will perform the validation that you need to ensure the file is what you expect it to be. Your calling code can rescue the ArgumentError if it can recover from it, or just let it fail :-)
Note: Updated to reflect error noted in the comments below. Be sure to set the :return_headers option when calling CSV::foreach.
If you are worried about the minimal overhead of calling header_row? for each of the row entries, you can construct a CSV instance and use shift to manually check the first row before continuing. For instance:
def ReadLongFile(longFile)
File.open(longFile) do |file|
reader = CSV.new(file, {:col_sep => '|', :headers => true, :return_headers => true})
header_row = reader.shift
raise ArgumentError, "Bad file headers" unless header_sane?(header_row)
reader.each do |row|
p row
end
end
end
Implemented as above, the following behavior holds true:
[4] pry(main)> def header_sane? row
[4] pry(main)* true
[4] pry(main)* end
=> nil
[5] pry(main)> ReadLongFile("file.csv")
#<CSV::Row "date":"2010-01-01" "time":"07:00:00" "account":"A1">
#<CSV::Row "date":"2010-01-01" "time":"07:00:01" "account":"A2">
=> nil
[6] pry(main)> def header_sane? row
[6] pry(main)* false
[6] pry(main)* end
=> nil
[7] pry(main)> ReadLongFile("file.csv")
ArgumentError: Bad file headers
from (pry):7:in `block in ReadLongFile'