Appending an element to associative array awk -

i've got input file (input.txt) few fields:

a1  b1  c1  d1  e1 a2  b2  c2  d1  e2 a3  b3  c3  d2  e3 a4  b4  c4  d2  e4 

and want append elements of associative array,

awk '{a[$4]=a[$4] $5; print a[$4]} end {for(b in a) {print a[b]}}' input.txt 

i think output should (ie e2 concatenated e1, , e4 concatenated e3):

e1 e2 e3 e4 

but instead output is:

e2 e4 

i'm not sure what's wrong code?

your output isn't consistent command, assume want following:

• build list of 5th-column values each unique 4th-column value
• print these lists, preceded respective 4th-column value

a naïve fix want 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 2 things note:

• the accumulated 5-th column values have leading space - happens grouped output in case.
• due enumerating keys for (b in a), 4th-column values not appear in order appear in input, because order in awk enumerates keys of [always associative] arrays based on internal hash values, has no guaranteed relationship order in array elements added (nor guarantee particular order in general).