Collapse rows based on a key in a csv file using awk or perl

Question

Input csv file :

key,c1,c2,c3......,cn
1,car,phone,cat,.....,kite
2,abc,def,hij,.......,pot
1,yes,no,is,.........,hello
2,hello,yes,no,......,help

Output csv file:

Key,c1,c2,c3,.......,cn
1,caryes,phoneno,catis,.....,kitehello
2,abchello,defyes,hijno,....,pothelp

The input file has 14 million rows. Can someone help with an efficient way of doing this? Thanks in advance.

Answer 1

I don't know how efficient it will be, but you could do something like this with a hash of anonymous arrays, using a map to apply the string concatenation assignment .= to each element of each hash value:

perl -F, -nle '
  $k = shift @F;
  map { $h{$k}[$_] .= $F[$_] } 0..$#F 
  }{ 
  for $k (sort { $a <=> $b } keys %h) {
    print join ",", $k, @{ $h{$k} }
  }' file
key,c1,c2,c3......,cn
1,caryes,phoneno,catis,..............,kitehello
2,abchello,defyes,hijno,.............,pothelp