Printing every other line from input text

Question

I am trying to do some text manipulation in Mathematica. After importing a text file (in.txt) as a string, how would you then output every other line? In the command line using awk for instance one would do

awk 'NR % 9 != 8' in.txt

However, after looking through the documentation Mathematica doesn't seem to have any current line number querying mechanism. Do you need to split the input string on every new line and then work with the list of strings? Or is Mathematica not the best choice for manipulating text?

Answer 1

You could fashion yourself a tool, for instance the following:

printlines[filename_, start_: 1, end_: All, every_: 1] := Module[
  {lines},
  lines = MapIndexed[
     {First@#2, #1} &,
     ReadList[filename, "String"]
    ];
  Grid[lines[[start ;; end ;; every]], Alignment -> {{Right, Left}}]
]

You can pass a filename to open, and option arguments to specify the first and last lines to print, and the "pitch" (i.e. printing every 2nd, 3rd, ... line). For instance:

file = "what's in a name.txt";
printlines[file]

printlines[file, 6, 7]

printlines[file, 6, All, 3]

Answer 2

The awk approach you gave as an example is fundamentally different to what you ask. With awk you do some text processing ON THE FLY (at the same time each line is read), it is not equivalent to 1/read ALL the text first, then 2/ do the filtering, processing on the whole text saved in memory. These involve completely different strategies.

You can actually do something similar to awk in Mathematica using Low level functions:

To read every other line on the fly, you can do for example:

(* Here is the way to create the dummy file, "file.txt", example I use in the test:
Table["Blabla # "<>ToString[i]<>"\n", {i, 10}]//StringJoin//Export["file.txt", #] &
*)


stream = OpenRead["file.txt"];

NestWhileList[
 (Skip[stream, Record]; Read[stream, Record])&,
 (Read[stream, Record]),
 # =!= EndOfFile &,
 1, Infinity, -1]

Close[stream]

{"Blabla # 1", "Blabla # 3", "Blabla # 5", "Blabla # 7", "Blabla # 9"}

In case you don't know, it is also possible to run directly from Mathematica the awk's code (or some other external command) and retrieve the result, for example here:

ReadList["!awk 'NR % 2!=0' file.txt", String]

{"Blabla # 1", "Blabla # 3", "Blabla # 5", "Blabla # 7", "Blabla # 9"}