miller/docs6/docs/data/unsparsify.mlr

# First pass:
# Remember all unique key names:
for (k in $*) {
  @all_keys[k] = 1;
}
# Remember all input records:
@records[NR] = $*;

# Second pass:
end {
  for (nr in @records) {
    # Get the sparsely keyed input record:
    irecord = @records[nr];
    # Fill in missing keys with empty string:
    map orecord = {};
    for (k in @all_keys) {
      if (haskey(irecord, k)) {
        orecord[k] = irecord[k];
      } else {
        orecord[k] = "";
      }
    }
    # Produce the output:
    emit orecord;
  }
}