Which AWK program can do this manipulation?

Question

Given a file containing a structure arranged like the following (with fields separated by SP or HT)

4 5 6 2 9 8 4 8
m d 6 7 9 5 4 g
t 7 4 2 4 2 5 3
h 5 6 2 5 s 3 4
r 5 7 1 2 2 4 1
4 1 9 0 5 6 d f
x c a 2 3 4 5 9
0 0 3 2 1 4 q w

Which AWK program do I need to get the following output?

  4 5
  m d
  t 7
  h 5
  r 5
  4 1
  x c
  0 0
  6 2
  6 7
  4 2
  6 2
  7 1
  9 0
  a 2
  3 2
  9 8
  9 5
  4 2
  5 s
  2 2
  5 6
  3 4
  1 4
  4 8
  4 g
  5 3
  3 4
  4 1
  d f
  5 9
  q w

Thanks in advance for any and all help.

Postscript

Please bear in mind,

1. My input file is much larger than the one depicted in this question.

2. My computer science skills are seriously limited.

3. This task has been imposed on me.

Answer 1

awk -v n=4 '
    function join(start, end,    result, i) {
        for (i=start; i<=end; i++)
            result = result $i (i==end ? ORS : FS)
        return result
    }
    {
        c=0
        for (i=1; i<NF; i+=n) {
            c++
            col[c] = col[c] join(i, i+n-1)
        }
    }
    END {
        for (i=1; i<=c; i++)
            printf "%s", col[i]  # the value already ends with newline
    }
' file

The awk info page has a short primer on awk, so read that too.

---

Benchmarking

1. create an input file with 1,000,000 columns and 8 rows (as specified by OP)

   #!perl
   my $cols = 2**20; # 1,048,576
   my $rows = 8;
   my @alphabet=( 'a'..'z', 0..9 );
   my $size = scalar @alphabet;
   
   for ($r=1; $r <= $rows; $r++) {
       for ($c = 1; $c <= $cols; $c++) {
           my $idx = int rand $size;
           printf "%s ", $alphabet[$idx];
       }
       printf "
   ";
   }
   

   $ perl createfile.pl > input.file
   $ wc input.file
          8  8388608 16777224 input.file
   

2. time various implementations: I use the fish shell, so the timing output is different from bash's

   • my awk

     $ time awk -f columnize.awk -v n=4 input.file > output.file
     
     ________________________________________________________
     Executed in    3.62 secs   fish           external
        usr time    3.49 secs    0.24 millis    3.49 secs
        sys time    0.11 secs    1.96 millis    0.11 secs
     
     $ wc output.file
      2097152  8388608 16777216 output.file
     

   • Timur's perl:

     $ time perl -lan columnize.pl input.file > output.file
     
     ________________________________________________________
     Executed in    3.25 secs   fish           external
        usr time    2.97 secs    0.16 millis    2.97 secs
        sys time    0.27 secs    2.87 millis    0.27 secs
     

   • Ravinder's awk

     $ time awk -f columnize.ravinder input.file > output.file
     
     ________________________________________________________
     Executed in    4.01 secs   fish           external
        usr time    3.84 secs    0.18 millis    3.84 secs
        sys time    0.15 secs    3.75 millis    0.14 secs
     

   • kvantour's awk, first version

     $ time awk -f columnize.kvantour -v n=4 input.file > output.file
     
     ________________________________________________________
     Executed in    3.84 secs   fish           external
        usr time    3.71 secs  166.00 micros    3.71 secs
        sys time    0.11 secs  1326.00 micros    0.11 secs
     

   • kvantour's second awk version: Crtl-C interrupted after a few minutes

     $ time awk -f columnize.kvantour2 -v n=4 input.file > output.file
     ^C
     ________________________________________________________
     Executed in  260.80 secs   fish           external
        usr time  257.39 secs    0.13 millis  257.39 secs
        sys time    1.68 secs    2.72 millis    1.67 secs
     
     $ wc output.file
      9728 38912 77824 output.file
     

     The $0=a[j] line is pretty expensive, as it has to parse the string into fields each time.

   • dawg's python

     $ timeout 60s fish -c 'time python3 columnize.py input.file 4 > output.file'
     [... 60 seconds later ...]
     $ wc output.file
      2049  8196 16392 output.file
     

3. another interesting data point: using different awk implementations. I'm on a Mac with GNU awk and mawk installed via homebrew

   • with many columns, few rows

     $ time gawk -f columnize.awk -v n=4 input.file > output.file
     
     ________________________________________________________
     Executed in    3.78 secs   fish           external
        usr time    3.62 secs  174.00 micros    3.62 secs
        sys time    0.13 secs  1259.00 micros    0.13 secs
     

     $ time /usr/bin/awk -f columnize.awk -v n=4 input.file > output.file
     
     ________________________________________________________
     Executed in   17.73 secs   fish           external
        usr time   14.95 secs    0.20 millis   14.95 secs
        sys time    2.72 secs    3.45 millis    2.71 secs
     

     $ time mawk -f columnize.awk -v n=4 input.file > output.file
     
     ________________________________________________________
     Executed in    2.01 secs   fish           external
        usr time  1892.31 millis    0.11 millis  1892.21 millis
        sys time   95.14 millis    2.17 millis   92.97 millis
     

   • with many rows, few columns, this test took over half an hour on a MacBook Pro, 6 core Intel cpu, 16GB ram

     $ time mawk -f columnize.awk -v n=4 input.file > output.file
     
     ________________________________________________________
     Executed in   32.30 mins   fish           external
        usr time   23.58 mins    0.15 millis   23.58 mins
        sys time    8.63 mins    2.52 millis    8.63 mins