How to extract multiple columns from a csv file with ruby?

Since row is an array, your question boils down to how to pick certain elements from an array; this is not related to CSV.

You can use values_at:

row.values_at(5,6,9,10)

returns the fields 5,6,9 and 10.

If you want to present these picked fields in a different order, it is however easier to map each index explicitly:

output_row = Array.new(row.size) # Or row.dup, depending on your needs
output_row[1] = row[6]
# Or, if you have used row.dup and want to swap the rows:
output_row[1],output_row[6] = row[6],row[1]
# and so on
out_csv.puts(output_row)

This assumes that you have defined before

out_csv=CSV.new(STDOUT)

since you want to have your new CSV be created on standard output.

Let's first create a (header-less) CSV file:

enum = 1.step
FNameIn = 't_in.csv'

CSV.open(FNameIn, "wb") { |csv| 3.times { csv << 5.times.map { enum.next } } }
  #=> 3   

I've assumed the file contains string representations of integers.

The file contains the three lines:

File.read(FNameIn).each_line { |line| p line }
"1,2,3,4,5\n"
"6,7,8,9,10\n"
"11,12,13,14,15\n"

Now let's extract the columns at indices 1 and 3. These columns are to be written to the output file in that order.

cols = [1, 3]

Now write to the CSV output file.

arr = CSV.read(FNameIn, converters: :integer).
          map { |row| row.values_at(*cols) }
  #=> [[2, 4], [7, 9], [12, 14]] 

FNameOut = 't_out.csv'
CSV.open(FNameOut, 'wb') { |csv| arr.each { |row| csv << row } }

We have written three lines:

File.read(FNameOut).each_line { |line| p line }
"2,4\n"
"7,9\n"
"12,14\n"

which we can read back into an array:

CSV.read(FNameOut, converters: :integer)
  #=> [[2, 4], [7, 9], [12, 14]]

A straightforward transformation of these operations is required to perform these operations from the command line.