Re^3: Trouble skipping lines using Perl

LeBran:

It didn't give you any warnings because the expression $chromosome = /^chrM/ is perfectly fine. It just doesn't do what you want it to. Instead of checking whether $chromosome starts with "chrM", it instead checks whether $_ starts with "chrM", and then sets $chromosome to a true value if it does, and a false value otherwise. Since you're not using $_ while parsing your lines, it never starts with "chrM" and always returns a false value.

It's a common enough mistake that I could see a case being made for "if ($var = /rex/)" generating a warning, as I expect that "if ($var = ($_ =~ /rex/))" is pretty uncommon (at least, when looking at my code).

...roboticus

When your only tool is a hammer, all problems look like your thumb.

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Re^4: Trouble skipping lines using Perl by LeBran (Initiate) on Nov 21, 2017 at 17:03 UTC
Ah, thank you, So presumably `next if ($_ = /^chrM/);` would also be correct? Cheers	[reply] [d/l]
Re^5: Trouble skipping lines using Perl by AnomalousMonk (Archbishop) on Nov 21, 2017 at 17:23 UTC
So presumably `next if ($_ = /^chrM/);` would also be correct? No! The `$_ = /^chrM/` expression matches the regex against `$_` (update: by default, since the `//` match is not explicitly bound to any other scalar by a `=~` binding operator) and then assigns the result of the ~~comparison~~ \| match to `$_` (which at least gives `$_` some defined value, so I guess it's not all bad :) Matching against `$_` (or any other scalar variable) is only semantically correct if that variable has first been given some meaningful value as in a `while (<FILE>) { ... }` loop. Stick to `next if ($line =~ /^chrM/);` Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]