Add Extract A Column Of Data From A CSV File as a ruby TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1161 TILs and counting..._
+_1162 TILs and counting..._
 
 ---
 
@@ -915,6 +915,7 @@ _1161 TILs and counting..._
 - [Evaluating One-Off Commands](ruby/evaluating-one-off-commands.md)
 - [Exclude Values From An Array](ruby/exclude-values-from-an-array.md)
 - [Expect A Method To Be Called And Actually Call It](ruby/expect-a-method-to-be-called-and-actually-call-it.md)
+- [Extract A Column Of Data From A CSV File](ruby/extract-a-column-of-data-from-a-csv-file.md)
 - [FactoryGirl Sequences](ruby/factory-girl-sequences.md)
 - [Fail](ruby/fail.md)
 - [Find The Min And Max With A Single Call](ruby/find-the-min-and-max-with-a-single-call.md)

ruby/extract-a-column-of-data-from-a-csv-file.md

@@ -0,0 +1,39 @@
+# Extract A Column Of Data From A CSV File
+
+Let's say I have just downloaded a medium-sized CSV of data onto my hard drive.
+I want to exact one column of data from that CSV to inject into some other
+tool/process.
+
+Here is how I'd go about doing that with Ruby.
+
+```ruby
+require 'csv'
+
+filename = "#{Dir.home}/Downloads/file.csv"
+column_index = 2 # zero-indexed column of interest
+
+# an array of collecting the values I want to extract
+col_data = []
+
+# read in the CSV into memory
+csv_data = CSV.read(filename)
+
+# pop headers out of the top of the array
+csv_data.shift
+
+# grab the column of interest from each row
+csv_data.each { |row| col_data << row[column_index] }
+
+# do something with the extract column of data
+comma_separated_list = col_data.join(', ')
+system "command -v pbcopy >/dev/null 2>&1 && echo '#{comma_separated_list}' | pbcopy"
+```
+
+All but the last two lines are pretty standard. We identify the file and column
+of interest. Read in the CSV from that file and ditch the headers. Then we grab
+that column's value for every entry in the CSV.
+
+Then we need to do something with that data.
+
+In my case, I want to turn those values into a comma-separated list and put it
+on my clipboard. Those last two lines do just that.