Re: Line number problem with foreach

for takes a list, so it reads the entire file before iterating the loop. So, by the time you get to printing the lines within the loop, $. is set to the line number of the last line in the file. Using while instead of for will ensure that you process the file line by line.

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Re^2: Line number problem with foreach by davido (Cardinal) on Feb 28, 2009 at 06:52 UTC
BrowserUK is exactly right. The `foreach()` loop asks for a list. The `<>` diamond operator behaves differently in list context than it does in scalar context. In scalar context, each time the diamond operator is evaluated it hands you the next line of the file. In list context, the entire file is slurped into memory at once and a list is created (and enumerated for the foreach loop). The `$.` special variable tells you what line of a file you most recently read. The foreach loop only evaluates the <> operator one time -- when it slurps in the entire file. Each time through the loop the foreach operator hands you the next line of the list, but the entire file has already been read into memory, and the $. variable tells you this by telling you that you're on line 4. This is a valuable lesson on how to quietly build a piece of code that cannot scale well when file sizes grow. If it weren't for the fact that the $. variable did something you didn't anticipate, you might have left the foreach loop in your code. Then one day when the file is 2GB huge, your client would call and ask why the program is locking up his system. Sticking to the `while()` construct will eliminate that issue. Dave	[reply] [d/l] [select]

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Re^2: Line number problem with foreach
by davido (Cardinal) on Feb 28, 2009 at 06:52 UTC

BrowserUK is exactly right. The foreach() loop asks for a list. The <> diamond operator behaves differently in list context than it does in scalar context. In scalar context, each time the diamond operator is evaluated it hands you the next line of the file. In list context, the entire file is slurped into memory at once and a list is created (and enumerated for the foreach loop).

The $. special variable tells you what line of a file you most recently read. The foreach loop only evaluates the <> operator one time -- when it slurps in the entire file. Each time through the loop the foreach operator hands you the next line of the list, but the entire file has already been read into memory, and the $. variable tells you this by telling you that you're on line 4.

This is a valuable lesson on how to quietly build a piece of code that cannot scale well when file sizes grow. If it weren't for the fact that the $. variable did something you didn't anticipate, you might have left the foreach loop in your code. Then one day when the file is 2GB huge, your client would call and ask why the program is locking up his system.

Sticking to the while() construct will eliminate that issue.

Dave

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