Advent Calendar - December 11, 2020

Friday, Dec 18, 2020| Tags: Perl

Advent Calendar 2020


The gift is presented by Adam Russell. Today he is talking about his solution to the task Lines Range of “The Weekly Challenge - 072”. This is re-produced for Advent Calendar 2020 from the original post by Adam Russell.


You are given a text file name $file and range $A - $B where $A <= $B.

Write a script to display lines range $A and $B in the given file.

Example

Input:

    $ cat input.txt
    L1
    L2
    L3
    L4
    ...
    ...
    ...
    ...
    L100
$A = 4 and $B = 12

Output:

    L4
    L5
    L6
    L7
    L8
    L9
    L10
    L11
    L12

I was a bit surprised to realize how this could be done with a one liner. In the example below input.txt is a plain text file which has 100 lines that all look like LX for 1 <= X <= 100.

$ perl -s -n -e 'print if $. >= $A && $. <= $B'  -A=4 -B=12 < input.txt

L4
L5
L6
L7
L8
L9
L10
L11
L12

The use of the special variable $. to track input line numbers is a common Perl idiom. What was surprising to me was that I was unfamiliar with the -s command line option. This option allows you to set variables on the command line. Anything after the — is interpreted to be a variable initialized to the given value. You can see that in the example above where -A=4 and -B=12 creates variables $A and $B initialized to 4 and 12 respectively.

If you do not set the variable to something then it is just initialized as true. For example:

$ perl -s -e 'print "$x\n";'  -x
1

If you have any suggestion then please do share with us perlweeklychallenge@yahoo.com.

Advent Calendar 2020

SO WHAT DO YOU THINK ?

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