Question

For example, suppose I run the following command:

gawk -f AppendMapping.awk Reference.tsv TrueInput.tsv

Assume the names of files WILL change.
While iterating through the first file, I want to create a mapping.

map[$16]=$18

While iterating through the second file, I want to use the mapping.

print $1, map[$2]

What’s the best way to achieve this behavior (ie, different behavior for each input file)?

Answer 1

As you probably know NR stores the current line number; as you may or may not know, it’s cumulative – it doesn’t get reset between files. FNR, on the other hand, is specific to the file, so you can use those two to see whether you’re in the first file (beyond the second you’ll need to keep your own counter).

# In case you want to keep track of the file number
FNR == 1 { fileno++ }*emphasized text*

NR == FNR {
    # First file
}
NR != FNR {
    # Second or later file
}

You could also use getline in the BEGIN block to loop through it manually.

BEGIN {
    file = ARGV[1]
    while(getline < file) {
        # Process line
    }
    delete ARGV[1]
}