Appending an element to associative array awk

Question

I\'ve got an input file (input.txt) with a few fields:

A1  B1  C1  D1  E1
A2  B2  C2  D1  E2
A3  B3  C3  D2  E3
A4  B4  C4  D2  E4

And I wa

Answer 1

Your output isn't consistent with your command, but I assume that you want the following:

• build up a list of 5th-column values for each unique 4th-column value
• print these lists, preceded by the respective 4th-column value

A naïve fix to get what you want would be:

$ awk '{a[$4]=a[$4] " " $5} END {for (b in a) { print b; print a[b]}}' input.txt
D1
 E1 E2
D2
 E3 E4

but there are two things to note:

• The accumulated 5-th column values will have a leading space - which happens to help with grouped output in this case.
• Due to enumerating the keys with for (b in a), the 4th-column values will NOT appear in the order they appear in the input, because the order in which awk enumerates keys of its [always associative] arrays is based on internal hash values, which has no guaranteed relationship to the order in which array elements were added (nor does it guarantee any particular order in general).