Variable assignment after logical OR

rq-2102 has asked for the wisdom of the Perl Monks concerning the following question:

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Re: Variable assignment after logical OR by FunkyMonk (Bishop) on Oct 02, 2007 at 18:58 UTC
It's all to do with the order in which operators get evaluated (precedance). In your open statement, `\|\|` is evaluated before the comma, so your statement is parsed as `open FILE, (">test.txt" \|\| $failed_flag = 1);` giving the syntax error. Perl has some super-low precedance operators (and, or and not). `and` and `or` can be used as statement modifiers. So, you can write: `open FILE, ">test.txt" or $failed_flag = 1;` [download] Or, if you add parentheses to your original statement, you can write: `open( FILE, ">test.txt" ) \|\| $failed_flag = 1;` [download] That said, if you're using a modern perl, you are better off using the three-argument open and a lexical filehandle: `open my $FILE, ">", "test.txt" or $failed_flag = 1;` [download] See here to read more about precedance update: missed a quote. Thanks toolic. `not` can't be used as a modifier.	[reply] [d/l] [select]
Re^2: Variable assignment after logical OR by jwkrahn (Abbot) on Oct 02, 2007 at 20:03 UTC
In your open statement, `\|\|` is evaluated before the comma, so your statement is parsed as `open FILE, (">test.txt" \|\| $failed_flag = 1);` giving the syntax error. Actually the `\|\|` operator also has higher precedence than the `=` operator so it is parsed as `open FILE, (">test.txt" \|\| $failed_flag) = 1;` which is why you get the error `Can't modify constant item in scalar assignment`	[reply] [d/l] [select]
Re^3: Variable assignment after logical OR by FunkyMonk (Bishop) on Oct 02, 2007 at 20:40 UTC
You're absolutely right of course and I should have noticed that. Thanks for pointing it out.	[reply] [d/l] [select]
Re: Variable assignment after logical OR by ikegami (Patriarch) on Oct 02, 2007 at 19:03 UTC
You have a precedence problem. General rule: Use `or` to join statments together (where you'd otherwise use an `if`). Use `\|\|` when you'll use the result of the operation (i.e. in an assignment, to pass to a function, etc). Omitting parens from function calls has a few pitfalls, including this one. You might be better off using them.	[reply] [d/l] [select]
Re: Variable assignment after logical OR by snopal (Pilgrim) on Oct 02, 2007 at 18:43 UTC
Try this: `open FILE, ">test.txt" or $failed_flag = 1;`	[reply] [d/l]
Re: Variable assignment after logical OR by johngg (Canon) on Oct 02, 2007 at 19:08 UTC
snopal has pointed you towards using `or` instead of `\|\|` because the former is a lower-precedence equivalent of the latter. Another way to solve your problem would be to use parentheses with your open so the `\|\|` doesn't bind the file name and the flag together. `open(FILE, ">test.txt") \|\| $failed_flag = 1;` [download] It is often recommended these days to use the three-argument form of `open` with lexical filehandles, like this. `open my $testFH, '>', 'test.txt' or $failed_flag = 1;` [download] Cheers, JohnGG Update: I really must learn to type faster :(	[reply] [d/l] [select]
Re^2: Variable assignment after logical OR by rq-2102 (Acolyte) on Oct 02, 2007 at 19:36 UTC
Thanks all for the help and advice! For now I've switched from "\|\|" to "or" but I'll definitely try to make a habit of using the new open statement as well, in this case it's not an option but it's good to know. --------- brian	[reply]
Re^3: Variable assignment after logical OR by ikegami (Patriarch) on Oct 02, 2007 at 20:47 UTC
Why is it not an option?	[reply]
Re: Variable assignment after logical OR by Eimi Metamorphoumai (Deacon) on Oct 03, 2007 at 19:07 UTC
Others have told you what's wrong with your code, but I'd like to mention that there are other options besides a flag value and a subroutine. In particular, I would suggest something like `unless(open FILE, ">test.txt"){ #error handling code goes here }` [download]	[reply] [d/l]
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