Stack multiple columns

Question

I would like to change a file so that column N+1 would be put at the end of column N for each N = odd number. For instance, if I had a file with 4 columns, the column 2 would be pasted at the end of column 1 and the column 4 at the end of column 3.

Example file:

1 2 3 4
1 2 3 4
1 2 3 4

Output file:

1 3
1 3
1 3
2 4
2 4
2 4

My files have ~100 lines and ~60000 columns.

Answer 1

With bash:

#!/bin/bash

for c in 0 1; do
  while read -r -a columns; do
    for((i=$c; i<${#columns[@]}; i+=2)); do
      echo -n "${columns[$i]} "
    done
    echo
  done < file
done

Output:

1 3 
1 3 
1 3 
2 4 
2 4 
2 4 

Answer 2

Here's some fairly cryptic awk:

awk '{for (i=1; i<=NF; i+=2) {printf "%s ", $(i + (NR > FNR))}; print ""}' file file

The first time you process the file, NR > FNR will be false, or 0.
The second time you process the file, NR > FNR will be true, or 1.

---

This one requires a single pass through the file, but you have to keep half the file in memory:

awk '
    {
        for (i=1; i<NF; i+=2) {
            printf "%s ", $i
            row[NR] = row[NR] $(i+1) " "
        }
        print ""
    }
    END {
        for (i=1; i<=NR; i++) print row[i]
    }
' file