How to use empty "<>" (Diamond Operator) inside While loop

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

I am not getting "<>" diamond operator right. I want to understand how the "<>" (empty Diamond operator)" works in a "while"loop.

Here is my code ( the script is saved as diamond.pl):

#!/usr/bin/perl -w
while (<>)
{
    print ;
}
[download]

Now i have two files called colors.txt and seasons.txt in the same directory as that of the above script. so if i run below command, it start printing the individual lines of both the .txt files:

perl diamond.pl colors.txt seasons.txt
[download]

and I understand above.

However, If I run below comamnds, it errors out;

perl diamond.pl these are words and not the file names
[download]

I get below eerror:

# perl diamond.pl these are words and not the file names
Can't open these: No such file or directory at diamond.pl line 2.
Can't open are: No such file or directory at diamond.pl line 2.
Can't open words: No such file or directory at diamond.pl line 2.
Can't open and: No such file or directory at diamond.pl line 2.
Can't open not: No such file or directory at diamond.pl line 2.
Can't open the: No such file or directory at diamond.pl line 2.
Can't open file: No such file or directory at diamond.pl line 2.
Can't open names: No such file or directory at diamond.pl line 2.
[download]

My undestanding was (and this is mentioned on numerous pages on hte internet), the empty "<>" checks for the @ARGV. So in above command, I was hoping to see the output as one word of the sequence "these are words and not the file names" printed on each line.

I am pretty sure am doing something really dumb here. could you explain how an "<>" (empty) operators works in a while loop. An example of the script and the command would be of great help.

Thanks.

Comment on How to use empty "<>" (Diamond Operator) inside While loop Select or Download Code

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Re: How to use empty "<>" (Diamond Operator) inside While loop by choroba (Cardinal) on Jun 25, 2012 at 09:04 UTC
No, that is not how the diamond operator works. It checks for arguments, but only to treat them as file names. If there are no arguments, it reads its input from STDIN. You can try that with `echo This is the standard input \| perl diamond.pl` [download] See I/O Operators	[reply] [d/l]
Re^2: How to use empty "<>" (Diamond Operator) inside While loop by zwon (Abbot) on Jun 25, 2012 at 13:08 UTC
treat them as file names to be exact, it passes them to two-argument open, which may treat them as file names or as something else. See magic-diamond <> behavior -- WHAT?!.	[reply]
Re: How to use empty "<>" (Diamond Operator) inside While loop by CountZero (Bishop) on Jun 25, 2012 at 09:19 UTC
Thus sayeth the docs: The null filehandle `<>` is special: it can be used to emulate the behavior of sed and awk, and any other Unix filter program that takes a list of filenames, doing the same to each line of input from all of them. Input from `<>` comes either from standard input, or from each file listed on the command line. Here's how it works: the first time `<>` is evaluated, the `@ARGV` array is checked, and if it is empty, `$ARGV[0]` is set to "-", which when opened gives you standard input. The `@ARGV` array is then processed as a list of filenames. The loop `while (<>) { ... # code for each line }` [download] is equivalent to the following Perl-like pseudo code: `unshift(@ARGV, '-') unless @ARGV; while ($ARGV = shift) { open(ARGV, $ARGV); while (<ARGV>) { ... # code for each line } }` [download] except that it isn't so cumbersome to say, and will actually work. It really does shift the `@ARGV` array and put the current filename into the `$ARGV` variable. It also uses filehandle `ARGV` internally. `<>` is just a synonym for `<ARGV>`, which is magical. (The pseudo code above doesn't work because it treats `<ARGV>` as non-magical.) Alwyas go back to the source before you trust random info on the internet. CountZero A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James My blog: Imperial Deltronics	[reply] [d/l] [select]
Re: How to use empty "<>" (Diamond Operator) inside While loop by Athanasius (Archbishop) on Jun 25, 2012 at 09:09 UTC
See I/O Operators in perlop in perldoc. Athanasius <°(((>< contra mundum	[reply]
Re: How to use empty "<>" (Diamond Operator) inside While loop by tobyink (Canon) on Jun 25, 2012 at 09:57 UTC
`these are words and not the file names` [download] How do you expect Perl to tell the difference between words which are filenames and words which are not filenames? They're all treated as filenames. That's how `<>` works. `perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'`	[reply] [d/l] [select]
Re: How to use empty "<>" (Diamond Operator) inside While loop by Anonymous Monk on Jun 25, 2012 at 09:55 UTC
See also Iterator::Diamond/Iterator::Files, or FMTYEWTK About Mass Edits In Perl, opening many files, Tutorials: Perl documentation documentation, Searching Perl Documentation	[reply]
Re: How to use empty "<>" (Diamond Operator) inside While loop by slayedbylucifer (Scribe) on Jun 27, 2012 at 03:04 UTC
Hi Everyone, Thanks for your responses. I see now where I was going wrong. Thank you very much.	[reply]