The intention is to grab $index from the headerline of the .txt file (which only appears once on line-1). It's nothing more than a set of tab-delimited headers:

Strand    Type    Pos    Length    Form    Adjustment

However it can sometimes take the form:

ID   Strand    Type    Pos    Length    Form    Adjustment

In reply to Re^2: Memory usage while tallying instances of lines in a .txt file by TJCooper
in thread Memory usage while tallying instances of lines in a .txt file by TJCooper

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