Print 3x repetitions in second column in awk

Question

I want to print the whole line, when my program finds in the second column exactly 3x repetition. How to do that?

This is the input

DGD1 SOL401
DGD2 SOL4717
DGD2 SOL4847
DGD3 SOL401
DGD3 SOL4991
DGD3 SOL329
DGD4 SOL4991
DGD5 SOL4717
DGD5 SOL4717
DGD5 SOL401

This is the expected output

DGD1 SOL401
DGD2 SOL4717
DGD3 SOL401
DGD5 SOL4717
DGD5 SOL4717
DGD5 SOL401

question from:https://stackoverflow.com/questions/65860690/print-3x-repetitions-in-second-column-in-awk

Answer 1

You don't need to read the input file twice:

awk '
    { vals[$2] = vals[$2] $0 ORS }
    END {
        for (i in vals) {
            if ( gsub(ORS,"&",vals[i]) == 3 ) {
                printf "%s", vals[i]
            }
        }
    }
' eq9_1.ndx

Note that the above will work even for input coming from a stream.