smknjoe has asked for the wisdom of the Perl Monks concerning the following question:

Hello,

I have the following code. It works and does what I pretty much want it to do, but it doesn't look the greatest. I was wondering if there is a better, cleaner approach to doing the same thing. Originally the $string had $i, but that errored out due to the fact that $i wasn't defined yet and because I needed it to be evaluated inside the for loop to expand it. Also, I was hoping to simply eval the string, but it isn't real Perl code so that wasn't a viable option.

#!/usr/bin/env perl

use strict;
use warnings;

### This eval code snippet works ###
eval(my $ref2 = "parent_ref");
####################################

my $string = "first_string ref[i] second_string $ref2.ref[i] third_str
+ing";
my $new_string;
my $i;

for ($i=0; $i<5; $i++) {
    print "$i : ";
    $new_string = $string; # don't clobber original string, else searc
+h-and-replace fails after first loop
    $new_string =~ s/\[i\]/\[$i\]/g;
    print "$new_string\n";
}

[download]

And the output looks like this:

0 : first_string ref[0] second_string parent_ref.ref[0] third_string
1 : first_string ref[1] second_string parent_ref.ref[1] third_string
2 : first_string ref[2] second_string parent_ref.ref[2] third_string
3 : first_string ref[3] second_string parent_ref.ref[3] third_string
4 : first_string ref[4] second_string parent_ref.ref[4] third_string

[download]

Anyways, I'm just hoping there's a better way to possibly do this that is probably more advanced than my Perl skillset allows.

Thanks, Joe W.

Replies are listed 'Best First'.
Re: Evaluating variables within a string
by NetWallah (Canon) on Dec 06, 2017 at 00:26 UTC
    You are over-thinking it. No Eval needed. This code produces your output: 
    perl -E "my $ref2 = q|parent_ref|; for my $i(0..4){say qq|first_string
+ ref[$i] second_string $ref2.ref[$i] third_string|}"

    [download]

                    All power corrupts, but we need electricity.

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Re: Evaluating variables within a string
by nysus (Parson) on Dec 06, 2017 at 00:27 UTC

    Throw a "my" declaration in the loop statement:

    for (my $i=0; $i<5; $i++) {
  print "$i : first_string ref[$i] second_string parent_ref.ref[$i] th
+ird_string\n";
}

    [download]

    Update:

    Forgot the $ref2 bit. So:

    my $ref2 = 'parent_ref';

for (my $i=0; $i<5; $i++) {
  print "$i : first_string ref[$i] second_string $ref2.ref[$i] third_s
+tring\n";
}

    [download]

    $PM = "Perl Monk's";
    $MCF = "Most Clueless Friar Abbot Bishop Pontiff Deacon Curate Priest";
    $nysus = $PM . ' ' . $MCF;
    Click here if you love Perl Monks

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[d/l]
[select]
Re: Evaluating variables within a string
by AnomalousMonk (Archbishop) on Dec 06, 2017 at 00:58 UTC
    eval(my $ref2 = "parent_ref");

    Like NetWallah and nysus, I'm puzzled by this statement: Is this supposed to represent some much more complicated or tricksey code?

    Give a man a fish:  <%-{-{-{-<

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[d/l]
[select]
      I knew I shouldn't have included that piece of code in the example. I was simply playing around with the eval command, and left it in. I know a little bit about "evil eval", but sometimes it is useful.
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Re: Evaluating variables within a string
by Dallaylaen (Chaplain) on Dec 06, 2017 at 05:14 UTC
    As it stands, a simple sprintf would work just fine: 
    my $string = "first_string ref[%u] second_string %s.ref[%u] third_stri
+ng";

for (my $i=0; $i<5; $i++) {
    my $new_string = sprintf $string, $i, "parent_ref", $i; 
    print "$i: $new_string\n";
}

    [download]
    Maybe the real case requires more sophisticated template engine, something like String::Formatter, but from the snippet it doesn't look like that.
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