CODEWITHSUNDEEP
perleof
intiha
intiha

Reputation: 319

How can I check for eof in Perl?

So I have a bit of problem figuring what Perl does in the following case:

while(1){
$inputLine=<STDIN>

#parse $inputLine below
#BUT FIRST, I need to check if $inputLine = EOF

}

before I get the obvious answer of using while(<>){}, let me say that there is a very strong reason that I have to do the above (basically setting up an alarm to interrupt blocking and I didnt want that code to clutter the example).

Is there someway to compare $inputLine == undef (as I think that is what STDIN returns at the end).

Thanks.

Upvotes: 5

Views: 18406

Answers (4)

cheard
cheard

Reputation: 1

The following will have problems with input files that have lines which only have a line feed or as in the case that was giving me problems a FF at the beginning of some lines (Form Feed - the file was the output from a program developed at the end of the 70s and still has formatting for a line printer and is still in FORTRAN - I do miss the wide continous paper for drawing flow diagrams on the back).

open (SIMFIL, "<", 'InputFileName') or die "Can´t open InputFileName\n" ;
open (EXTRDATS, ">>", 'OutputFileName' ) or die "Can´t open OutputFileName\n";
$Simfilline = "";
while (<SIMFIL>) {
    $Simfilline = <SIMFIL>;
    print EXTRDATS $Simfilline;
    $Simfilline = <SIMFIL>;
    print EXTRDATS $Simfilline;
    }
close SIMFIL;
close EXTRDATS;

` The following is when eof comes in handy - the expression: "while ()" can return false under conditions other than the end of the file.

open (SIMFIL, "<", 'InputFileName') or die "Can´t open InputFileName\n" ;
open (EXTRDATS, ">>", 'OutputFileName' ) or die "Can´t open OutputFileName\n";
$Simfilline = "";
while (!eof SIMFIL) {
    $Simfilline = <SIMFIL>;
    print EXTRDATS $Simfilline;
    $Simfilline = <SIMFIL>;
    print EXTRDATS $Simfilline;
    }
close SIMFIL;
close EXTRDATS;

This last code fragment appears to duplicate the input file exactly.

Upvotes: 0

Greg Bacon
Greg Bacon

Reputation: 139491

Inside your loop, use

last unless defined $inputLine;

From the perlfunc documentation on defined:

defined EXPR
defined

Returns a Boolean value telling whether EXPR has a value other than the undefined value undef. If EXPR is not present, $_ will be checked.

Many operations return undef to indicate failure, end of file, system error, uninitialized variable, and other exceptional conditions. This function allows you to distinguish undef from other values. (A simple Boolean test will not distinguish among undef, zero, the empty string, and "0", which are all equally false.) Note that since undef is a valid scalar, its presence doesn't necessarily indicate an exceptional condition: pop returns undef when its argument is an empty array, or when the element to return happens to be undef.

Upvotes: 11

Vivin Paliath
Vivin Paliath

Reputation: 95518

You can use eof on the filehandle. eof will return 1 if the next read on FILEHANDLE is an EOF.

Upvotes: 1

mob
mob

Reputation: 118605

defined($inputLine)

Also, see the 4 argument version of the select function for an alternative way to read from a filehandle without blocking.

Upvotes: 4

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