Merge values for same key

Question

Is that possible to use awk to values of same key into one row? For instance

a,100
b,200
a,131
a,102
b,203
b,301

Can I convert them to a file like this:

a,100,131,102
b,200,203,301

Answer 1

If Perl is an option,

perl -F, -lane '$a{$F[0]} = "$a{$F[0]},$F[1]"; END{for $k (sort keys %a){print "$k$a{$k}"}}' file

These command-line options are used:

• -n loop around each line of the input file
• -l removes newlines before processing, and adds them back in afterwards
• -a autosplit mode – split input lines into the @F array. Defaults to splitting on whitespace.
• -e execute the perl code
• -F autosplit modifier, in this case splits on ,

@F is the array of words in each line, indexed starting with $F[0]
$F[0] is the first element in @F (the key)
$F[1] is the second element in @F (the value)
%a is a hash which stores a string containing all matches of each key

Answer 2

You can use awk like this:

awk -F, '{a[$1] = a[$1] FS $2} END{for (i in a) print i a[i]}' file

a,100,131,102
b,200,203,301

We use -F, to use comma as delimiter and use array a to keep aggregated value.

Reference: Effective AWK Programming