Filtering rows of a dataframe with AWK to only show entries in a given field with X instances

Question

I have a large dataframe, in which one of the fields("ID") correspond to a unique identifier. However, there are multiple instances of that value, depending on the binary combination of additional measurements("strucureA" and "structureB"). I want to devise a way to filter the rows using awk to shows measurements associated with an ID that only has 4 instances in the initial dataframe.

I can envision doing this in two parts, first counting the number of instances using uniq -c or a awk array and saving to a file, and then using that new file to filter the initial dataframe with awk arrays and FNR==NR. But I was hoping that there may be a way for awk to tally the count and then print a group of rows that meet a criteria (e.g. an entry appearing X number of times ).

> Initial table
ID  structureA  structureB
sample_1    1   1
sample_2    1   1
sample_2    2   1
sample_2    1   2
sample_2    2   2
sample_3    1   1
sample_3    2   1
sample_3    3   1
sample_4    1   1
sample_4    2   1
sample_4    3   1
sample_4    4   1

> Desired table
ID  structureA  structureB
sample_2    1   1
sample_2    2   1
sample_2    1   2
sample_2    2   2
sample_4    1   1
sample_4    2   1
sample_4    3   1
sample_4    4   1

Answer 1

Thanks to Ed Morton for improvement of the awk script:

awk -v n=$1 '
NR==1
{a[$1]=($1 in a ? a[$1] RS : "") $0}
END{for(i in a){if(gsub(RS,"&",a[i])==n-1){print a[i]}}}
' "$2"

NR==1 prints the header. The last statement invokes split function just to check how many lines there are in each array element. If they are 4, then they are printed.

You call it with ./script 4 file (the 1st argument is the number of repetitions, the 2nd is the file name) after making it executable.

This answer does not impose that lines with same first field must be next to each other in the input file.

Answer 2

$ awk -v n=4 '
    NR==1 { print; next }
    $1 != prev { if (cnt==n) printf "%s", buf; prev=$1; cnt=0; buf="" }
    { cnt++; buf=buf $0 ORS }
    END { if (cnt==n) printf "%s", buf }
' file
ID  structureA  structureB
sample_2    1   1
sample_2    2   1
sample_2    1   2
sample_2    2   2
sample_4    1   1
sample_4    2   1
sample_4    3   1
sample_4    4   1