Question

I want to parse and filter a file that looks like this:

@@1 Row one. 
@@2 Row two.

I have been able to do the filtering of the rows with the following code:

(defn parse-text-cms [sel-row]
  (let [f_data  (st/split  #"@@" (slurp "cms/tb_cms.txt"))] 
  ;(prn (map #(take 1 %) f_data))))
  (filter  #(= (first (take 1 %)) sel-row) f_data)))

However, this codes gives me (if sel-row=1):

1 Row one.

I would like to chop off that 1 and the space after, so to have:

Row one.

I think there is some sequence magic to do this. I just can’t come up with an elegant solution.

Answer 1

I would define the function the following way:

(defn parse-text-cms [sel-row]
  (with-open [input (clojure.java.io/reader "cms/tb_cms.txt")]
    (first
     (for [[_ number line] (map (partial re-find #"@@(\d)+\s+(.*)")
                                (line-seq input))
           :when (= number (str sel-row))]
       line))))

The combination of line-seq and reader gives me a sequence of lines from the input file. with-open ensures that the file is properly closed when I’m done. I apply a regex to each line that looks for @@ followed by a number and some spaces.

re-find returns a vector with three items:

• the whole matched line
• the number (the first group in the regex)
• the rest of the line (the second group in the regex)

I bind these to number and line using destructuring in a for statement (I’m not interested in the whole matched line, so I ignore that). I filter for the selected sel-row using :when and yield only the (rest of the) line.

Since I only expect one match in the file, I return just the first item from the sequence built by for. Because of the laziness of for, map and line-seq, this also stops reading of the file after the item is found.

If you do a lot of lookups for rows, I would suggest loading the whole file into memory instead of reading it every time, though.